A Guidebook to Mechanism in Organic Chemistry

Halogenation

The reaction may also be initiated in the dark by heating but

considerably elevated temperatures are required to effect CI—Cl-»-

Cl*+ *C1; thus the rate of chlorination of ethane in the dark at 120°

is virtually indetectable. The reaction becomes extremely rapid,

however, on the introduction of small quantities of Pb(Et) 4 which

undergoes decomposition at this temperature to yield ethyl radicals

(cf. p. 235) capable of acting as initiators:

Et-+Cl—CI -* Et—C1+-C1

As is well known, the hydrogen atom on a tertiary carbon is more
readily displaced than those on a secondary carbon and these, in their
turn, more readily than those on a primary carbon; this reflects the
relative stability of the radical, R, that will be formed in the first
instance (p. 234). The difference is often not sufficiently great, how­
ever, to avoid the formation of mixtures of products from hydro­
carbons containing more than one position that may undergo attack;
further, what preferential attack there is may be in large part negatived
by a statistical effect. Thus, in isobutane, (CHa)gCH, although the
hydrogen atom on the tertiary carbon is more readily attacked than
those on the primary, there are no less than nine of the latter to
attack compared with only one of the former, thus further limiting
the preparative, i.e. selective, use of photochemical chlorinStion.
The reaction is, however, also influenced by polar factors, for the
electronegative Hal as well as being a radical is at the same time an
electrophilic reagent and will tend therefore to attack preferentially
at a site where the electron density is high. Radical halogenation thus
tends to be retarded by the presence of electron-withdrawing
atoms or groups, e.g. a second halogenation on a carbon atom that
has already been substituted is more difficult than the first.
If the carbon indirectly attacked is asymmetric, e.g. RR'R'CH.then
a racemic chloride is obtained. This racemisation does not constitute
proof of the planar nature of the radical formed, however, (cf. p. 239),
for the same result would be obtained with a radical having a pyra­
midal configuration provided it could rapidly and reversibly turn
itself 'inside out' as can the pyraHlrtal molecules of ammonia and
amines:

C R R' R'

R R' R' C

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