A Guidebook to Mechanism in Organic Chemistry

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Carbonium Ions, Electron-deficient N and O Atoms

CH 3 —CH 2 —Mc + AlBr 3 ^ CH 2 —CH 2 —Me • HAlBr 3

* *

which may happen in cases such as the above where it is only hydro­

gen that has apparently moved.

The elimination reactions of carbsnium ions will be discussed

further below (p. 191^when elimination reactions in general are dealt

with, but their rearrangement merits further study.

THE REARRANGEMENT OF CARBONIUM IONS

Despite the apparent confusion introduced above by the isomerisa-

tion of propane, the rearrangement reactions of carbonium ions can

be divided essentially into those in which a change of actual carbon

skeleton does, or does not, take place; the former are the more impor­

tant but the latter will be briefly mentioned first.

(i) Without change in carbon skeleton

(a) Allylic rearrangements: A classical example of this variety may

occur where the carbonium ion formed is stabilised by delocalisation,

e.g. in the SNl solvolysis of 3-chlorobut-l-ene,Me-CHCl-CH=CH 2 ,

in ethanol. After formation of the carbonium ion

MeCHCICH=CH 2 ^i: Cle +

MeCH-^CH=*=CH 2

Me-CH=CH—CH,

complex which carries its positive charge on a secondary, rather than

e+ «-

a primary, carbon atom, i.e. Me 2 CHGaBr 4 rather than Me-CH 2 *

CH 2 GaBr 4 , again results in a hydride shift so that the major product

of the reaction is actually isopropylbenzene.

That such rearrangements need not always be quite as simple as

they look, however, i.e. mere migration of hydrogen, is illustrated by

the behaviour with AIBr 3 of propane in which a terminal carbon

atom is labelled with^13 C, when partial transfer of the labelled carbon

to the 2-position occurs. This is presumably due to

Me—CH 2 —CH 3 + AlBr 3 ^ (M^CH^CH 2 HAlBr 3