A Guidebook to Mechanism in Organic Chemistry

Rearrangement of Hydrocarbons

Me

Me—C—CH—/ X

I I

Me CI

Me

-ch£^

Me—C

I

Me

Me ©

I

Me—C—CH

X

Me

(XII)

etc.

(b) Rearrangement of hydrocarbons: Wagner-Meerwein type re­

arrangements are also encountered in the cracking of petroleum

hydrocarbons where catalysts of a Lewis acid type are used. These

generate carbonium ions from the straight-chain hydrocarbons (cf.

the isomerisation of 1SC labelled propane above), which then tend to

rearrange to yield branched-chain products. Fission also takes place,

of course, but the branching is of importand?as the branched hydro­

carbons produced cause less knocking in the cylinders of internal

combustion engines than do their straight-chain isomers. It should,

however, be mentioned that cracking can also be brought about by

catalysts that promote reaction via radical intemediates (p. 236).

Rearrangement of unsaturated hydrocarbons takes place (geadily

in the presence of acids:

This tendency can be a nuisance when acid reagents, e.g. hydrogen
halides, are being added preparatively to olefines: mixed products
that are difficult to separate may result or, in unfavourable cases,
practically none of the desired product may be obtained.