Organic Chemistry

Section 4.10 Addition of Radicals • The Relative Stabilities of Radicals 169

When HBr reacts with an alkene in the absence of a peroxide, the electrophile—the
first species to add to the alkene—is In the presence of a peroxide, the electrophile
is In both cases, the electrophile adds to the carbon that is bonded to the
greater number of hydrogens, so both reactions follow the general rule for elec-
trophilic addition reactions:The electrophile adds to the carbon that is bonded to
the greater number of hydrogens.
Because the addition of HBr in the presence of a peroxide forms a radical interme-
diate rather than a carbocation intermediate, the intermediate does not rearrange. Rad-
icals do not rearrange as readily as carbocations.

As just mentioned, the relative stabilities of primary, secondary, and tertiary alkyl
radicals are in the same order as the relative stabilities of primary, secondary, and ter-
tiary carbocations. However, energy differences between the radicals are quite a bit
smaller than between the carbocations.

The relative stabilities of primary, secondary, and tertiary alkyl radicals are reflect-
ed in the transition states leading to their formation (Section 4.3). Consequently, the
more stable the radical, the less energy is required to make it. This explains why the
bromine radical adds to the carbon of 1-butene that is bonded to the greater num-
ber of hydrogens to form a secondary alkyl radical, rather than adding to the other
carbon to form a primary alkyl radical. The secondary radical is more stable than the
primary radical; therefore, the energy barrier to its formation is lower.
The following mechanism for the addition of HBr to an alkene in the presence of a
peroxide involves seven steps. The steps can be divided into initiation steps, propaga-
tion steps, and termination steps:

+

CH 2

CH 2

Br

Br

CH 3 CCHCH 2 3 C

CH 3 CH 3

Br

Br

H

CH 3 C

CH 3

+

propagation

steps

3.

4. CH 3 C CH 2 H Br Br

CH 3

+

initiation steps

2. RO + H Br ROH + Br

RO OR 2 RO

sp^2

sp^2

>

R

RC

R

tertiary radical

>

R

RC

H

secondary radical

>

H

RC

H

primary radical

H

HC

H

methyl radical

least

stable

most

stable

CH 3 CH 3

CH 3 CHCH

3-methyl-1-butene 1-bromo-3-methylbutane

CH 2 HBr CH 3 CHCH 2 CH 2 Br

peroxide

+

carbon skeleton is

not rearranged

sp^2

Br–. sp^2

H+.

Radical intermediates do not rearrange.