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

2. Examples of Carbocation rearangement

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

4. Examples:

5. 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

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

7. Alkylbenzene
syntheses:
Friedel-Crafts alkylation

8. 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

9. 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

10. 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

11. Mechanism:

12. Alkenylbenzenes
syntheses:
1. modification of side chain

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

14. Reactions of Alkenylbenzenes
1- oxidation

15. 2- reduction
3- Addition

16. 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

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

18. Answers:
1. P- chlorostyrene

19. 2. P-bromobenzoic acid
3. Allylbenzene