2 یلآ یمیش - staff.guilan.ac.ir · Unusually stable - heat of hydrogenation 150 kJ/mol less...
Transcript of 2 یلآ یمیش - staff.guilan.ac.ir · Unusually stable - heat of hydrogenation 150 kJ/mol less...
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2شیمی آلی
به نام خدا
2شیمی آلی
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1. McMurry, Organic Chemistry, 7th ed., Brooks/Cole,
Monterey, 2008.
2. Morrison and Boyd, Organic Chemistry, 6th ed.,
Benjamin Cummings,1992.
3. Carey and Giuliano, Organic Chemistry, 10th ed.,
McGraw-Hill Education, 2016.
4. Vollhardt and Schore, Organic Chemistry, 7th ed.,W.H.
Freeman, 2014.
5. Wade and Simek, Organic Chemistry, 9th ed.; Pearson
Higher Ed, 2016.
6. Clayden, Greeves, and Warren, Organic Chemistry, 2nd
ed. Oxford University Press, 2012.
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نمره10:کوئیز
نمره20: نظم و ترتیب و حضور و غیاب
نمره70: امتحان نیم ترم
نمره100: امتحان آخر ترم
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شیمی آلی جلد دوم
دکترعیسی یاوری: ترجمه
1391تهران -نشر نوپردازان
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15 Benzene and Aromaticity
16 Chemistry of Benzene: Electrophilic Aromatic Substitution
17 Alcohols and Phenols
18 Ethers and Epoxides; Thiols and Sulfides
> A Preview of Carbonyl Compounds
19 Aldehydes and Ketones: Nucleophilic Addition Reactions
20 Carboxylic Acids and Nitriles
21 Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution
Reactions
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15. Benzene and
Aromaticityبنزن و خصلت آروماتیکی
Based on McMurry’s Organic Chemistry, 7th edition
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Aromatic Compounds
Aromatic was used to described some fragrant compounds in early 19th century
Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition)
Current: distinguished from aliphatic compounds by electronic configuration
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cherries,
peaches,
and almondscoal distillate
Tolu balsam
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steroids
Cholesterol
lowering
drug
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Why this Chapter?
Reactivity of substituted aromatic compounds
is tied to their structure
Aromatic compounds provide a sensitive
probe for studying relationship between
structure and reactivity
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15.1 Sources and Names of
Aromatic Hydrocarbons From high temperature distillation of coal tar قطران ذغال سنگ
Heating petroleum at high temperature and pressure over
a catalyst
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Some aromatic hydrocarbons found in coal tar
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two large reservoirs of organic material
petroleumAromatic compounds are
separated as such from coal tar قطران ذغال سنگ
Aromatic compounds are
synthesized from the
alkanes of petroleumabsence
of air
Coal
volatile compounds residue
cokecoal gas coal tar
smelting of iron to steeldistillation
benzene, 1 kg
toluene, 250 g
xylenes, 50 g
phenol, 250 g
cresols, 1 kgnaphthalene, 2.5 kg
a ton of soft coal
60 kg
benzene
toluene
xylenes
catalytic reforming
dehydrogenation
cyclization
isomerization
hydrodealkylation
ذغال سنگ نفت
گاز ذغال
به فوالدگدازگری آهن گرم کردن مخلوط کانیها به منظور )
(ذوب و جداکردن بخش فلزی آنها
تبدیل کاتالیزی مواد نفتی به
محصوالت فرارتر
آلکیل زدایی با هیدروژن
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Discovery and early theories of
benzene’s structure
Discovered by Michael Faraday (1825)
Analysis showed a molecular formula of
C6H6
Many structures were proposed,
culminating with Kekule’s “cyclohexatriene” of 1865.
Kekule realized that the ring bonds must be identical,
proposing a rapid equilibration.
These are now described as resonance forms of
benzene.
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1829-1896
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Isomers of the composition C6H6
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Naming Aromatic Compounds Many common names (toluene = methylbenzene;
phenol = hydroxybenzene; aniline = aminobenzene)
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Naming Aromatic Compounds
Monosubstituted benzenes systematic names as hydrocarbons with –benzene
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Alkyl-substituted benzenes are sometimes referred to as arenes
Calk < 6
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Naming Aromatic Compounds
Monosubstituted benzenes systematic names as hydrocarbons with –benzene
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Alkyl-substituted benzenes are sometimes referred to as arenes
Calk < 6
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The Phenyl Group
When a benzene ring is a substituent, the term
phenyl is used (for C6H5 _)
You may also see “Ph” or “f” in place of “C6H5”
“Benzyl” refers to “C6H5CH2_”
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Calk > 6
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The Phenyl Group
When a benzene ring is a substituent, the term
phenyl is used (for C6H5 _)
You may also see “Ph” or “f” in place of “C6H5”
“Benzyl” refers to “C6H5CH2_”
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Calk > 6
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Disubstituted Benzenes
Relative positions on a benzene ring
ortho- (o) on adjacent carbons (1,2)
meta- (m) separated by one carbon (1,3)
para- (p) separated by two carbons (1,4)
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Disubstituted Benzenes
Relative positions on a benzene ring
ortho- (o) on adjacent carbons (1,2)
meta- (m) separated by one carbon (1,3)
para- (p) separated by two carbons (1,4)
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Disubstituted Benzenes
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Describes reaction pattern—product orientation:
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Naming Benzenes With More
Than Two Substituents Choose numbers to get lowest possible values
List substituents alphabetically with hyphenated numbers
Common names, such as “toluene” can serve as root name (as in TNT)
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Naming Benzenes With More
Than Two Substituents Choose numbers to get lowest possible values
List substituents alphabetically with hyphenated numbers
Common names, such as “toluene” can serve as root name (as in TNT)
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15.2 Structure and Stability of
Benzene: Molecular Orbital Theory
Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H)
This is substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier
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Heats of Hydrogenation (DHohydrog )
as Indicators of Stability
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The addition of H2 to C=C normally gives off about 118 kJ/molTwo conjugated double bonds in cyclohexadiene add 2H2 to give
off 230 kJ/mol3 double bonds would give off 356kJ/molBenzene has 3 unsaturation sites but gives off only 206 kJ/mol on
reacting with 3H2 molecules
Therefore it has about 150 kJ more “stability” than an isolated set of
three double bonds
Benzene is actually 24 kJ more stable than cyclohexadiene!
230 – 206 = 24
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Benzene’s Unusual Structure
All its C-C bonds are the same length: 139 pm —between single (154 pm) and double (134 pm) bonds
Electron density in all six C-C bonds is identical
Structure is planar, hexagonal
C–C–C bond angles 120°
Each C is sp2 and has a p orbital perpendicular to the plane of the six-membered ring
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Drawing Benzene and Its Derivatives
The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds
This does indicate the number of electrons in the ring but reminds us of the delocalized structure
We shall use one of the resonance structures to represent benzene for ease in keeping track of bonding changes in reactions
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Molecular Orbital Description of
Benzene
The 6 p-orbitals combine to give
Three bonding orbitals
with 6 electrons
Three antibonding with
no electrons
Orbitals with the same energy are
degenerate
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The linear combination of p orbitals to form π orbitals: (a) ψa antibonding
(out of phase overlap); (b) ψb bonding (in-phase overlap); and (c) the wave
character of ψa and ψb. The subscripts a and b refer to antibonding and
bonding orbitals, respectively.
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Molecular Orbital Description
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Isolated Double bond
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Conjugated Double bonds
The bonding -orbitals are made from 4 p orbitals that provide greater
delocalization and lower energy than in isolated C=C
electrons in 1,3-butadiene are delocalized over the bond system
delocalization leads to stabilization
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15.3 Aromaticity and the
Hückel 4n+2 Rule Unusually stable - heat of hydrogenation 150 kJ/mol
less negative than a cyclic triene
Planar hexagon: bond angles are 120°, carbon–
carbon bond lengths 139 pm
Undergoes substitution rather than electrophilic
addition
Resonance hybrid with structure between two line-
bond structures
These properties (and others) are labeled “aromatic”
or “aromaticity”.
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Aromaticity and the 4n + 2 Rule
Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2 electrons (n is 0,1,2,3,4)
For n=1: 4n+2 = 6; benzene
is stable and the electrons are
delocalized
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1896-1980
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Compounds With 4n Electrons Are Not
Aromatic (May be Antiaromatic)
Planar, cyclic molecules with 4n electrons are
much less stable than expected (antiaromatic)
They will distort out of plane and behave like
ordinary alkenes
4- and 8-electron compounds are not delocalized
(single and double bonds)
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Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder reaction at low temperature
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Cyclooctatetraene has four double bonds, reacting with Br2, KMnO4, and HCl as if it were four alkenes
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Cyclobutadiene
wasn’t prepared
until 1965, by Pettit,
but it dimerizes
(Diels-Alder) even
at -78oC:
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1927-1981
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Willstatter (1911) made cyclooctatraene
Not aromatic; non-planar:
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1872-1942
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15.4 Aromatic Ions
The 4n + 2 rule applies to ions as well as neutral species
Both the cyclopentadienyl anion and the
cycloheptatrienyl cation are aromatic
The key feature of both is that they contain 6 electrons
in a ring of continuous p orbitals
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Aromatic Ions: experimental evidence
H
H
B:-1
+ BH
pKa = 16 (!!!)
H
Br
AgBF4BF4
-1
stable salt !!!
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Aromaticity of the Cyclopentadienyl
Anion
1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalization
Removal of H+ at the CH2 produces a cyclic 6-electron system, which is stable
Relatively acidic (pKa = 16) because the anion is stable
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Resonance in cyclopentadienyl
anion
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Cycloheptatriene
Cycloheptatriene has 3 conjugated double bonds joined by a CH2
Removal of “H-” leaves the cation
The cation has 6 electrons and is aromatic
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Cycloheptatrienyl Cation
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15.5 Aromatic Heterocycles:
Pyridine and Pyrrole
Heterocyclic compounds contain elements other
than carbon in a ring, such as N,S,O,P
Aromatic compounds can have elements other than
carbon in the ring
There are many heterocyclic aromatic compounds
and many are very common
Cyclic compounds that contain only carbon are
called carbocycles (not homocycles)
Nomenclature is specialized
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Pyri
din
e A six-membered heterocycle with a nitrogen atom in its ring
electron structure resembles benzene (6 electrons)
The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital)
Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity
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Pyrr
ole
A five-membered heterocycle with one nitrogen
electron system similar to that of cyclopentadienyl anion
Four sp2-hybridized carbons with 4 p orbitals perpendicular to the ring and 4 p electrons
Nitrogen atom is sp2-hybridized, and lone pair of electrons occupies a p orbital (6 electrons)
Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base
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Nitrogen Heterocycles:
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Nucleic Acid Bases:
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Deoxyribonucleotides:
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DNA Structure:
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When electrons fill the various molecular orbitals, it takes two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of nsucceeding energy levels
This is a total of 4n + 2
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Polycyclic Aromatic Compounds
Aromatic compounds can have rings that share a set
of carbon atoms (fused rings)
Compounds from fused benzene or aromatic
heterocycle rings are themselves aromatic
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Naphthalene Orbitals
Three resonance forms and delocalized electrons
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15.8 Spectroscopy of Aromatic
Compounds
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IR
UV
1H NMR
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Benzene:30 Vibrational modes
Aromatic ring
C–H stretching
at 3030 cm1
And
peaks 1450 to 1600 cm1
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15.8 Spectroscopy of Aromatic
Compounds
IR: Aromatic ring C–H stretching at 3030 cm1
and peaks 1450 to 1600 cm1(See Figure 15-13)
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15.8 Spectroscopy of Aromatic
Compounds
UV:Peak near 205 nm and a less intense peak in 255-275
nm range
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15.8 Spectroscopy of Aromatic
Compounds
1H NMR:
Aromatic H’s strongly deshielded by ring and absorb
between 6.5 and 8.0
Peak pattern is characteristic of positions of
substituents
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Ring Currents
Aromatic ring oriented perpendicular to a strong magnetic field, delocalized electrons producing a small local magnetic field
Opposes applied field in middle of ring but reinforces applied field outside of ring
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Spectroscopy of Aromatic Compounds
UV: Peak near 205 nm and a less intense peak in 255-275 nm range
1H NMR: Aromatic H’s strongly deshielded by ring and absorb between 6.5 and 8.0
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Aryl and Benzylic protons:
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13C NMR of Aromatic Compounds
Carbons in aromatic ring absorb at 110 to 140
Shift is distinct from alkane carbons but in same
range as alkene carbons
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Summary of Spectroscopy
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