Limitations on Friedel-Crafts Alkylation

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

+ CH3CH2CH2-Br, AlCl3

CHCH3H3C

AlCl3+

C CH3H3C

CH3

isopropylbenzene

tert-butylbenzeneCH3CHCH2-Br

CH3

Examples of Carbocation rearangementExamples of Carbocation rearangement

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

Y

RXAlCl3

SO3H NO2 NR3+

CF3 CCl3

C N

CHO

CRO

COHO

CORO

CNH2

O

+ No reaction

When Y Equals Any of These Groups, the BenzeneRing Does Not Undergo Friedel-Crafts Alkylation

Examples:

Br

+ CH3CH2-Br, AlCl3

Br Br

CH2CH3

CH2CH3+

COOH

+ CH3-Br, AlCl3 NR

+ CH3-Br, AlCl3 NR

NO2

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.

1.Alkylbenzenes

2. Alkenylbenzenes

3. Alkynylbenzenes

CH=CH2 styrene

C CH Phenylacetylene

Alkylbenzene

syntheses:1. Friedel-Crafts alkylation

H3CH2C Br+

AlBr3

CH2

H3C

2-Acylation of Aromatic Rings then reduction of the carbonyl group by Zn(Hg)/HCl

Met

hod A

giv

es

100 %

10 al

kylb

enze

ne

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 ringConverts an alkylbenzene into a benzoic acid, ArR ArCO2H

2- Free radical halogenation in side chain:

Br2 / light

Note: halogen atom adds to the benzyl carbon atom (the carbon atom directly bonded with benzene ring

Benzyl carbon

CH2CH3

benzyl free radical > 3o > 2o > 1o > CH3

CHCH3

CHCH3CHCH3 CHCH3.

.

.

.

X2 2 X.

+ X .

Mechanism:

Alkenylbenzenes

syntheses:1. modification of side chain

CHCH3

CHCH3

C

CH=CH2

CH

Br

OHstyrene

H+, heat

H2, Pd-C

KOH

+ CH2=CH-Br no reactionAlBr3

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

Vinyl bromide

Reactions of Alkenylbenzenes

1- oxidation

CH=CH2

CH=CH2

CH=CH2

CHCH2

COOH

CH=O

OHOH

+ CO2

+ O=CH2

KMnO4

heat

1. O3

2. Zn, H2O

KMnO4

2 -reduction

CH=CHCH3 CH2CH2CH3

H2, Ni

CHCHCH3

CHCH2CH3

CHCH2CH3

Br

Br

Br

OSO3H

Br2, CCl4

HBr

H2SO4

CH=CHCH3

3 -Addition

CH=CH2 + Cl2, Fe CHCH2

Cl Cl

CH2CH3 CH2CH3CH=CH2

Cl

CHCH3Cl2, FeCl

ClCl

Cl2, hvKOH(alc)

p-chlorostyrene

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

CH2CH3 CH2CH3

CH=CH2

Cl

CHCH3

CH3-CH2Cl

AlCl3

Cl2, FeCl3

Cl

ClCl

Cl2, hv

p-chlorostyrene

KOH

1. P- chlorostyrene

Answers:

+ CH2=CHCH2-Br, AlCl3 CH2CH=CH2

allylbenzene

2. P-bromobenzoic acid

3. Allylbenzene