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Aromatic Substitution Reactions
Chapter 17 Aromatic Substitution Reactions OrgChem-Chap17
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17.1 Mechanism for Electricphilic Aromatic Substitution
Organic Chemistry II Fall 1999 17.1 Mechanism for Electricphilic Aromatic Substitution Arenium ion resonance stabilization OrgChem-Chap17 Chapter 18
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Example 1. Example 2. OrgChem-Chap17
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Example 2. Mechanism of the nitration of benzene
OrgChem-Chap17
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Addition reaction vs. Electrophilic aromatic substitution
OrgChem-Chap17
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Stability Ga < Gs Bezene is very stable so it is very diificult to break the resonance stabilization OrgChem-Chap17
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Is the addition reaction possible for a benzene ?
Very difficult because of the stability of the product resonance stabilization OrgChem-Chap17
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17.2 Effect of Substituent Why ? Resonance stabilization
17 times faster than the substitution of benzene Why ? Resonance stabilization OrgChem-Chap17
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Ortho attack Meta attack Para attack Meta and para attack is favored
CH3 is an ortho/para directing group OrgChem-Chap17
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Nitration of anisole (methoxy benzene)
10,000 times faster than the substitution of benzene Why ? Resonance stabilization OrgChem-Chap17
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The effect of methoxy group
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. Therefore, any group that has an unshared pair of electrons is the ortho/para director OrgChem-Chap17
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Nitration of nitrobenzene
times slower than the substitution of benzene 2. meta director OrgChem-Chap17
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OrgChem-Chap17
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Activating group (elecron donating group): ortho/para director
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 OrgChem-Chap17
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F is highly electronegative, therefore inductive withdrawing effect is stronger than the resonance effect 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 ! OrgChem-Chap17
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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 ! OrgChem-Chap17
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See P 680 OrgChem-Chap17
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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! OrgChem-Chap17
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17.4 Nitration OrgChem-Chap17
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Preparation of NO2+ OrgChem-Chap17
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A problem occurs with amino substitution
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! OrgChem-Chap17
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Amide group: much less basis, still activator and o/p director
Example, OrgChem-Chap17
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17.5 Halogenation Mechanism Same as the nitration
Resonance stabiliztion, Activating group faciliate the reaction + AlCl3 + HCl OrgChem-Chap17
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OrgChem-Chap17
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17.6 Sulfonation Fuming sulfuric acid OrgChem-Chap17
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Mechanism OrgChem-Chap17
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17.7 Friedel-Craft Alkylation
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Mechanism of the Friedel-Craft Alkylation
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Drawbacks The alkyl groups that is added to the ring is an activated group: a large amount of products w/ two or more alkyl groups Aromatic compound w/ strongly deactivating groups cannot be alkylated. Rearrangement Because OrgChem-Chap17
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Other ways to generate carbocations
Strong acid, TsOH, can eliminate water, then CH3-ph-CH2+ can be generated Other examples Lewis acid is used OrgChem-Chap17
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Synthetic detergents OrgChem-Chap17
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BHT and BHA are anti oxidant added to food prepared by Friedel-Crafts alkylation reactions
OrgChem-Chap17
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17.8 Friedel-Craft Acylation
Generation of acyl cation OrgChem-Chap17
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Drawback: like the alkylation, this reaction does not work with strongly deactivated substrates (m directors) Examples OrgChem-Chap17
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Examples OrgChem-Chap17
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17.9 Electrophilic Substitution of Polycyclic Aromatic Compounds
Why the 1 position is preferred? OrgChem-Chap17
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Containing stable benzene ring
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17.10 Nucleophilic Aromatic Substitution; Diazonium ion
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Examples OrgChem-Chap17
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17.11 Nucleophilic Aromatic Substitution; Addition-Elimination
OrgChem-Chap17
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Not SN2 but Addition-Elimination
Mechanism Not SN2 but Addition-Elimination OrgChem-Chap17
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The order of leaving group ability
Examples OrgChem-Chap17
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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 OrgChem-Chap17
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Mechanism OrgChem-Chap17
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Benzyne The existence of benzyne OrgChem-Chap17
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17.13 Some Additional Useful Reactions
Reduction of nitro group to amine using hydrogen and a catalyst or by using acid and a metal (Fe, Sn, or SnCl2) Application OrgChem-Chap17
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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 OrgChem-Chap17
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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 Oxidation of alkyl groups bonded to the aromatic ring If the carbon bonded to the ring is not tertiary OrgChem-Chap17
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17.14 Synthesis of Aromatic Compound
OrgChem-Chap17
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Preparation of m-chlorobenzene and p-chlorobenzene
Preparation of o-bromophenol OrgChem-Chap17
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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 OrgChem-Chap17
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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 OrgChem-Chap17
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Preparation of OrgChem-Chap17 bezene
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