Limitations on Friedel-Crafts Alkylation There are three major limitations on Friedel-Crafts alkylations. 1. carbocation rearrangements are common.

Examples of Carbocation rearangement

2. alkylation fails on benzene rings bearing one or more of these strongly electron-withdrawing groups.

Examples:

Not Friedel-Crafts reactive 3. alkylation fails on benzene rings bearing amine groups. Basic amino groups (–NH2,–NHR, & –NR2) react with lewis acids (AlCl3 ) Not Friedel-Crafts reactive

Arenes: Hydrocarbons contain both aliphatic and aromatic parts. Alkylbenzenes 2. Alkenylbenzenes 3. Alkynylbenzenes

Alkylbenzene syntheses: Friedel-Crafts alkylation

2-Acylation of Aromatic Rings then reduction of the carbonyl group by Zn(Hg)/HCl 100 % 10alkylbenzene Method A gives Method B gives Mixture of alkylbenzene Note: method A: acylation then reduction method B: friedel crafts alkylation

Reactions of Alkylbenezene on Side chain 1- oxidation: Alkyl side chains can be oxidized to CO2H by strong reagents such as KMnO4 and Na2Cr2O7 if they have a C-H next to the ring Converts an alkylbenzene into a benzoic acid, ArR  ArCO2H

2- Free radical halogenation in side chain: Benzyl carbon Br2 / light Note: halogen atom adds to the benzyl carbon atom (the carbon atom directly bonded with benzene ring

Mechanism:

Alkenylbenzenes syntheses: 1. modification of side chain

Note1: Friedel-Crafts alkylation not normally used for Note1: Friedel-Crafts alkylation not normally used for alkenylbenzene synthesis Vinyl bromide

Reactions of Alkenylbenzenes 1- oxidation

2- reduction 3- Addition

Note 2:alkenes are more reactive than aromatic rings In syntheses of alkenylbenzenes, the carbon-carbon double bond must be synthesized after any EAS reactions If is started by EAS in the presence of double bond the addition will take place in stead of halogenation

How we can prepare: 1. P- chlorostyrene 2. P-bromobenzoic acid 3. Allylbenzene

Answers: 1. P- chlorostyrene

2. P-bromobenzoic acid 3. Allylbenzene