Transcript Learning materials

Conjugated dienes, aromaticity.
Allylic and benzylic reactivity.
Phenols.
Required background:
Alkenes
Electrophilic addition to alkenes
Molecular shapes
Stability of intermediates
Resonance structures
Acidity of alcohols
Essential for:
1. Chemistry of the carbonyl group
2. Understanding of antioxidants
3. Enols and enolates
Outline
1. Conjugated and non-conjugated dienes. Chemistry of
vision
2. Conjugated addition to 1,3-dienes
3. Aromaticity
4. Derivatives of benzene
5. Aromatic electrophilic substitution
6. Allylic and benzylic oxidation
7. Phenols
Heats of combustion:
CH2=C=CH-CH2-CH3
3251 kJ/mol
Least stable
Weak s-p* conjugation
CH2=CH-CH2-CH=CH2
3217 kJ/mol
Better s-p* conjugation
CH2=CH-CH=CH-CH3
3186 kJ/mol
Most stable
Both s-p* and p-p*
conjugations
=C= is a new example of sp-hybridized carbon
Lowest p-orbitals for 1,4- and 1,3-dienes
Each additional double bond in conjugation increases the wavelength of
absorption
Consequence:
1. Absorption at some point crosses from the UV into the visible area of light
Ability of polyenes to absorb visible light enables retinol to change shape
upon illumination by light and send a signal to the brain
Consequences:
1. Our eyes are able to sense light
2. Absorption shifts toward longer wavelength and at some point crosses from the UV
into the visible area
In the cone cell there are three different
types of Opsins: Red Opsin, Blue Opsin
and Green Opsin. The wavelength where
Retinal absorbs light is different for each.
The Blue Rhodopsin absorbs at 420
nanometers; Green Rhodopsin absorbs at
530 nanometers and Red Rhodopsin
absorbs at 570 nanometers.
There may be only one chromophore
(Retinal), but because the proteins are a
little different, the way the Retinal is
attached to a protein has a great deal of
affect on its wavelength of absorption.
The aldehyde group of retinal enables its attachment to opsins to form rhodopsins
Outline
1. Conjugated and non-conjugated dienes. Chemistry of
vision
2. Conjugated addition to 1,3-dienes
3. Aromaticity
4. Derivatives of benzene
5. Aromatic electrophilic substitution
6. Allylic and benzylic oxidation
7. Phenols
or
CH2
H
Br
Br
-
Br
CH3
+
-
CH3
CH
H2C
H2C
CH3
H2C
25 oC
+
CH3
Br
H2C
Br
Product of 1,2-addition formes faster (kinetic
control) and becomes a major product at -80 oC
(because Br- is closer to the reaction site).
Product of 1,4-addition is more stable
(thermodinamic control) and becomes a major
product at 25 oC (because of the equilibrium).
Polymerization of 1,3-dienes
n CH2=CH-CH=CH2 => -(-CH2-CH=CH-CH2-)nH3C
Na
H3C
...
Isoprene
...
H
n
CH 2
...
CH 2
Artificial rubber
CH2
H2C
CH 2
Rubber tree
H
...
H
H3C
CH 2
Natural rubber
Sulphur, heat
Commercial rubber
n
Outline
1. Conjugated and non-conjugated dienes. Chemistry of
vision
2. Conjugated addition to 1,3-dienes
3. Aromaticity
4. Derivatives of benzene
5. Aromatic electrophilic substitution
6. Allylic and benzylic oxidation
7. Phenols
An aromatic system is formed from a polyene p-system with an odd number of
occupied orbitals (2n+1), bearing 4n+2 electrons (Huckel’s rule of aromaticity)
An anti-aromatic system is formed from a polyene p-system with an even number
of occupied orbitals (2n), bearing 4n electrons (Huckel’s rule of anti-aromaticity)
The Huckel’s rule is valid for planar monocyclic systems.
Examples of aromatic species:
Examples of anti-aromatic species:
-
CH
+
CH
n=4
n=6
n=6
+
CH
-
CH
n=4
N
n=2
O
n=4
H
N
N
n=6
n=6
n=6
Reactions that demonstrate the concept of aromaticity
Br
CH2
Br2
Br
CH3
Pt, heat
+ 4H2
H3C
- H2
- H2
- H2
- H2
Outline
1. Conjugated and non-conjugated dienes. Chemistry of
vision
2. Conjugated addition to 1,3-dienes
3. Aromaticity
4. Derivatives of benzene
5. Aromatic electrophilic substitution
6. Allylic and benzylic oxidation
7. Phenols
Nomenclature and physical properties of substituted benzenes
Substituents in the benzene ring are ranked and numbered in the following order:
-COOH > -CHO > -OH > -Alkyl > others
Examples:
NO 2
HO
HO
CHO
NO 2
2-Hydroxybenzaldehyde
(ortho-hydroxybenzaldehyde)
3-Nitrophenol
(meta-nitrophenol)
C2H5
4-Nitroethylbenzene
(para-nitroethylbenzene)
(1-Ethyl-4-nitrobenzene)
Note: Hydroxy-derivatives of benzene are called phenols, not alcohols.
Some derivatives also have common names:
HO
H2C=HC
HO
Toluene
meta-Xylene
Phenol
Styrene
OH
Hydroquinone
Boiling and points of substituted benzenes and cyclohexenes are very close.
Benzene:
m.p. 5.5 oC, b.p. 80.1 oC
Cyclohexene: m.p. 6.6 oC, b.p. 80.7 oC
Melting points of para-derivatives usually much higher, than of meta- and ortho-derivatives,
because the molecules of para-derivatives are packed easier in the crystal lattice.
Br
Br
NO2
Cl
Br
O2 N
Solid
Br
Liquid
Solid
Cl
Cl
Cl
Cl
Liquid
H2, Pt
2. Reactivity of benzene versus alkenes
1. O 3. 2. Reduction
O
Cl2, light
Cl
O2, V 2O5, heat
O
O
O
H
H
O
Outline
1. Conjugated and non-conjugated dienes. Chemistry of
vision
2. Conjugated addition to 1,3-dienes
3. Aromaticity
4. Derivatives of benzene
5. Aromatic electrophilic substitution
6. Allylic and benzylic oxidation
7. Phenols
Electrophilic substitution (SEAr).
H
E
+
+
E- Y
H- Y
Leaving group (-H, -I, -SO3H)
a. Bromination
Br
Br
+
+
Br
FeBr 3
Br
FeBr 3
H
Proceeds through a p-complex
Br
Br
H
Br
H
+
Br 2
+
CH
FeBr 3
H
HC
+
CH
s-complex with the positive charge, distributed only between
ortho- and para-positions
Br
Br
Br
Br
Addition
-
+
-H
Br
Substitution (aromaticity is restored)
b. Chlorination (same mechanism as for bromination)
Iodination does not occur this way.
Fluorination proceeds through a different mechanism.
Nitration
H
+
HNO3
NO2
H2SO4
Nitronium-cation
HNO3
+
H2SO4
H
NO2
+
+
HSO4-
+
H2O
Proceeds through a p-complex
O2N
+
-H
O2 N
O2N
H
H
+
O2N
+
CH
HC
+
CH
H
Alkylation and acylation
H
AlCl3
+
(Large excess)
Cl
(Example)
Cl
+
+
AlCl3
AlCl3
Cl
H
Proceeds through a p-complex
+
-H
H
H
+
H
+
CH
HC
+
CH
Alkylation, catalyzed by aluminium chloride, can be complicated by carbocation rearrangements
Cl
H
AlCl3
+
+
27%
+
H
49%
+
+
C
H
AlCl3 or H 2SO4
Proceeds through a p-complex
+
-H
H
H
+
H
+
CH
HC
+
CH
O
O
H
AlCl3
+
R
Cl
R
Cl
Cl
O
+
+
AlCl3
R
AlCl3
O
R
H
Proceeds through a p-complex
O
R
R
+
-H
O
R
H
+
O
R
H
H
O
+
CH
HC
+
CH
Electronic effect of substituents at the benzene ring
a. Inductive acceptor. The atom of the substituent, connected to the benzene ring, has higher
electronegativity, than H.
Examples: -OCH3, -NH2, -Cl, -NO2
b. Resonance acceptor. Conjugation between p-orbitals is depicted by resonance structures
with the positive charge in the benzene ring.
Examples: -COR, -NO2, -SO3H
O
-
O
R
+
CH
R
..........
c. Resonance donor. Conjugation between p-orbitals is depicted by resonance structures
with the negative charge in the benzene ring.
Examples: -OCH3, -NH2, -Cl, -phenyl
NH2
-
CH
+
NH2
..........
d. Hyperconjugative donor. Conjugation, involving s-orbitals, is depicted by
non-classical resonance structures (allowing breaking s-bonds) with the negative charge
in the benzene ring.
Examples: -CH3, -Alkyl
H
H
H
H
H
-
+
CH
H
..........
e. Hyperconjugative acceptor. Conjugation, involving s-orbitals, is depicted by
non-classical resonance structures (allowing breaking s-bonds) with the positive charge
in the benzene ring.
Examples: -CF3
F
F
F
F
+
CH
F
-
F
..........
Substituent effect on reactivity of substituted benzenes
Electron donors increase reactivity in SEAr
Examples: -CH3, -NR2, -OR, -CH=CH2
Electron acceptors decrease reactivity in SEAr
Examples: -NO2, -NH3+, -COR, -Cl
For substituents with opposite effects, the resonance effect overrides all other effects,
except for chlorine and bromine, where the inductive effect is the strongest.
+
Br
FeBr 3
Br 2
OH
OH
Br
Br
+
Br 2
Diluted solution in water
Br
Example of activation
Directing effect of substituents (already present at the benzene ring before substitution)
on substitution
(Note: the signs of resonance refers to the structures before the substituent is added)
E
H
+
Donor
H
E
H
E
+
C
HC
+
C
Stabilization
Donor
Stabilization
Donor
Lack of stabilization
All electron donors direct the incoming substituent
to the ortho- and para-positions (regardless of any accepting effects)
Examples: -CH3, -NR2, -OR, -Cl, -Br, -CH=CH2
E
H
C
Donor
+
H
E
Donor
HC
Ortho-substitution (favored)
C
!!!
E
Para-substitution (favored)
Donor
+
+
H
H
H
E
CH
H
E
+
HC
C
Donor
H
E
+
+
Donor
Donor
!!!
E
Meta-substitution (disfavored)
H
H
E
CH
+
Donor
HC
C
H
+
H
E
Donor
+
Donor
(Note: the signs of resonance refers to the structures before the substituent is added)
E
H
+
Acceptor
H
E
H
E
+
C
HC
+
C
Destabilization
Acceptor
Destabilization
Acceptor
Lack of destabilization
Pure electron acceptors direct the incoming substituent to the meta-position
Examples: -NO2, -NH3+, -COR, -CF3
E
H
C
Acceptor
+
H
E
Acceptor
HC
Ortho-substitution (disfavored)
C
!!!
E
Para-substitution (disfavored)
Acceptor
+
+
H
H
H
E
CH
H
E
+
HC
C
Acceptor
H
E
+
+
Acceptor
Acceptor
!!!
E
H
H
E
CH
+
HC
Meta-substitution (favored)
Acceptor
C
H
+
H
E
Acceptor
+
Acceptor
Outline
1. Conjugated and non-conjugated dienes. Chemistry of
vision
2. Conjugated addition to 1,3-dienes
3. Aromaticity
4. Derivatives of benzene
5. Aromatic electrophilic substitution
6. Allylic and benzylic oxidation
7. Phenols
Allylic (benzylic) oxidation
NH3, O 2, heat, catalyst
CN
Cl
Cl
+
KMnO4, H 2O, H , heat
KMnO4, H 2O, heat
Mg(NO 3)2
COOH
O
+
KMnO4, H 2O, H , heat
COOH
Outline
1. Conjugated and non-conjugated dienes. Chemistry of
vision
2. Conjugated addition to 1,3-dienes
3. Aromaticity
4. Derivatives of benzene
5. Aromatic electrophilic substitution
6. Allylic and benzylic oxidation
7. Phenols
Acidity of phenols
Phenols are much more acidic, than alcohols because of stabilization of the conjugate base
(phenoxide) due to conjugation with the aromatic ring.
-
OH
O
O
+
-H
O
O
-
-
CH
HC
-
CH
pKa = 10
-
OH
O
-H+
pKa = 17
Substituents at the ortho- and para-positions further increase acidity of phenols.
OH
OH
OH
OH
NO2
O2 N
NO2
O2 N
pKa = 10
pKa = 8
pKa = 7
O2 N
pKa = 1 (Picric acid)
As opposed to NaOH, NaHCO3 is a too weak base to bring phenol in aqueous solution.
OH
NaHCO3
ONa
NaOH
Compare:
NaHCO3
RCOONa
NaOH
RCOOH
RCOONa
6. Oxidation of phenols
OH
O
Ag 2O
OH
O
1,4-benzoquinone, 90%
OH Ag 2O
O
OH
O
4-methyl-1,2-benzoquinone, 68%
OH
Ag 2O
O
Ag 2O
Messy mixture
OH
O
50%
O
H
O
+
R.
.
+
RH
stable phenoxy-radical
Phenols are able to intercept free radicals and inhibit free-radical reactions
HO
HO
O
a food preservative
Vitamin E