Benzaldehida

Seperti: dalam lingkungan alkalin yang kuat, benzaldehida sendiri reaksi disproporsionasi redoks, menghasilkan asam dan alkohol benzil:

Persiapan Obat

Benzaldehida banyak hadir dalam tanaman, terutama di Rosaceae tersebut, terutama dalam bentuk glikosida hadir di pabrik kulit batang, daun atau biji, misalnya, almond pahit amygdalin. Metode produksi industri benzaldehida Ada dua kategori utama: masing-masing, toluena dan benzena sebagai bahan baku. Persiapan Laboratorium dapat diterapkan untuk katalitik (paladium / barium sulfat) benzoil metode reduksi klorida.

1 klorinasi toluena dan metode hidrolisis dengan toluena sebagai bahan baku dalam terang klorinasi, dengan campuran benzil klorida. Konsumsi bahan baku tetap: toluena 1700kg / t, klorin 3000kg / t, soda 1500kg / t.

(2) oksidasi benzyl alcohol benzil klorida hidrolisis benzil alkohol, benzaldehida diperoleh melalui oksidasi.

3 oksidasi langsung toluena, benzena, formaldehida, toluena oksidasi intermediet asam benzoat. Benzyl alcohol, toluene benzaldehida asam benzoat.

4 dengan benzena sebagai bahan baku di bawah tekanan dan di bawah aksi aluminium klorida, benzena direaksikan dengan karbon monoksida dan hidrogen. Konten benzaldehida Industri di atas 98,5%.

Pemurnian

1 Pasang sejumlah benzaldehida dilarutkan dalam eter dan kemudian dicuci dengan larutan Na2CO3. Solusi organik dicuci dikeringkan di atas Na2SO4 anhidrat, dan pelarut telah dihapus oleh penguapan rotary (eter). The diperlakukan benzaldehida sebagai sejumlah kecil bubuk seng, distilasi pengurangan tekanan.

(2) pertama dengan 10% natrium hidroksida atau natrium karbonat, dan kemudian dicuci dicuci dengan natrium berair sulfit. Magnesium sulfat atau natrium sulfat: yang terakhir untuk bergabung disulfida nitrogen distilasi vakum. [2]

PENANGANAN Penyimpanan

Penanganan Tindakan pencegahan: operasi tertutup, ventilasi penuh. Operator harus melalui pelatihan khusus, ketaatan pada aturan. Operator Usulan memakai saringan penyerapan diri respirator (masker setengah), memakai kacamata keselamatan pelindung kimia, mengenakan overall pelindung pada infiltrasi, mengenakan sarung tangan karet. Jauhkan dari, panas, merokok di tempat kerja api. Gunakan sistem ventilasi ledakan-bukti dan peralatan. Mencegah kebocoran uap ke udara tempat kerja. Hindari kontak dengan agen oksidasi, asam. Penanganan dalam nitrogen. Penanganan cahaya ketika cahaya muat, kemasan dan wadah untuk mencegah kerusakan. Dilengkapi dengan berbagai dan jumlah peralatan kebakaran dan kebocoran peralatan darurat yang sesuai. Wadah kosong mungkin residu berbahaya.

Penyimpanan Perhatian: Simpan di tempat sejuk, berventilasi gudang. Jauhkan dari api dan panas. Packing segel, tidak bisa kontak dengan udara. Haruskah oksidan, asam, bahan kimia makanan yang disimpan secara terpisah dan menghindari pencampuran waduk. Ledakan-bukti pencahayaan, fasilitas ventilasi. Melarang penggunaan mudah untuk menghasilkan percikan peralatan mekanik dan alat-alat. Area penyimpanan harus dilengkapi dengan peralatan respon tumpahan dan bahan host yang cocok. [3]

Tujuan.

Penggunaan penting bahan baku kimia yang digunakan dalam pembuatan aldehida lauril, asam laurat, asetaldehida dan benzene, benzil benzoat, juga digunakan sebagai bumbu

Penggunaan untuk pengukuran ozon, fenol, alkaloid dan terletak di samping metilen reagen karboksil

Pernyataan Keselamatan sebagai header rempah-rempah khusus harum, rumus bunga untuk jejak, seperti ungu, Prynne, melati, violet, akasia, bunga matahari, manis dadih, plum, orange blossom, dll. Sabun juga bisa digunakan. Bisa juga digunakan sebagai bumbu makanan almond, buah, krim, ceri, kelapa, aprikot, peach, kemiri, plum besar, vanili, rempah-rempah dan rasa lainnya. Rasa anggur seperti rum, brandy dan jenis lainnya juga digunakan

Laporan Risiko benzaldehida liar Yan herbisida kering dan pengatur tumbuh intermediet inabenfide

Keselamatan Laporan GB 2760 - 1996 menyediakan untuk penggunaan sementara rasa makanan. Terutama digunakan untuk persiapan almond, ceri, persik, kacang-kacangan dan rasa lainnya, jumlah hingga 40%. Fu sebagai kaleng cherry agen flavoring sirup, dosis per kg gula 3ml

Menggunakan enam farmasi, pewarna, rempah-rempah intermediet. Sekelompok untuk produksi benzaldehida, asam laurat, aldehida laurat, lampu hijau, benzil benzoat, anilin benzylidene, aseton benzylidene. Untuk pencampuran rasa sabun, rasa makanan, dll

Tujuh] [Penggunaan sintesis organik, pelarut, ozon diukur dalam metilen sebelah gugus karbonil, fenol dan uji alkaloid, persiapan aroma [4]

Risiko

Bahaya Kesehatan

Sepasang mata dan mukosa pernafasan memiliki efek merangsang. Karena volatilitas yang rendah, efek merangsang yang cukup untuk menyebabkan bahaya serius.

Bahaya Ledakan

Barang-barang yang mudah terbakar, beracun, mengiritasi. [3]

Tindakan

Tindakan pertolongan pertama

Kulit hubungi: Lepaskan pakaian yang terkontaminasi, air ponsel bilas.

Kontak mata: Apakah kelopak mata, air irigasi ponsel atau garam. Dokter

Synthesis and Characterization of a Novel Series ofMeso(Nitrophenyl) andMeso(CarboxyPhenyl) Substituted PorphyrinsMarco A. Schiavona, Lidia S. Iwamotoa, Antnio G. Ferreirab, Yassuko Iamamotoa, Maria V. B. Zanonicand Marilda das D. Assisa*aDepartamento de Qumica, Faculdade de Filosofia, Cincias e Letras de Ribeiro Preto,Universidade de So Paulo, Av. Bandeirantes, 3900, 14040-901, Ribeiro Preto - SP, Brazil.bDepartamento de Qumica, Universidade Federal de So Carlos,Rodovia Washington Luis Km 235, 13595-905, So Carlos - SP, Brazil.cInstituto de Qumica, Universidade Estadual Paulista, CP 355, 14801-970, Araraquara - SP, BrazilAs porfirinas aninicas 5,10,15-tris(4-carboxifenil), 20-mono(2-nitrofenil) porfirina(1), 5,10(ou 15)-bis(4-carboxifenil), 15(or 10),20-bis(2-nitrofenil)porfirina (2) and 5-mono(4-carboxifenil), 10,15,20-tris(2-nitrofenil)porfirina (3),foram sintetizadas diretamente atravs da reao de pirrol com os benzaldedos substitudos em meio de cido propinico/nitrobenzeno. A relao molar dos benzaldedos foi controlada para otimizar a sntese e purificao das porfirinas desejadas. Esta nova srie de porfirinas foi caracterizada por cromatografia em camada delgada, espectrometria de massas (FAB MS), RMN1H, UV/Vis, IV e eletroqumica. As porfirinas 5,10,15,20-tetrakis(4-carboxifenil)porfirina (4)e 5,10,15,20-tetrakis(2-nitrofenil)porfirina (5) tambm foram estudadas para comparao, tornando a srie completa. A reduo eletroqumica das porfirinas base livre e correspondentes ferro(III) porfirinas foi investigada em eletrodos de carbono e mercrio. Os potenciais de reduo mostraram a dependncia esperada do nmero de grupos nitro, fortemente retirador de eltrons, presentes no anel porfirnico, fornecendo evidncias adicionais para a caracterizao dos compostos sintetizados.The anionic 5,10,15-tris(4-carboxyphenyl), 20-mono(2-nitrophenyl) porphyrin (1), 5,10(or 15)-bis(4-carboxyphenyl), 15(or 10),20-bis(2-nitrophenyl)porphyrin (2) and 5-mono(4-carboxyphenyl), 10,15,20-tris(2-nitrophenyl)porphyrin (3) were sinthesized directly by reaction of pyrrole with substituted benzaldehydes in nitrobenzene/propionic acid media. The benzaldehydes molar ratio was controlled to optimize the synthesis and purification of the desired porphyrins. This new series of porphyrins was characterised by TLC, mass spectrometry (FAB MS),1H NMR, UV/Vis, IR and electrochemistry. 5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin (4)and5,10,15,20-Tetrakis(2-nitrophenyl)porphyrin (5)were also characterised for comparative purposes, completing the series The electrochemical reduction was investigated for the free base and corresponding iron(III) porphyrins on glassy carbon and mercury electrodes. The reduction potentials showed the expected dependence on the number of electron-withdrawing nitro groups present on the porphyrin ring providing additional evidences for the characterisation of the synthesised compounds.Keywords:anionic porphyrins, synthesis of porphyrins, metalloporphyrins, electrochemistry of porphyrinsIntroductionThe synthesis of porphyrins has gained special attention in recent years because of their importance in bioorganic and bioinorganic chemistry1and their applications in biomedical sciences. Porphyrins are used, for example, to mimic the function of hemeproteins such as cytochrome P-450 in oxidation catalysis2, as photosensitizers in photodynamic therapy of cancer (PDT)3, in electron transport chains4and as building blocks in molecular devices5. Each research area requires porphyrins with different and specific structural features, bearing a variety of different substituents.Carboxy substituted porphyrins are attractive synthetic targets for three reasons: firstly, these substituents are present in natural porphyrins such as protoporphyrin IX, the prosthetic group of many important biological molecules like hemoglobin6. Secondly, the carboxy group confers an amphyphylic character to the porphyrins, and this is very important, for example, to improve the selectivity in tumour localization in PDT3. Lastly, carboxy groups can act as linkers to attach porphyrins to other materials, for example, they can be anchored to solid supports by amidation7.We are interested inmeso-para-carboxy andmeso-ortho-nitro phenyl substituted porphyrins. The carboxy groups when ionised provide the charges necessary to enable their intercalation into lamellar double hydroxides (LDH)8or to support them on cationic functionalized silicas through electrostatic binding. They can also be covalently linked to amino-functionalized silica to give heterogeneous porphyrin catalysts7. Theortho-nitro groups are important, since they remove electron density from the porphyrin ring and provide steric hindrance improving the stability of the metalloporphyrin catalysts in oxidation reactions9. The nitro group can improve the ability of porphyrin systems to act as radiosensitizers10and, like carboxy groups, they can act as linkers to attach porphyrins to other materials7.The purpose the present study was to combine the advantages offered by both functional groups into one porphyrin ligand. The corresponding metalloporphyrins will be used as catalysts for hydrocarbon oxidations in homogeneous and supported systems.In this paper, we report the synthesis and purification of anionic tetraarylporphyrins bearing carboxy- and nitro- substituents on the phenyl rings (Figure 1), using the method of Gonsalveset al11. The characterisation of water-soluble porphyrins is much more difficult than the characterisation of porphyrin derivatives soluble in organic solvents because of their salt-like character. UV/Vis and1H NMR spectra are severely complicated by aggregation effects and a quantitative interpretation is rarely possible. The results of elemental analyses are not definitive because the substances are strongly hygroscopic12. Therefore, the synthesized porphyrins were characterised by combining thin layer chromato-graphy (TLC), mass spectrometry (FAB MS),1H NMR, UV/Vis and IR spectroscopies and electrochemistry and by comparison with the commercial tetra-(para-carboxy-phenyl) and tetra-(ortho-nitrophenyl) porphyrins.

ExperimentalMaterials and methodsCommercially available chemicals and solvents were from Aldrich, Fisons, Sigma and Merck, unless otherwise stated. Silica gel (230-400 mesh) was purchased from Merck and Florisil (magnesia silicate 60-100 mesh) from J.T. Baker. 5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin (4)and5,10,15,20- tetrakis(2-nitrophenyl)porphyrin (5)and 5,10,15,20-tetraphenylporphyrin (6) were purchased from Midcentury and used as received.UV/Vis spectra were recorded on a Hewlett Packard 8452 Diode Array spectrophotometer. The spectra were recorded in 2 mm path length quartz cells (Hellma). The FTIR spectra were recorded on a FT spectrometer-BOMEN B-100 (4000 - 400 cm-1) in KBr pellets; the FeP : KBr molar ratio was aproximately 1:2000. FAB mass spectra were recorded on a V.G. Analytical Autospec Instrument, using FAB+mode. 4-Nitrobenzyl alcohol was used as the matrix and caesium ion bombardment to generate the ions. The1H NMR measurements were recorded on a BRUKER 400.13 MHz, DRX400 spectrometer operating at 303 K using a 5 mm BBO probe with temperature control. The1H NMR spectra of2,3and5were obtained in CD2Cl2solution (2,5 x 10-2mol dm-3) and for1and4CD3OD solutions of the same concentration were used. All spectra were acquired with spectral widths of approximately 16000 Hz and 64 K data points for acquisition using TMS as internal reference. All electrochemical experiments were carried out with a Metrohm Polarecord E-506 coupled to a Metrohm 663 VA Stand which was connected to a computer for data storage and handling. A multimode electrode system was used in both the dropping mercury electrode for differential pulse polarographic measurements and the glassy carbon electrode for cyclic voltammetric measurements. The glassy carbon electrode was polished with alumina before each experiment, rinsed with water, cleaned in a ultrasound bath and dried at room temperature. The three-electrode system was completed with a glassy carbon auxiliary electrode and an Ag/AgCl (3 mol dm-3KCl) reference electrode. A pulse amplitude of 50 mV was used for differential pulse polarography. Tetra-n-butyl-ammonium tetraphenylborate (TBATFB) was obtained from Eastman Chemicals and was used as received. Porphyrin solutions were 1x10-4mol dm-3, prepared by dissolving the purified substance in CH2Cl2.Synthesis and purification of porphyrins and metalloporphyrins5,10,15-Tris(4-carboxyphenyl)20-mono(2-nitrophenyl) porphyrin (1), 5,10(or 15)-bis(4-carboxyphe-nyl)15(or 10),20-bis(2-nitrophenyl) porphyrin (cis-ortrans-isomer) (2) and 5-mono(4-carboxyphenyl)10,15,20-tris(2-nitrophenyl) porphyrin (3) were prepared according to the method of Gonsalveset al11. The procedure was repeated with different ratios of 2-nitrobenzaldehyde to 4-carboxybenzaldehyde in order to optimize the yields of the desired porphyrin.Synthesis of12-Nitrobenzaldehyde (1.47 g, 9.8 mmol) and 4-carboxybenzaldehyde (3.04 g, 20.2 mmol) were mixed with propionic acid (105 cm3) and nitrobenzene (45 cm3) and pyrrole (2.08 cm3, 30 mmol) was added. The reaction vessel was shielded from ambient light and the mixture was heated at 120 C for 1 h. After cooling and solvent removal under vacuum the crude porphyrin was adsorbed onto 10.5 g of Florisil. This mixture was applied to a dry-packed Florisil column and eluted with solvents of increasing polarity: dichloromethane, dichloromethane/methanol (9 : 1), dichloromethane/methanol (1 : 1), and methanol. The last two fractions which contained porphyrins1and2, were filtered to eliminate the Florisil particles. The solvent was removed under vacuum and the solid porphyrins were finally purified by column chromatography on silica with dichloromethane/acetone/acetic acid (8 : 2 : 0.1) as eluent to give 297 mg (3.75 x 10-4mol, 5% yield) of pure1,max/nm (CH3OH) 416 (/dm3mol-1cm-12.3 x 105), 514 (1.3 x 104), 546 (6.9 x 103), 590 (5.7 x 103), 644 (3.7 x 103);m/z(M+) 792.Synthesis of2This compound was obtained as a by-product from the syntheses of1and3,max/nm (CH3OH) 416 (/dm3mol-1cm-12.1 x 105), 514 (1.3 x 104), 548 (5.5 x 103), 590 (4.0 x 103), 648 (2.2 x 103);m/z(M+) 793.Synthesis of32-Nitrobenzaldehyde (1.81 g, 12 mmol) and 4-carboxy-benzaldehyde (0.45 g, 3 mmol) were mixed with propionic acid (52.5 cm3) and nitrobenzene (22.5 cm3) and pyrrole (1.04 cm3, 15 mmol) was added. The reaction and purification were similar for1giving 148 mg (1.86 x 10-4mol, 5 % yield) of pure3,max/nm (CH3OH) 418 (/dm3mol-1cm-11.0 x 105), 514 (8.7 x 103), 550 (4.6 x 103), 590 (4.0 x 103), 650 (1.9 x 103);m/z(M+) 794.Synthesis of iron(III)porphyrins, FeP (1-Fe,2-Fe,3-Fe,4-Fe,5-Fe)These compounds were prepared by refluxing the free base porphyrins (amounts corresponding to 0.2 mmol) with iron(II) chloride tetrahydrate (amount corresponding to 2 mmol) in N,N'-dimethylformamide (15 cm3) under nitrogen by the method of Adleret al13. The reaction was monitored by UV/Vis spectroscopy and TLC. The iron porphyrins were precipitated from the reaction medium by adding HCl 0.10 mol dm-3(15 cm3), and cooling the reaction flask in an ice bath. The precipitates were recovered by centrifugation and the supernatant, which contained DMF and excess iron(II) salt, was discarded. The iron porphyrins were washed with HCl 0.10 mol dm-3(30 cm3) and purified by silica column chromatography. The solvents used to elute firstly the free base porphyrin followed by the iron porphyrins were: methanol and methanol/acetic acid mixture (10:0.1) for1-Fe; a dichloromethane/acetone/acetic acid mixture (8 : 2: 0,1) for2-Fe; dichloromethane and a dichloromethane/methanol mixture (9:1) for3-Fe. These procedures gave the amounts corresponding to metallation yields of1-Fe- 64%;max/nm (CH3OH) 338 (/dm3mol-1cm-12.2 x 104), 416 (6.2 x 104), 582 (3.5 x 103), 656 (shoulder).m/z(M+) : 845.2-Fe- 55%;max/nm (dichloroethane) 372 (/dm3mol-1cm-13.2 x 104), 422 (6.2 x 104), 510 (1.0 x 104), 582 (4.5 x 103), 670 (broad, 3 x 103).m/z(M+): 846;3-Fe- 72%;max/nm (dichloroethane) 368 (/dm3mol-1cm-13.8 x 104), 422 (7.2 x 104), 510 (1.1 x 104), 582 (5.4 x 103);m/z(M+) 847.Results and DiscussionPorphyrins synthesesThe greatest advance in the development of methods for porphyrin synthesis since the classic Rothemund process14has been the procedure of Lindsey et al15.The gentle conditions of this synthesis provide means for converting a large range of pre-functionalized benzal-dehydes into the corresponding porphyrins. For this reason, the method has been widely employed over recent years. However, Lindsey's method fails when the porphyrin synthesis involves benzaldehydes bearing ionic substituents which are insoluble in the reaction solvents dichloromethane or chloroform, unless the ionic group is masked. In this way, the synthesis of porphyrins bearing carboxy groups often requires the acid groups to be masked as esters. This consequently involves an extra step of group deprotection after the porphyrin has been formed16. We decided to use the method of Gonsalveset al11, which employs propionic acid and nitrobenzene medium, in the synthesis of theortho-nitrophenyl andpara-carboxyphenyl substituted porphyrin series. Whereas in Lindsey's method an expensive highly potent quinone is used as the oxidant, in the method of Gonsalves the oxidant is the nitrobenzene/propionic acid solvent. The latter also leads to the precipitation of the porphyrin from the reaction medium, thus facilitating its isolation and purification.A mixed aldehyde condensation using stoichiometric amounts ofortho-substituted benzaldehyde (A),para-substituted benzaldehyde (B) and pyrrole affords a mixture of six different porphyrins which can be separated chromatographically5a. In order to decrease the number of porphyrins obtained in the synthesis and to avoid extremely tedious chromatographic purification processes the system was optimized with small scale reactions using different ratios of the two aldehydes. In this way each of the porphyrins A3B, A2B2(cisandtrans) and AB3were prepared.In the synthesis of1(AB3) porphyrin, the initial ratio of pyrrole:ortho-nitrobenzaldehyde:para-carboxybenzal -dehyde used was 4:0.8:3.2, rather than the stoichiometric 4:1:3 ratio. This synthesis gave a crude yield of 36% (estimated by the absorption of the Soret band in the UV/Vis spectrum of the reaction mixture) of the two porphyrins4and1at a 13:1 ratio as determined through the mass of porphyrins after purification. In a second experiment, taking into consideration the higher reactivity ofpara-carboxybenzaldehyde, the relative amount ofortho-nitrobenzaldehyde was increased and the reagents ratio used was 4:1.6:2.4. This synthesis led to a high crude yield (60% estimated by UV/Vis) of the mixture of the three porphyrins1,2and3. In a third experiment, the relative amounts of the two benzaldehydes (4:1.3:2.7) were fitted to obtain a high crude porphyrin yield (60%, estimated by UV/Vis) corresponding to the mixture of1and2. This was the optimized condition used to obtain1. The final yield of1after purification was 5%, based on pyrrole.To synthesize the (A3B) porphyrin3, we used pyrrole :o-nitrobenzaldehyde :p-carboxybenzaldehyde at a molar ratio of 4 : 3.2 : 0.8. In this synthesis, the porphyrins5,3and2were detected and the desired porphyrin3was obtained in 5% yield based on the starting pyrrole, after purification. Such yields are good, considering the losses in purification and if compared to other similar preparations involving mixed substituents3b,3c,17,18.For porphyrins purification, a careful study of the total polarity of the eluent to be employed was necessary. This study was carried out using TLC on silica gel and the ternary solvent mixtures initially used, such as benzene/methanol/acetic acid, water/acetonitrile/p-dioxane or 2,6-lutidine/water/ammonia (gas) described in the literature for natural and synthetic systems containing different numbers of carboxylic groups19. However, these water/solvent mixtures were not effective in the present study. Winkelman's20quaternary solvent system, pyridine/chloroform/water/acetic acid (2:1:1:1) employed for the separation of porphyrins containing different numbers of SO3H groups, obtained from the sulfonation of4, was also ineffective for separating theo-nitro- andp-carboxy-substituted porphyrins. Finally, by using the ternary solvent mixture, chloroform/acetone/acetic acid (8 : 2 : 0,1) proposed by Haradaet al21for the separation of themeso-mono-(p-carboxyphenyl)tri-phenylporphyrin from the mixture of this porphyrin andmeso-tetraphenyl-porphyrin, good separations were obtained. By substituting chloroform for dichloromethane, we obtained the ideal solvent mixture for the purification of the porphyrins to be studied. Porphyrins4and5were also analysed in this solvent mixture as references. A difference of 0.10 0.20 in the Rf values per added carboxy group was observed (seeTable I), with the exception of4, which did not elute from the origin. The fraction containing2was probably a mixture of thecisandtransisomers. However it was not possible to separate them under the conditions used.

In conclusion the mixedortho-nitrophenyl andpara-carboxyphenyl substituted porphyrins can be obtained directly by the Gonsalves method. Furthermore, by controlling the molar ratio of the benzaldehydes, it is possible to optimize the synthesis and purification of the desired porphyrins.Infrared spectroscopyInfrared spectroscopy was very useful for the characterisation of the studied porphyrin series, due to the presence of the NO2and COOH substituents, whose absorption bands are defined and easy to assign.Compounds containing the NO2group have strong absorption bands from symmetric and asymmetric deformations (1389 - 1259 cm-1and 1661 - 1499 cm-1, respectively)22. The exact position of these bands depends on the substitutions and unsaturations within the NO2group. Collmanet al23assigned the 1542-1526 cm-1and 1356-1340 cm-1absorption ranges to the asymmetrical and symmetrical vibrations, respectively, for the NO2groups in 5,10,15,20-tetrakis-(o-nitrophenyl)porphyrin),5. Goldberget al24reported these absorptions at 1500 and 1330 cm-1, respectively, for the same porphyrin.Table 2shows the absorption frequencies observed for these groups on the porphyrins synthesized in this study. The values for4and5are also included for comparison purposes.

Porphyrins containing NO2groups as substituents on themeso-phenyl rings (Table 2) show the characteristic absorption bands of this group which agree with Collman's results23. The relative absorption intensities decrease in the order5,3,2and1, as would be expected upon decreasing the number of NO2groups in these porphyrins (Figure 2).

The carboxylic acids are well assigned in FTIR spectroscopy to regions near 1700, 1400 and 1250 cm-1of the spectrum22and the study of all these regions provide a reliable identification of these acids. The absorption near 1700 cm-1is assigned to C=O stretching vibration. Carboxylic acids are capable of forming hydrogen bonds between the carbonyl and hydroxyl groups of the two molecules and this carbonyl frequency is accordingly reduced25. In this study, the porphyrins bearing carboxy substituents show carbonyl frequencies near 1690 cm-1(Table 2andFigure 2) indicating aggregation through hydrogen bonds.The two other absorptions, near 1400 cm-1(1440 1395 cm-1) and near 1250 cm-1(1320 1210 cm-1) have been associated by many workers to the single bond C-O stretching vibration25. These bands appear in the spectrum of4as very intense and large bands centred at 1402 and 1268 cm-1, which agree with data reported by Datta-Gupta and Bardos for this free-base porphyrin16. The intensity of these bands decreases in the order:4>1>2>3, as expected, due to the decrease in the number of carboxylic acid groups in these porphyrins (Figure 2).The others bands, not assigned here, are atributed to the vibration of the ring porphyrinic and phenyls groupmeso-substituents, and are well established in the literature16,26.1H NMR spectroscopyThe high frequency region of the1H NMR spectra shows the chemical shifts for pyrrole hydrogens (H) and these data were used to characterise the porphyrins.Table 3shows the chemical shifts and the multiplicities of the signals .

The chemical shifts (8.69 br s) for Hhydrogens (close to the nitrophenyl groups) of5, are at lower frequency than the Hhydrogens (close to the carboxyphenyl groups) of4(8.88 br s). This effect is caused by the electron withdrawing effect of theortho-nitro groups on the phenyl ring, which leads to less deprotection by the anisotropy effect on these-pyrrole hydrogens.For1, one signal (multiplet,8.78 8.83) was observed for Hhydrogens between the carboxyphenyl substituents (Hc,Figure 3) and two signals (doublets) for the other Hhydrogens. The latter arise from hydrogens Ha, which are close to the nitrophenyl group (8.69); and the former from Hb (8.80) next to the carboxyphenyl group (Figure 3). The chemical shift differences for hydrogens Ha and Hb reflect the difference in the electron current ring, which induces different anisotropic effects on them. However, for hydrogens Hc, we expected to see a singlet and not a multiplet27. This is probably because the molecular aggregation leads to a non-equivalency between hydrogens Hc. Unfortunately, the N-H pyrrole hydrogens were not observed for this compound because the NMR spectrum was obtained in methanol d-4 since this porphyrin is insoluble in dichloromethane. Integration of the areas corresponding to H(close to carboxy) and H(close to nitro) hydrogens of3,2and1gave the expected 3:1, 2:2 and 1:3 (H-NO2/H-COOH) ratios, respectively. For2,1and4we can also observe a broad hump above the signal corresponding to hydrogens H(close to nitro) and H(close to carboxy), which we again suppose are due to intermolecular aggregation. It is well known that porphyrins bearing carboxylic groups can aggregate through hydrogen bonds and also the polar nitro substituent can act as a proton acceptor in hydrogen bonds24. The aggregation was indicated also by infrared spectra, which show broad signals corresponding to the carboxyl group.

The chemical shifts of the N-H hydrogens in the centre of the porphyrin are also affected by the number of nitrophenyl groups. Increasing the number of nitro groups leads to the expected high frequency shift of the N-H signal. In the region of the phenyl hydrogens the multiplet signals overlap makes the assignments difficult.Electrochemical characterizacionThe electrochemical reduction of free base and ironortho nitro andpara-carboxy substituted porphyrins was studied both on a glassy carbon electrode and a mercury electrode by cyclic voltammetry and differential pulse polarography, respectively, in CH2Cl2; 0.1 mol dm-3tetra-n-butylammonium tetraphenylborate (TBATFB).The electroreduction of the free bases 5,10,15,20-(tetraphenyl)porphyrin (6) and tetrakis(o-nitrophenyl) porphyrin (5) and the corresponding iron porphyrins,6-Fe,5-Fe, were also investigated in order to understand the influence of substituents on the electrochemical behaviour of these compounds and for comparison purposes. The free base porphyrins1,4and corresponding iron porphyrins were not investigated by this technique due to their insolubility in dichloromethane which was used as solvent in these studies.The voltammetric reduction of6has two characteristic reversible one-electron transfer reactions (Figure 4,Table 4) as judged by wave analysis of ip/1/2= constant, Epa-Epc = 59 mV and ipa/ipc = 0.9-1.0 for(scan rate) = 10-2000 mV s-1. These reductions correspond to the production of-anion and di-anion radicals as well established in literature28. The cyclic voltammograms for nitro-substituted compounds,2(Figure 5),3and5, exhibit two reduction processes at very close potentials (Table 4). Only one anodic peak associated with the reoxidation of product generated in the second reduction step is seen on the reverse scan, but the ipa/ipc ratio is always smaller than one. In addition, both peak potentials show cathodic shifting as a function of scan rate increasing, indicating that nitro-substituted compounds follow a reduction mechanism involving chemical reactions subsequent to the electron transfer29. In general, all nitro-substituted free base porphyrins are easier to reduce relative to the parent unsubstituted porphyrin (seeTable 4).

A comparison of the electrochemical behaviour with other reported data30,31leads us to suppose that the redox reactions of the nitro-porphyrins reported here also involve the porphyrin-ring system. Considering the predominant effect of the nitro groups on the electrochemical properties, all results were analysed as a function of the number of these groups.Electrochemical characterizationIn order to confirm these results, differential pulse polarographic experiments were carried out with the aim to improve wave separation. The same trends are observed in the polarographic behaviour of these compounds, as shown inTable 5. Differential pulse polarograms show well defined reduction waves and the corresponding peak potentials indicate that the reductions are easier, increasing the number of nitro groups on the porphyrin ring. Although the electrochemical behaviour in both electrode systems was similar, a negative shifting in the reduction steps was noticed as compared to that obtained with a glassy carbon electrode, indicating that the nature of the electrode material affects the reduction process.

A typical cyclic voltammogram obtained for nitro substituted iron(III) porphyrins is shown inFigure 6and compared to that for6-Feobtained under the same conditions (Figure 7). All potential values obtained from voltammetric reduction of the iron porphyrins are shown inTable 4. As expected, the compounds containing the metal centre show two extra peaks at less negative potentials than that observed for reduction of the free base porphyrins. These peaks were assigned to the successive reduction of the iron (III)-iron(II) and iron (II)-iron (I) couples in the porphyrin complex32,33. However, the cyclic voltammograms are significantly changed in morphology and potential magnitude as the number of nitro groups is increased in the macrocycle.

All investigated FeP undergo a reduction process involving a reversible one electron transfer of the metal centre to yield Fe(II), as demonstrated by analysis of the scan rate influence, which shows typical virtually constant values of the ip/1/2ratio and Epa-Epcvalues always around 60 mV. Concomitantly, the peak potential shifts to less negative potential for FeP upon going from6-Feto2-Fe,3-Feand then to5-Fe, increasing the number of nitro groups in the macrocycle. The results are confirmed by polarographic techniques, as shown by peak potentials obtained at the mercury electrode (Table 5).The metal centre second reduction process in the6-Fealso shows a characteristic reversible reduction involving one electron transfer. Nevertheless, the three nitro derivatives investigated exhibit cyclic voltammograms with the second reduction peak at less negative potential than that required for reduction of the iron(II)-iron(I) couple in6-Fe. The difference between the first and second reduction potentials decreases as the number of nitro substituents increases. These data may indicate that the second reduction potential is markedly more sensitive to changes in the electronic nature of the porphyrin macrocycle. The cathodic shift of the peak potential could be interpreted as an increased interaction of the iron(II) with the porphyrin ring. The absence of an anodic peak on the reverse potential scan at a slow scan rate indicates that the second reduction process is complicated by fast chemical reactions which probably consume the generated product, assigned as the iron(I) porphyrin complex, under these conditions. This chemical reaction could be the loss or exchange of the axial ligand34or self protonation of the porphyrin35.In addition, the reduction potential of the porphyrin ring system for all iron complexes is shifted up to 690 mV with the introduction of nitro groups, as shown inTables 4and5. As previously observed, the reduction of free base nitro-porphyrins or corresponding iron porphyrins showed the same general trends, indicating that the ring system reduction strongly depends on the number of nitro substituents in the porphyrin macrocyle.In conclusion, our findings suggest that the electronwithdrawing NO2groups decrease the electron density in both metal centre and conjugated porphyrin-ring system and this leads to easier reduction. On the other hand, the stability of the product electrochemically generated by the reduction of the Fe(II)-Fe(I) couple is decreased for the nitro-substituted porphyrins investigated in this study. Further work is now in progress in our laboratory in order to clarify the reduction mechanism of these compounds.ConclusionsDespite great difficulties in working with ionic porphyrins due to aggregation, we have synthesized, isolated and characterised a new series of porphyrins containing mixed substituents, nitro and carboxy, in the meso-phenyl rings, with estimated overall porphyrin yield as high as 60%. We have demonstrated that it is possible to optimise the benzaldehyde molar ratio in order to obtain fewer porphyrin isomers and higher concentrations of the desired porphyrins, which facilitates the purification process. These porphyrins are important as possible precursors of systems of the self-assembly type and they are potentially good catalysts due to the versatility of the carboxy and nitro groups which can be used to support them in different materials. Further studies are currently under way to explore the catalytic activities of these compounds in oxidation reactions.AcknowledgementsThis work was supported by CAPES, CNPq and FAPESP. We thank J. R. Lindsay Smith for discussions and for the FAB MS spectra.References1. (a) Lindsey, J. S.Metalloporphyrins-Catalyzed Oxidations,Montanari, F. and Casella, L. Eds., Academic Publishers, The Netherlands, 1994. [Links](b) Milgron, L.R.The Colours of Life, Oxford University Press, New York, 1997.[Links]2. (a) Ortiz de Montellano, P. R.Cytochrome P450: Structure, Mechanism and Biochemistry, 2nded., Plenum Press, New York, 1995. [Links](b) Dolphin, D.; Traylor, T.G.; Xie, L. Y.Acc. Chem. Res.1997,30, 251.[Links]3. (a) Bonnett, R.Chem. Soc. Rev.1995, 19. [Links](b) Gaud, O.; Granet, R.; Kaouadji, M.; Krausz, P.; Blais, J. C.; Bolbach, G.Can. J. Chem.1996,74, 481. [Links](c) Driaf, K.; Granet, R.; Krausz, P.; Kaouadji, M.; Thomasson, F.; Chulia, A. J.; Verneuil, B.; Spiro, M.; Blais, J. C.; Bolbach, G.Can. J. Chem.1996,74, 1550.[Links]4. Arimura, T.J. Synthetic Org. Chem. Japan1997,55, 557.[Links]5. (a) Lindsey, J. S.; Prathapan, S.; Johnson, J. E.; Wagner, R. W.Tetrahedron1994,50,8941. [Links](b) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D.Angew. Chem. Int. Ed.1998,37, 2344. [Links](c) Vicente, M. G. H.; Cancilla, M. T.; Lebrilla, C. B.; Smith, K. M.Chem. Commun.1998,2355. [Links](d) Biemans, H. A. M.; Rowan, A. E.; Verhoeven, A.; Vanoppen, P.; Latterini, L.; Schenning, J.; Meijer, E. W.; Schryver, F. C.; Nolte, R. J. M.J. Am. Chem. Soc.1998,120,11054. [Links](e) Osuka, A.; Shimidzu, H.Angew. Chem. Int. Ed. Engl.1997,36, 135.[Links]6. Kaim, W.; Schwederski, B.Bioinorganic Chemistry: Inorganic Elements in the Chemistry of Life, John Wiley & Sons, Chichester, 1995.[Links]7. Lindsay Smith, J. R.Metalloporphyrins in Catalytic Oxidations, Sheldon, R. A. Ed., Marcel Dekker, New York, 1994.[Links]8. (a) Bedioui, F.Coord. Chem. Rev.1995,144, 39. [Links](b) Mansuy, D.; Battioni, P.; Battioni, J. P.Eur. J. Biochem.1989,184, 267.[Links]9. (a) Assis, M. D.; Serra, O. A.; Iamamoto, Y.Inorg. Chim. Acta1991,187, 109. [Links](b) Assis, M. D.; Mello, A. J. B.; Serra, O. A.; Iamamoto, Y.J. Mol. Catal A: Chemical1995,97, 41.[Links]10. Meng, G. G.; James, B. R.; Skov, K. A.Can. J. Chem.1994,72, 1894, and references therein.[Links]11. Rocha Gonsalves, A. M. d'A.;Varejo, J. M. T. B.; Pereira, M. M.J. Heterocyclic Chem.1991,28, 635.[Links]12. (a) Buchler, J. W.; Kunzel, F. M.; Mayer, U.; Nawra, M.Fresenius J. Anal. Chem.1994,348, 371. [Links](b) Herrmann, O.; Medhi, H.; Corsini, A.Can. J. Chem.1978,56, 1084.[Links]13. Adler, A. D.; Longo, F. R.; Kampas, F.; Kim,J.J. Inorg. Nucl. Chem.1970,32, 2445.[Links]14. (a) Rothemund, P.J. Am. Chem. Soc.1936,58, 625. [Links](b) Rothemund, P.J. Am. Chem. Soc.193961, 2912.[Links]15. (a) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz, A. M.J. Org. Chem.1987,52, 827. [Links](b) Lindsey, J. S.; Wagner, R. W.J. Org. Chem.1989,54, 828.[Links]16. Datta-Gupta, N.; Bardos, T. J.J. Heterocyclic Chem.1966,3, 495.[Links]17. (a) Bigey, P.; Frau, S.; Lomp, C.; Claparols, C.; Bernadou, J.; Meunier, B.Bull. Soc. Chim. Fr.1996,13, 679. [Links](b) Assis, M. D.; Lindsay Smith, J. R.J. Chem. Soc., Perkin Trans.II1998,2221.[Links]18. Bonar-Law, R. P.J. Org. Chem.1996,61, 3623.[Links]19. (a) Grishima, L. E.; Brykina, G. D.; Shpigum, O. A.J. Anal. Chem.1995,50, 826. [Links](b) Jensen, J.J. Chromatog.1963,10, 236. [Links](c) Chu, T. C. and Chu, E. J.J. Chromatog.1966,21, 46.[Links]20. (a) Wilkelman, J.Cancer Research,1962,22, 589. [Links](b) J. Wilkeman, J.; Slater, G.; Grossman, J.Cancer Research1967,27, 2060.[Links]21. Harada, A.; Shiotsuki, K.; Yamaguchi, H.; Kamachi, M.Inorg. Chem.1995,34, 1070.[Links]22. Silverstain, R. M.; Bassier, G. C.; Morril, T. C.Spectrometric Identification of Organic Compounds, John Wiley and Sons, Inc., 3aed., 1974 .[Links]23. Collman, J. P.; Brauman, J. I.; Doxee, K. M.; Halbert, T. R.; Bunnenberg, E.; Linder, R. E.; Lanar, G. N.; Gaudio, J. D.; Spartalian, K.J. Am. Chem. Soc.1980,102, 4182.[Links]24. Kumar, R. K.; Balasubramanian, S.; Goldberg, I.Inorg. Chem.1998,37, 541.[Links]25. Bellamy, L. J.The Infrared Spectra of Complex Molecules, John Willey and Sons, New York, 1954, ch. 10.[Links]26. Thomas, D. W.; Martell, A. B.J. Am. Chem. Soc.1959,81, 5111.[Links]27. Meng, G. G.; James, B. R.; Skov, K. A.Can. J. Chem.1994,72, 1894.[Links]28. Fuhrlop, J. H.; Kadish, K. M.; Davis, D. G.J. Am. Chem. Soc.1982,95,5140.[Links]29. Bard, A. J.Electrochemical Methods, John Willey and Sons, New York,1980.[Links]30. Giraudeau, A.; Callot, H. J.; Gross, M.Inorg. Chem.1979,18, 201.[Links]31. Arao, T.; Maiya, B. G.Polyhedron1994,13,1863.[Links]32. Messet, M. J. M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S. E.; Walker, F. A.Inorg. Chem.1996,35,5188.[Links]33. Kadish, K. M.; Morrison, M. M.; Constant, L. A.; Dickens, L.; Davis, D. G.J. Am. Chem. Soc.1976,98, 8337.[Links]34. (a) Bottomley, L. A.; Kadish, K.Inorg. Chem.1981,20, 1348. [Links](b) Lexa, D.; Momenteau, M.; Mispeltier, J.; Lhoste, J.M.,Bioelectrochem. Bioenerg.1975,1, 108.[Links]35. (a) Davis, D. G. in "The Porphyrins", D. Dolphin Ed., v.V, ch. 4, 19. [Links](b) Constant, L. A.; Davis, D. G.Anal. Chem.1975,47, 2253.[Links]Received: May 31, 1999Published on the web: August 31, 2000FAPESP helped in meeting the publication costs of this article.

J. Braz. Chem. Soc.vol.15no.3So PauloMay/June2004http://dx.doi.org/10.1590/S0103-50532004000300010ARTICLEAnalysis of impurities in crude and highly-purified terephthalic acid by capillary electrophoresisMaria de Lourdes L. MoraesI; Joel C. RubimI,1; Rene R. RealpozoII; Marina F. M. TavaresI,*IInstituto de Qumica, Universidade de So Paulo, CP 26077, 05513-970 So Paulo-SP, BrazilIITereftalatos Mexicanos S.A., AP 204, Minatitln, Ver. 96700, Mxico

ABSTRACTIn this work, a simple and fast capillary electrophoresis method for the simultaneous analysis of 4-carboxybenzaldehyde (4-CBA),p-toluic acid (pTOL) and benzoic acid (BZ) in industrial batches of crude (CTA) and highly-purified (PTA) terephthalic acid was developed. The electrophoretic conditions comprise: 20 mmol L-1tetraborate buffer at pH 9, hydrodynamic injection (5 s/17 kPa), applied voltage of +30 kV and direct UV detection at 200 nm.The concentrations of 4-CBA and pTOL in five batches of CTA and four batches of PTA as determined by the proposed CE methodology were in agreement with the results from gas chromatography and polarography methods in current use in Tereftalatos Mexicanos S.A. (TEMEX). Levels of BZ were determined only by the CE methodology and ranged from 60 - 300 ppm in CTA and 5 - 7 ppm in PTA. Several impurities deriving from the incomplete oxidation ofp-xylene were investigated. So far the positive identification of 4-hydroxymethylbenzoic acid was accomplished by comparison with pure standards.Keywords:capillary electrophoresis, terephthalic acid, PTA, 4-CBA

RESUMONeste trabalho, foi desenvolvido um mtodo simples e rpido, utilizando eletroforese capilar (CE), para a anlise simultnea de 4-carboxibenzaldedo (4-CBA), cidop-toluico (pTOL) e cido benzico (BZ), em lotes industriais de cido tereftlico cru (CTA) e purificado (PTA). As condies eletroforticas de anlise foram: tampo tetraborato a 20 mmol L-1(pH 9), injeo hidrodinmica (5 s, 17 kPa), tenso aplicada de +30 kV e deteco direta em 200 nm.Foram analisados cinco lotes de CTA e quatro lotes de PTA nos teores de 4-CBA e pTOL pela metodologia proposta e as metodologias utilizadas na empresa Tereftalatos Mexicanos S.A. (TEMEX), que so cromatografia gasosa para pTOL e polarografia para 4-CBA, mostrando concordncia entre os valores encontrados. Os nveis de BZ foram determinados apenas pela metodologia CE, apresentando no CTA, valores compreendidos entre 60 e 300 ppm, enquanto que no PTA, 5 a 7 ppm. Vrias impurezas derivadas da oxidao incompleta dop-xileno foram investigadas, constatando-se a presena do cido 4-hidroximetilbenzico por comparao com padres.

IntroductionOne of the major chemistry industrial branches is the petrochemical segment. From ethene (obtained from nafta, petroleum derivatives or directly from natural gas), the petrochemistry can originate many raw materials that allow man to make new ones, substituting with advantage wood, animal furs and other natural products. Plastics and fibers are two important classes of synthetic products, most of them deriving from highly-purified terephthalic acid (PTA) and its esters.1About 50% of all PTA are used to make polyester fibers for tire cord, industrial filament, and protective apparel. The fastest growing use of PTA is, however, in the synthesis of polyethylene terephthalate (PET), better known as the number "1" recyclable material used in drink containers. Nearly 33% of PTA are used in this manner. Almost 8% of PTA and/or dimethylterephthalate (DMT) are used in the production of polyester film that has, until recently, been the material of choice for the audio recording industry. The remaining uses of PTA and DMT are vast and varied. A part of PTA is used in making polybutylene terephthalate and other engineering resins. Other esters of PTA are used in the production of liquid crystal displays. The acid itself is used in the producing ofKevlar, a particularly strong trademark DuPont material that is incorporated in sailing equipment, aircraft structures and bulletproof vests.2Terephthalic acid is produced industrially by oxidation ofp-xylene in acetic acid medium under the effect of a catalyst system based on Co, Mn and Br, at high temperatures (Figure 1).1The oxidation is a stepwise reaction involving several intermediates, whose presence as impurities in the final product is highly dependent upon the process parameters. Several side products have been reported and classified by means of their structural features. They comprise the derivatives of benzoic acid, phenol, terephthalic acid, diphenyl, fluorenone and anthraquinone as well as a variety of esters.3The presence of these impurities in terephthalic acid used in the production of polyester, for instance, is undesirable because they can slow down the polymerization process and also impart coloration to the polymer due to thermal instability. This effect is caused by the presence of monoprotic acids, suchp-toluic acid (pTOL), benzoic acid (BZ) and 4-carboxybenzaldehyde (4-CBA).

The impurities 4-CBA and pTOL are customarily monitored in industrial laboratories using polarographic and gas chromatographic methodologies, respectively.4,5The disadvantage of this procedure is that two independent techniques must be employed, which is a time consuming practice, demanding two distinct equipments and trained personnel in both techniques.Few reports based on reversed-phase liquid chromatography of terephthalic acid impurities can also be found.6-8In one of these reports,6the authors investigated the liquid-phase catalytic oxidation ofp-xylene to terephthalic acid, monitoring the product and impurities by gradient elution using methanol:acetonitrile:water mixtures as mobile-phase.Over the past decade, capillary electrophoresis (CE) has become a resourceful alternative technique for the analysis of a variety of compounds of industrial relevance.9Even though capillary electrophoresis has been employed in the evaluation of a series of aldehydes,10organic acids,11phenolic compounds,12,13and polycarboxylic acids,14,15few reports concerning specifically the analysis of terephthalic acid and the identification of its impurities have been published.16,17In this work, a simple, fast and reliable capillary electrophoresis methodology for the simultaneous analysis of major impurities in industrial batches of crude (CTA) and highly-purified (PTA) terephthalic acid has been implemented.ExperimentalEquipmentThe experiments were conducted in a capillary electrophoresis system (model 270A-HT, Perkin-Elmer, Applied Biosystems Div., Foster City, CA, USA), equipped with a variable UV-Vis detector set at 200 nm, a temperature control device maintained at 25C and a data acquisition and treatment software (TurbochromTM, PE Nelson, Cupertino, CA, USA). Samples were injected hydrodynamically (17 kPa pressure) and the electrophoresis system was operated under normal polarity and constant voltage conditions. A fused-silica capillary with dimensions 70 cm total length x 50 cm effective length and 75 m i.d. was used in the analysis of the crude product. A high-sensitivity optical cell (LC Packings, Amsterdam, The Netherlands), which consisted of a fused-silica capillary assembled in right angles, with dimensions 75m i.d., 22 cm short section, 82 cm long section from injection port and 3 mm pathlength, was used in the evaluation of impurities in the highly-purified product.Chemicals and samplesAll standards were prepared from reagent-grade chemicals (Aldrich/Sigma, St. Louis, MO, USA) and deionized water (Millipore Corporation, Bedford, MA, USA). Stock solutions of terephthalic acid, 4-carboxybenzaldehyde (4-CBA),p-toluic acid (pTOL), benzoic acid (BZ) and 4-hydroxymethylbenzoic acid (HMB) were prepared at 100 mg/L concentration in 0.15 mol L-1NaOH. Working solutions in the range of 0.50 to 30 mg/L were prepared by appropriate dilution of the stocks with deionized water. A certified standard of highly-purified terephthalic acid was obtained from AMOCO and prepared at 10000 mg/L concentration in 0.15 mol L-1NaOH. Samples of CTA and PTA were obtained from Tereftalatos Mexicanos (TEMEX) and prepared at 5000 mg/L and 10000 mg/L in 0.15 mol L-1NaOH, respectively. The electrolyte was a tetraborate buffer solution, prepared at 20 mmol L-1, pH 9, containing eventually 0.20 mmol L-1of cetyltrimethylammonium bromide (CTAB) as flow modifier.Analytical proceduresThe buffer solutions were prepared fresh daily and filtered through a 0.22 m membrane filter (Millipore) just prior to use. At the beginning of the day, the capillary was conditioned by a flush of 1 mol L-1NaOH solution (20 min), followed by a flush of deionized water (10 min) and a flush of the electrolyte solution (30 min). In between runs, the capillary was just replenished with fresh electrolyte solution (2 min flush).Results and DiscussionIn capillary electrophoresis, the determination of small anions derived from carboxylic acids, is well established in the literature.17-20It is generally conducted under inverse polarity and reversed electroosmotic flow.19-21It is also possible to carry out anion separations without flow reversal, even though this last approach often leads to longer analysis time.17,22Figure 2presents an analysis of a crude batch of terephthalic acid in both normal and reversed flow conditions. The sample is very concentrated and the electropherogram was blown, so that the contaminants can be readily visualized. The elution order is dependent on the type of analysis chosen, therefore, the anion of highest mobility,i.e.terephthalate, elute first when flow reversal is employed. Due to the severe peak tailing in borate/CTAB electrolyte medium, even more pronounced due to the compound large concentration, a partial overlap between terephthalic acid and some of its impurities occurs (Figure 2A). Of course, conditions can be optimized to expand the elution window and perhaps have all impurities of interest eluting away from the terephthalic acid peak. However, any attempt to improve the separation will result in a longer analysis time and perhaps, some amount of tailing will persist, compromising further quantitative work of the impurities. Therefore, the analysis under normal flow conditions seems to be an appropriate choice for the discrimination of the impurities in the presence of a large amount of the major compound (Figure 2B). In this situation, the monocarboxylic acids elute first followed by the terephthalic acid. In borate buffer, the terephthalic acid peak also tails (not shown), but the elution window comprised between the system peak (peak 1 inFigure 2B) and the sharp edge of the terephthalic acid peak (peak 10) can be manipulated easily to achieve separation of the impurities of interest.

Figure 3presents a further optimized separation of crude terephthalic acid and also an analysis of the highly-purified product, in borate buffer under normal flow conditions, in regular 75 m capillaries. The CTA sample was diluted by a factor of two, so that the resolution between 4-CBA and pTOL was improved (Figure 3A). The major impurities can still be visualized for quantitative treatment. Besides 4-CBA and pTOL, BZ and HMB (peaks labeled as 5 and 2, respectively) are readily identified in the electropherogram. Minor impurities (peaks 6, 7 and 8) observed in the previous condition (Figure 2B), decreased considerably and are not seen in the electropherogram due to sample dilution. By decreasing the base width of the terephthalic acid peak, sample dilution affects the migration time of all other sample components.

Table 1presents a quantitative evaluation of five industrial batches of crude terephthalic acid. A comparison between the CE analysis and the analytical data from Tereftalatos Mexicanos (TEMEX) for 4-CBA, using polarography, is also given inTable 1. The results indicated agreement better than 20%. Data for TEMEX analysis of pTOL and BZ for the same batches are not available for comparison.The PTA electropherogram (Figure 3B) shows that the relative concentration of impurities is altered, for instance, the presence of 4-CBA is hardly visualized, confirming that the industrial purification of the crude product is very successful in removing this compound. Notice that in the electropherogram ofFigure 3B, the sample was prepared at 10000 mg/L concentration. Any attempt to increase area of the 4-CBA peak, either by further concentrating the sample or injecting a larger volume of sample will compromise resolution.A way to enhance detectability without manipulating sample concentration is to use a high-sensitivity optical cell (HS cell), which presents an enlargement of the optical pathlength for detection.Figure 4shows the electropherogram of a PTA sample analyzed in the HS cell, confirming the possibility of identification and quantitative determination of minor impurities, including 4-CBA.

Table 2and3present statistical parameters of the calibration curves used in the evaluation of limits of detection (LOD) for 4-CBA, pTOL and BZ standards, analyzed in a regular 75 mm i.d. capillary and in the HS cell, respectively. The analytical curve equations and coefficients of determination were calculated using linear least-square regression analysis. The results showed an excellent linearity (r2>0.99) between peak area and concentration, and an intercept close to the origin. The limits of detection decreased roughly by a factor of ten, when the HS cell was used. Since the expected levels of 4-CBA in the highly-purified terephthalic acid range from 10 to 20 ppm, which corresponds to 0.10 to 0.20 mg/L in the injected sample (due to low solubility of terephthalic acid in water, the impurities are diluted by a factor of 100, during sample preparation), the use of a regular capillary might not be applicable to any industrial sample. Notice that the LOD for 4-CBA is 0.31 mg/L. Therefore, the use of a high-sensitivity cell is definitely advisable for quantitative purposes.Table 4shows the results of a capillary electrophoresis analysis of four PTA batches in the HS cell. Correspondingly TEMEX results for 4-CBA and pTOL, when available, are also presented and compared to the CE data. The CE analyses were conducted with one calibration standard, the AMOCO certified standard, whose impurity levels are depicted in the table. The level of benzoic acid in the AMOCO standard was not available, therefore, it was determined electrophoretically in this work using a pure standard (Sigma). As observed previously for the analysis of the crude product, CE and TEMEX methodologies are in good agreement, better than 20% relative deviation. BZ is not currently analyzed in TEMEX laboratories.In order to characterize the CE methodology using the high-sensitivity optical cell, ten consecutive injections of a PTA sample were performed. This experiment showed a 0.52% repeatability in migration time (7.999 0.042 min), 4.8% in peak area and 2.4% in peak height for the pTOL contaminant and a 0.51% repeatability in migration time (8.262 0.043 min), 14.8% in peak area and 7.4% in peak height for the 4-CBA contaminant.Significance tests were carried out to verify whether there is a statistically significant difference between the results obtained from the proposed CE methodology and the methodologies previously implemented in TEMEX for the analysis of PTA. In order to compare the accuracy and precision of the methods,ttest andFtest were used, respectively.23Thettest is used to compare the means obtained with two methods or by two persons or by the same person but under different experimental conditions. It is indispensable in analytical chemistry for testing the validity of an analysis. For this purpose, the method variance is calculated, using the following equation:

whereandare the variances of the CE and the TEMEX method under consideration, respectively;n1andn2are the number of determinations in each method. Thetvalue is calculated by the equation:

whereandare the mean concentration obtained by each method andSis the standard deviation based in both methods (calculated from equation 1). Values oftare available in statistical tables for (n1+ n2- 2) degrees of freedom for a given confidence level.23If the experimentaltvalue is greater than the theoreticaltvalue, it indicates that there is a significant difference between the means and, consequently, the methods differ in accuracy.TheFtest is used to determine whether there is a difference in the precision of two methods. The parameterFis defined as:

Values ofFare available in statistical tables for (n1- 1) and (n2- 1) degrees of freedom for a given confidence level. When the experimentalFvalue is greater than the theoreticalFvalue, it is concluded thatS12is statistically different fromS22and thus the methods differ in precision.The results for comparison of accuracy and precision between the proposed CE methodology and the TEMEX methodologies are compiled inTable 5. The statistical results show that for both impurities, the methods are equally accurate. However, in terms of precision, gas chromatography and CE differ (pTOL analysis), whereas, polarography and CE are equivalently precise (4-CBA analysis). Differences in precision between the electrophoretic and chromatographic methods might be attributed to the larger injection volume fluctuations of the GC-FID equipment used in the analysis.

ConclusionsThe use of capillary electrophoresis for the simultaneous analysis of major impurities in industrial batches of crude and highly-purified terephthalic acid has been demonstrated. The advantages of the proposed methodologies over the adopted methodologies in the industrial quality control laboratories (gas chromatography and polarography) is the possibility to determine simultaneously all relevant contaminants with a single technique in a substantially shorter analysis time (less than 7 min).AcknowledgementsThe authors wish to acknowledge the Conselho Nacional de Desenvolvimento Cientfico e Tecnolgico (CNPq) and the Fundao de Amparo Pesquisa do Estado de So Paulo (FAPESP) of Brazil for financial support (FAPESP 95/2396-9) and fellowships (FAPESP 97/04595-4, CNPq 301201/94-3).References1. Sheehan, R.J.;Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid,Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH: Germany, 2002.[Links]2.http://www.kevlar.com/; accessed at July 11th, 2003.[Links]3. Roffia, P.; Calini, P.; Motta, L.; Tonti, S.;Ind. Eng. Chem. Res. Dev.1984,23, 629.[Links]4. Koshy, V.J.; Prasad, J.V.; Kalpana, G.; Satish, S.;Anal. Chim. Acta1995,307, 55.[Links]5. Cao, G.; Cavalien D'Oro, P.; Danoczy, E.; Roffia, P.;Oxidation Comm.1980,1, 153.[Links]6. Viola, A;. Cao, G.;J. Chromatogr. Science1996,34, 27.[Links]7. Cao, G.; Servida, A; Pisu, M.; Morbidelli, M.;AIChE J.1994,40, 1168.[Links]8. Cao, G.; Pisu, M.; Morbidelli, M.;Chem. Eng. Sci.1994,49, 5775.[Links]9. Beale, S.C.;Anal. Chem.1998,70, 279R.[Links]10. Pereira, E.A.; Cardoso, A.A.; Tavares, M.F.M.;J. AOAC International1999,82, 1562.[Links]11. Klampfl, C.W.; Burchberger, W.; Haddad, P.R.;J. Chromatogr. A2000,88, 1357.[Links]12. Moane, S.; Park, S.; Lunte, C.E.; Smyth, M.R.;Analyst1998,123, 1931.[Links]13. Fonseca, F.N.; Kato, M.J.; Oliveira Jr, L.; Pinto Neto, N.; Tavares, M.F.M.;J. Microcolumn Sep.2001,13, 227.[Links]14. Buchberger, W.; Winna, K.;J. Chromatogr.1996,739, 389.[Links]15. Jones, W.R.; Jandik, P.;J. Chromatogr.1992,608, 385.[Links]16. Moring, S.E.; Pairaud, C.; Albin, M.; Locke, S.; Thibault, P.; Tindall, G.W.;Am. Lab.1993, 7.[Links]17. Chiou, C.; Shih, J.;Analyst1996,121, 1107.[Links]18. Jandik, P.; Bonn, G.;Capillary Electrophoresis of Small Molecules and Ions, VCH Publishers, Inc.: New York, 1993.[Links]19. Volger, D.; Zemann, A.J.; Bonn, G.K.; Antal Jr, M.J.;J. Chromatogr. A1997,758, 263.[Links]20. Zemann, A.J.;J. Chromatogr. A1997,787, 243.[Links]21. Colombara, R.; Tavares, M.F.M.; Massaro, S.; Rubim, J.C.;J. Capillary Electrophor.1996,3, 147.[Links]22. Tindall, G.W.; Perry, R.L.;J. Chromatogr. A1995,696, 349.[Links]23. Devore, J.L.;Probability and Statistics for Engineering and the Sciences, 2nded., Brooks/Cole: Monterey, 1987.[Links]Received: January 30, 2002Published on the web: May 10, 2004FAPESP helped in meeting the publication costs of this article*e-mail:mfmtavar@iq.usp.br1Present address: Instituto de Qumica, Universidade de Braslia, CP 04478, 70919-970 Braslia-DF, Brazil