Organic Chemistry

Summary of Reactions 355

Summary

Alkanesare called saturated hydrocarbonsbecause they
do not contain any double or triple bonds. Since they also
have only strong bonds and atoms with no partial
charges, alkanes are very unreactive. Alkanes do undergo
radical substitution reactions with chlorine or
bromine at high temperatures or in the presence of
light, to form alkyl chlorides or alkyl bromides. The substi-
tution reaction is a radical chain reactionwith initiation,
propagation, and termination steps. Unwanted radical re-
actions are prevented by radical inhibitors—compounds
that destroy reactive radicals by creating unreactive radicals
or compounds with only paired electrons.
The rate-determining step of the radical substitution
reaction is hydrogen atom abstraction to form a radical.
The relative rates of radical formation are benzylic
allyl vinyl methyl. To determine the
relative amounts of products obtained from the radical halo-
genation of an alkane, both probability and the relative rate
at which a particular hydrogen is abstracted must be taken
into account. The reactivity–selectivity principlestates that
the more reactive a species is, the less selective it will be. A

73 ° 72 ° 71 ° 7 '

'

(Br 2 )

(Cl 2 )

s

bromine radical is less reactivethan a chlorine radical, so a

bromine radical is more selectiveabout which hydrogen

atom it abstracts. N-Bromosuccinimide (NBS) is used to

brominate allylic positions. Cyclic compounds undergo the

same reactions as acyclic compounds.

If a reactant does not have an asymmetric carbon, and a

radical substitution reaction forms a product with an asym-

metric carbon, a racemic mixture will be obtained. A

racemic mixture is also obtained if a hydrogen bonded to an

asymmetric carbon is substituted by a halogen. If a radical

substitution reaction creates an asymmetric carbon in a

reactant that already has an asymmetric carbon, a pair of

diastereomers will be obtained in unequal amounts.

Some biological reactions involve radicals formed by

the interaction of organic molecules with metal ions. The

reactions take place in the active site of an enzyme.

Strong circumstantial evidence implicates synthetic

chlorofluorocarbons as being responsible for the diminish-

ing ozone layer. The interaction of these compounds with

UV light generates chlorine radicals, which are the ozone-

removing agents.

Summary of Reactions

1.Alkanesundergo radical substitution reactions with or in the presence of heat or light (Sections 9.2, 9.3, and 9.4).

2.Alkyl-substituted benzenesundergo radical halogenation at the benzylic position (Section 9.5).

3.Alkenes undergo radical halogenation at the allylic positions. NBS is used for radical bromination at the allylic position

(Section 9.5).

∆ or h

NBS peroxide

Br

+

CH 2 RBr+ 2 CHR + HBr

h

Br

CH 3 CH 3 + Cl 2 CH 3 CH 2 Cl + HCl

∆ or h

∆ or h

excess

CH 3 CH 3 + Br 2 CH 3 CH 2 Br + HBr

excess

bromination is more selective than chlorination

Cl 2 Br 2

∆ or h

RCH 2 CH CH 2 + NBS peroxide RCHCH CH 2 + RCH CHCH 2 + HBr

Br Br