3-4. ALKENA (CnH2n), ALKUNA (CnH2n-2), STEREOKIMIA (Kimor I_Hadi)
AlkunaReaksi Alkuna : Adisi HX dan X 2 Reaksi adisi pada alkuna sama dengan yang bterjadi pada...
Transcript of AlkunaReaksi Alkuna : Adisi HX dan X 2 Reaksi adisi pada alkuna sama dengan yang bterjadi pada...
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Alkuna
Hidrokarbon yang mengandung karbon-karbonikatan rangkap tigaAsetilena (etuna), merupakan alkuna paling sederhana, dapat dihasilkan dari kalsiumkarbida (karbid) dengan air
Alkuna dapat diubah menjadi senyawa lain.
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Struktur Elektronik AlkunaIkatan Karbon-karbon rangkap 3, triple bond results from sporbital on each C forming a sigma bond and unhybridized pXand py orbitals forming π bonds.The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond.The bond is shorter and stronger than single or doubleBreaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)
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Electronic Structure of AlkynesCarbon-carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized pX and py orbitals forming π bonds.The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond.The bond is shorter and stronger than single or doubleBreaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)
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1. Penamaan LakunaSama seperti pada alkena dengan akhiran “una“.Penomoran dimulai dari ujung paling dekat ikatan rangkap 3.Ikatan rangkap tiga lebih dari satu : diuna, triuna dst..Ikatan rangkap dua dan tiga :Double and triple bonds are: enuna.
Penomoran dimulai dari ujung dekat ikatan rangkap (2 atau 3)Ikatan rangkap dua lebih diutamakan
6-metil-3-oktuna 4-metil-7-nonen-1-una
1-hepten-6-una
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2. Pembuatan Alkuna : Reaksi eliminasi DihalidaReaksi 1,2-dihalidoalkana dengan KOH atau NaOH, terjadieliminasi 2 HX (double dehydrohalogenation)Dihalida visinal dapat dihasilkan dari adisi brimin atau klorinpada alkena.
1,2-difenil etena(stilbena)
1,2-dibromo-1,2-difenil etena(dibromida visinal)
KOH, etanol
difenilsetilan
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2. Pembuatan Alkuna : Reaksi eliminasi Dihalida
Vinil halida (vinyl tersubstitusi pada ikatan rangkap C=C)
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3. Reaksi Alkuna : Adisi HX dan X2
Reaksi adisi pada alkuna sama dengan yang bterjadi pada alkena.Intermediat alkena bereaksi dengan reagen yang berlebih.Mengikuti Markovnikov
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Addition of Bromine and Chlorine
Initial addition usually gives trans intermediate
Can often be stopped at this stage if desired (1 eq. Br2)
Product with excess reagent is tetrahalide
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Addition of HX to Alkynes Involves Vinylic Carbocations
Addition of H-X to alkyne should produce a vinylic carbocation intermediate
Secondary vinyl carbocations are about as stable as primary alkyl carbocationsPrimary vinyl carbocations probably do not form at all
Nonethelss, H-Br can add to an alkyne to give a vinyl bromide if the Br is not on a primary carbon
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8.4 Hydration of AlkynesAddition of H-OH as in alkenes
Mercury (II) catalyzes Markovnikov oriented additionHydroboration-oxidation gives the non-Markovnikov product
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8.4 Hydration of AlkynesKeto-enol Tautomerism
Tautomerism = Isomeric compounds that rapidily interconvert by the movement of a proton and are called tautomersEnols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon
The keto form is usually so stable compared to the enol that only the keto form can be observed
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Mercury(II)-Catalyzed Hydration of Alkynes
Alkynes do not react with aqueous protic acidsMercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation
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Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes
The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone
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Hydration of Unsymmetrical AlkynesIf the alkyl groups at either end of the C-C triple bond are not the same, both products can form and this is not normally usefulIf the triple bond is at the first carbon of the chain (then H is what is attached to one side) this is called a terminal alkyneHydration of a terminal alkyne always gives the methyl ketone, which is useful
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Hydroboration/Oxidation of AlkynesBH3 (borane) adds to alkynes to give a vinylic boraneAnti-Markovnikov
Oxidation with H2O2 produces an enol that converts to the ketone or aldehydeProcess converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration
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Comparison of Hydration of Terminal Alkynes
Unhindered terminal alkynes add two boranesHydroboration/oxidation converts terminal alkynes to aldehydes because addition of water is non-Markovnikov
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Comparison of Hydration of Terminal Alkynes
The product from the mercury(II) catalyzed hydration converts terminal alkynes to methyl ketones
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8.5 Reduction of Alkynes
Addition of H2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction)The addition of the first equivalent of H2 produces an alkene, which is more reactive than the alkyne so the alkene is not observed
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Conversion of Alkynes to cis-AlkenesAddition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkeneThe two hydrogens add syn (from the same side of the triple bond)The Lindlar Catalyst will not reduce double bonds
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Conversion of Alkynes to trans-Alkenes
Anhydrous ammonia (NH3) is a liquid below -33 ºCAlkali metals dissolve in liquid ammonia and function as reducing agents
Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia
The reaction involves a radical anion intermediate
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8.6 Oxidative Cleavage of Alkynes
Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acids
Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide
Neither process is useful in modern synthesis – were used to elucidate structures because the products indicate the structure of the alkyne precursor
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Alkyne Acidity: Formation of Acetylide Anions
The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus
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Alkyne Acidity: Formation of Acetylide AnionsTerminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25. See Table 8.1 for comparisons))Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion
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8.8 Alkylation of Acetylide AnionsAcetylide ions can react as nucleophiles as well as bases (see Figure 8-6 for mechanism)
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8.8 Alkylation of Acetylide AnionsReaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes