Addition to Carbon-Carbon Double Bondsby a species containing positively polarised bromine: in this case the:
bromine molecule which becomes polarised on its close approach to
the IT electron layer of the define. The bromine first interacts with
the IT electrons to form a so-called w complex (IV) and .then breaks
through to form a a bond with carbon, yielding the carbonium ionThe addition is then completed by Bre attack to yield the dibro-
mide (VI); the evidence suggests that the formation of (V) is the rate-
determining step of the reaction. If an intermediate such as (V) is
indeed formed then mixed products should result if the addition of
bromine is carried out in the presence of other anions, for once (V)
has been formed, BrG will not then be in any specially privileged
position for attackingit to complete the addition. It is, in fact, found
that the presence oTXl^0 , NO 80 , etc., during bromination leads to
the formation of, for*8xample, C1CH 2 • CH 2 Br and OaN • CH 2 • CH 2 Br
in addition to (VI). That these products are not formed merely by
subsequent attack of C1Q and N0 3 e on (VI) is confirmed by the fact
that their formation proceeds much faster than do these substitution
reactions.
An alternative intermediate has also been suggested, namely the
bromonium ion (VII), which diners from (V) only in the distribution
of its electrons (it could indeed be derived by the attack of one of the
bromine atom's unshared electron pairs on the nearby carbonium
ion carbon atom):
The participation of such an intermediate would be expected to
have stereochemical implications.(V):Br(V) (VII)