A Guidebook to Mechanism in Organic Chemistry

6

ADDITION TO CARBON-CARBON DOUBLE

BONDS

As we have already seen (p. 6), a carbon-carbon double bond con­
sists of a strong a bond plus a weaker w bond, in a different position
(I):

H HHHHH

BN^™.-'H \. e/ \ / \. ./

" C—C <- C=C -* C—C : C

(I) dD (IN)

The pair of electrons in the IT orbital are less firmly held between
the carbon nuclei and so more readily polarisable than those of the
o bond, leading to the characteristic reactivity of such unsaturated
compounds. Addition to such compounds can oroceed through ionic
(II) or free radical (III) mechanisms, depending^!? the way in which
the w electrons become polarised either by the*^pproaching^€S|ent
or other causes. Thus the 'former tends to predominate in polar
solvents, the latter in non-polar solvents, especially in the presence
of other radicals or of light. Free radical addition reactions are dis­
cussed subsequently (p. 242) and attention will here be confined to the
ionic mechanism.
As the ir electrons are the most readily accessible feature of a
carbon-carbon double bond, we should expect them to shield the
molecule from attack by nucleophilic reagents. Thus the character­
istic reactions of such a system should be with cations, X®, or electron-
deficient species, i.e. with electrophilic reagents, and this is indeed
found to be the case.

ADDITION 9f BROMINE

The addition of halogens, e.g. bromine, is envisaged as taking place

not as a one-stage process, in which both bromine atoms become

attached simultaneously, but as a stepwise addition which is initiated