Arenes: compounds containing both aliphatic and aromatic parts. Alkylbenzenes Alkenylbenzenes Alkynylbenzenes Etc. Emphasis on the effect that one part has on the chemistry of the other half. Reactivity & orientation
Example: ethylbenzene EAS in the aromatic part -CH 2 CH 3 activates and directs ortho- & para- Free radical halogenation in the side chain -C 6 H 5 activates and directs benzyl
Alkylbenzenes, nomenclature: Special names
others named as “alkylbenzenes”:
Use of phenyl C 6 H 5 - = “phenyl” do not confuse phenyl (C 6 H 5 -) with benzyl (C 6 H 5 CH 2 -)
Alkenylbenzenes, nomenclature:
Alkynylbenzenes, nomenclature:
Alcohols, etc. , nomenclature:
Alkylbenzenes, syntheses: 1. Friedel-Crafts alkylation 2. Modification of a side chain: a) addition of hydrogen to an alkene b) reduction of an alkylhalide i) hydrolysis of Grignard reagent ii) active metal and acid c) Corey-House synthesis
Modification of side chain:
Friedel-Crafts: Ar-H + R-X, Al. Cl 3 Ar-H + R-OH, H+ Ar-R + H 2 O Ar-H + alkene, H+ Ar-R + HX
Friedel-Crafts limitations: a) Polyalkylation b) Possible rearrangement c) R-X cannot be Ar-X d) NR when the benzene ring is less reactive than bromobenzene e) NR with -NH 2, -NHR, -NR 2 groups
polyalkylation The alkyl group activates the ring making the products more reactive that the reactants leading to polyalkylation. Use of excess aromatic compound minimizes polyalkylation in the lab.
The electrophile in Friedel Crafts alkylation is a carbocation: R-X + Al. X 3 R+ R+ R-OH + H+ | | —C=C— + H+ R+ Carbocations can rearrange!
rearrangement
R-X cannot be Ar-X
NR with rings less reactive than bromobenzene -CHO, -COR -SO 3 H -COOH, -COOR -CN -NR 3+ -NO 2
NR with –NH 2, -NHR, -NR 2
Friedel-Crafts limitations: a) Polyalkylation b) Possible rearrangement c) R-X cannot be Ar-X d) NR when the benzene ring is less reactive than bromobenzene e) NR with -NH 2, -NHR, -NR 2 groups In syntheses it is often best to do Friedel-Crafts alkylation in the first step!
Alkylbenzenes, reactions: 1. Reduction 2. Oxidation 3. EAS a) nitration b) sulfonation c) halogenation d) Friedel-Crafts alkylation 4. Side chain free radical halogenation
Alkylbenezenes, reduction: CH 3 H 2, Ni 300 o. C, 100 atm. NR NR
Alkylbenezenes, oxidation: CH 3 KMn. O 4 heat NR NR NR
Oxidation of alkylbenzenes. 1) Syn 2) identification C 8 H 10:
Alkylbenzenes, EAS -R is electron releasing. Activates to EAS and directs ortho/para
Alkylbenzenes, free radical halogenation in side chain: benzyl free radical
Alkenylbenzenes, syntheses: 1. Modification of side chain: a) dehydrohalogenation of alkyl halide b) dehydration of alcohol c) dehalogenation of vicinal dihalide d) reduction of alkyne (2. Friedel-Crafts alkylation)
Alkenylbenzenes, synthesis modification of side chain
Alkenylbenzenes, synthesis Friedel-Crafts alkylation not normally used for alkenylbenzenes. an exception:
Alkenylbenzenes, reactions: 1. Reduction 2. Oxidation 3. EAS 4. Side chain a) add’n of H 2 h) oxymercuration b) add’n of X 2 i) hydroboration c) add’n of HX j) addition of free rad. d) add’n of H 2 SO 4 k) add’n of carbenes e) add’n of H 2 O l) epoxidation f) add’n of X 2 & H 2 O m) hydroxylation g) allylic halogenation n) ozonolysis o) vigorous oxidation
Alkenylbenzenes, reactions: reduction
Alkenylbenzenes, reactions oxidation
Alkenylbenzenes, reactions EAS?
Alkenylbenzenes, reactions side chain:
Benzyl carbocation
100 syn-oxidation; make a model!
Alkynylbenzenes, syntheses: Dehydrohalogenation of vicinal dihalides
Alkynylbenzenes, reactions: 1. Reduction 2. Oxidation 3. EAS 4. Side chain a) reduction e) as acids b) add’n of X 2 f) with Ag+ c) add’n of HX g) oxidation d) add’n of H 2 O, H+
Alkynylbenzenes, reactions: reduction anti- syn-
Alkynylbenzenes, reactions: oxidation
Alkynylbenzenes, reactions EAS?
Alkynylbenzenes, reactions: side chain:
Arenes: alkylbenzenes alkenylbenzenes alkynylbenzenes As expected, but remember that you cannot do EAS on alkenyl- or alkynylbenzenes.
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