A Guidebook to Mechanism in Organic Chemistry

Methods of Formation of Carbonium Ions

This reaction is, of course, reversible and the reverse reaction, the
acid-catalysed dehydration of alcohols, is probably more familiar.
A proton may also add on to a carbon-oxygen double bond

OH

H© \ ® Y9 v /

=0 yC—OH \c \

Y

as in the acid-catalysed addition of some anions, Y®, to an aldehyde
or ketone, the addition of proton to the ^>C=0 providing a highly
positive carWbn atom for attack by the anion. That such protonation
does indeed take place is confirmed by the fact that many ketones
showed double the theoretical freezing-point depression when dis­
solved in concentrated sulphuric acid due to:

^C=0+H,S04 ^ ^>C—OH + HSO.®

That the ketones undergo no irreversible change in the process
may be shown by subsequent dilution of the sulphuric acid solution
with water when the ketone may be recovered unchanged.
A similar result may also be obtained by the use of other electron
deficient species, i.e. Lewis acids:

^>C=< > + AlCls ^ >C—OA1C1,

Carbonium ions may also be generated where an atom containing

unshared electrons is protonated, the actual carbonium ion being

generated subsequently by the removal of this atom:

R—O—H + H® ^ R—O—H ^ R® + HaO
H
This is, of course, one of the steps in the acid-catalysed dehydration
of alcohols mentioned above. It may also be encountered in the acid-
catalysed decomposition of ethers, esters, anhydrides, etc.:

ROR+H® RO.R R® + HOR

H

RCO O R'+H® *± RCOOR' RCO+HOR*

• H

RCOOCOR+H® ^ RCOOCOR ^ RCO + HO.CR

H