A Guidebook to Mechanism in Organic Chemistry

Friedel-Crafts Reaction

cold is generally used instead; the more rapid reaction is, of course,
due to the concentration of free SOa that this acid contains.

FRIEDEL-CRAFTS REACTION

This can be conveniently divided into alkylation and acylation.

(i) Alkylation

The reaction of primary alkyl halides, e.g. MeCl, with aromatic

compounds «n the presence of Lewis acids—such as aluminium

halides, BF 3 , etc.—closely resembles the mechanism of catalysed

halogenation that has already been discussed:

8+ 8- •

Me—C1-A1CI,

*•

8+ 8-

Me—Cl-AlCls

+ HC1+A1CI,

That such polarised complexes between the halide and the aluminium

halide are undoubtedly formed is shown by the fact that the halogen

of isotopically-labelled aluminium halides is found to exchange with

that of the alkyl halide. With secondary and tertiary halides, how­

ever, the carbon atom carrying the halogen is increasingly more

able to accommodate positive charge (i.e. it will form a more

stable carbonium ion, cf. p. 84), and there is thus an increasing

tendency towards ionisation of the complex to yield R®, in an

ion pair {cf. p. 80), as the effective electrophilic species. No clear

distinction can be made between primary and other halides in

the extent to which they form actual carbonium ions, however,

as the nature of the catalyst used and of the halogen in the halide

also play a part. Thus with the halide Me 3 C-CH 2 Cl in the presence

of AICI3, benzene yields almost wholly Ph-CMej-OVMe,

due to isomerisation of the first-formed primary carbonium ion,

Me 3 C-CH 2 ®, to the tertiary carbonium ion, Me 2 C-CHa'Me (cf.