A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Carbon Double Bonds

The formation of the carbonium ion (XV) is the rate-determining

step in the reaction but whether this takes place directly or via the

rapid, reversible formation of a IT complex (XVI), followed by the

slow, rate-determining conversion of the latter to the carbonium

ion (XV)

/*'=^'<\ ' fast /*"\^'N\ slow*

H® H

(XVI) (XV)

is not wholly certain. Hydrogen halides are not nornfally used as

sources of proton because of their tendency to add on themselves, but

the HS0 4 s ions produced with sulphuric acid are only very weakly

nucleophilic and, even if they should add on, the alkyl hydrogen

sulphates (XVII) so produced are very readily hydrolysed by water:

(XVII)

The reaction is oftwfortance for converting petroleum fractions into
alcoiifllfcand is sometimes brought about by dissolving the alkenes in
concentrated sulphuric acid and then diluting the solution with
water. The orientation of addition, being proton-initiated, follows the
conventions already discussed and proceeds trans.

(ii) Carbonium ion addition

The carbonium ion intermediate that may result from initial pro-
tonation of the double bond in several of the above reactions can
itself, of course, act as an electrophile towards a second molecule of
alkene. Thus with isobutene (XVIII)

Me 2 C=CH 2 (XVIII) Me 3 C—CH=CMe 2 (XXII)

I A

H© _Hffi

Me 3 C ^CwXcMe, > Me 3 C—CH 2 —CMe 2 /cH^CMe 2

(XIX) (XX)

Me 3 C—CHif-CMej—CHss—CMe 2

(XXI)