14

2.5.

Friedel-Crafts Alkylation and Acylation

C-C bond formation / very important

2.5.1. Friedel-Crafts Alkylation

General reaction scheme:

Generation of electrophile:

Lewis acid (halogen carrier)

active species can be generated from an alcohol or an alkene using acid:

for haloalkanes
products:

this

leads

to

less

stable

carbocations

resulting

in

rearranged

other problems associated with Friedel-Crafts alkylation:

•

polyalkylation:monoalkylated arenes are more reactive than starting materials
(haloform reaction, alkylation of primary amines).

•
•

toluene is 25 times more reactive than benzene
small reactivity range
•haloarenes represent the lower reactivity range
•aromatic rings containing NH2, NHR or NR2bind to Lewis acid (through
their lone pairs) and kill reactivity (c.f. ammonium ion substituent).

Second removable group

R

R

R'

+R'-X

Lewis acid

+HX

AlCl3>FeCl3>BF3>TiCl3>ZnCl
2>SnCl4

Cl

H

+FeCl3

FeCl4

tight ion pair

relatively
stabilised
carbocation

NR2

AlBr3