orgchem- chap17 1 chapter 17 aromatic substitution reactions
TRANSCRIPT
OrgChem-Chap17
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Chapter 17Chapter 17
Aromatic Substitution Reactions
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Arenium ion
resonance stabilization
17.1 Mechanism for Electricphilic Aromatic Substitution
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Example 1.
Example 2.
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Example 2. Mechanism of the nitration of benzene
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Addition reaction vs.Electrophilic aromatic substitution
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H E
<
<
<
E
Stability
Ga < Gs Bezene is very stable so it is very diificult to break the resonance stabilization
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Is the addition reaction possible for a benzene ?
Very difficult because of the stability of the product
E
resonance stabilization
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17.2 Effect of Substituent
17 times faster than the substitution of benzene
Why ?
Resonance stabilization
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Ortho attack
Meta attack
Para attack
Meta and para attack is favored
CH3 is an ortho/para directing group
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Nitration of anisole (methoxy benzene)
10,000 times faster than the substitution of benzene
Why ? Resonance stabilization
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Therefore, any group that has an unshared pair of electrons is the ortho/para director
The effect of methoxy group
1.Inductive effect,
then as the oxygen is electronegative Methoxy is deactivating group not true
2. Resonance effect explanation is possible
This is what scientists are doing, you also should have this attitude, then find reasons. Otherwise no result at all.
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Nitration of nitrobenzene
1. 1017 times slower than the substitution of benzene
2. meta director
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Until now,
Activating group (elecron donating group): ortho/para director
Deactivationg group (elecron withdrawing group): meta dircectot
Exception: Halogens,
ortho/para derector + deactivating group
1. 17 times slower than the substitution of benzene
2. ortho/para director
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FF is highly electronegative, therefore inductive withdrawing effect is stronger than the resonance effect
Cl, Br, I
Cl, Br, and I are not very electronegative, while the resonance effect is not strong enough as the methoxy
Because the overlapping netween 2p AO of carbon and 3p(Cl), 4p(Br), 5p(I) AOs are not good. (2p AO for oxygen)
Still halogens are ortho/para director because there is the resonance effect although it is much weaker.
Nose ring theory !
Accurate experiment results are most important !
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@
Two ortho positions and one para position, therefore statistically the ratio or ortho to para products should be 2 to 1,
Which is generally true! (nitration of toluene)
Steric effect !
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See P 680
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17.3 Effect of Multiple Substituent
Methyl group controls the regiochemistry, because methyl group is a strong activating group
Rule: Groups that are closer to the top of Table 17.1 controls the regiochemistry!
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17.4 Nitration
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Preparation of NO2+
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N with unpaired electrons looks like a activating group and o/p director. But under acidic condition it can be protonated, then deactivating group and m director. Although the amine (strong activating group) conc. is very low, 18% is para product!
A problem occurs with amino substitution
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Amide group: much less basis, still activator and o/p director
Example,
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17.5 Halogenation
Same as the nitration
Resonance stabiliztion,
Activating group faciliate the reaction
Cl
+ AlCl3 + HCl
Mechanism
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17.6 Sulfonation
Fuming sulfuric acid
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Mechanism
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17.7 Friedel-Craft Alkylation
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Mechanism of the Friedel-Craft Alkylation
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Drawbacks
1. The alkyl groups that is added to the ring is an activated group: a large amount of products w/ two or more alkyl groups
2. Aromatic compound w/ strongly deactivating groups cannot be alkylated.
3. Rearrangement
CH3CH2CH2CH2Cl + AlCl3 CH3CH2CH2CH2 AlCl4
CH3CH2CHCH3
Because
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Other ways to generate carbocations
Other examples
Lewis acid is used
Strong acid, TsOH, can eliminate water,
then CH3-ph-CH2+ can be generated
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Synthetic detergents
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BHT and BHA are anti oxidant added to food prepared by Friedel-Crafts alkylation reactions
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17.8 Friedel-Craft Acylation
Generation of acyl cation
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Drawback: like the alkylation, this reaction does not work with strongly deactivated substrates (m directors)
Examples
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Examples
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17.9 Electrophilic Substitution of Polycyclic Aromatic Compounds
Why the 1 position is preferred?
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Containing stable benzene ring
Containing stable benzene ring
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17.10 Nucleophilic Aromatic Substitution; Diazonium ion
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Examples
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17.11 Nucleophilic Aromatic Substitution; Addition-Elimination
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Mechanism
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The order of leaving group ability
Examples
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17.12 Nucleophilic Aromati Substitution; Elimination-Addition
When there is no electron withdrawing group at o/p position, then elimination-addition occurs with very strong base (amide anion) or with weak base at high temperature
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Mechanism
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Benzyne
The existence of benzyne
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17.13 Some Additional Useful ReactionsReduction of nitro group to amine using hydrogen and a catalyst or by using acid and a metal (Fe, Sn, or SnCl2)
H3CH2COCO
Cl
NH2
Application
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Reduction of carbonyl group (aldehyde or ketone) to a methylene group
1. Clemmenson reduction
2. Wolff-Kishner reduction
3. Catalytic hydrogenation
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Oxidation of alkyl groups bonded to the aromatic ring
If the carbon bonded to the ring is not tertiary
H2/Pt reduction vs Wolff-Kishner and Clemmenson reduction
-H2/Pt works for the carbonyl attached to the aromatic ring
-Wolff-Kishner and Clemmenson reduction do not have this restriction
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17.14 Synthesis of Aromatic Compound
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Preparation of o-bromophenol
Preparation of m-chlorobenzene and p-chlorobenzene
HO
+ Br2
HO HO
Br
Br
+ Mixuture
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Preparation of m-bromochlorobenzene
Problem: both chloro and bromo groups are o/p directors
Solution: use NO2, a m director
Preparation of m-bromotoluene
Problem: methyl group is an o/p director
Solution: use NO2, the m director
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Preparation of m-butylbenzenesulfonic acid
Benzene sulfonic acid cannot be alkylated because the Friedel-Craft alkyl- or acylation does not work with deactivating group
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Preparation of
bezene