Section 4.10 Addition of Radicals • The Relative Stabilities of Radicals 169When 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 2CH 2BrBrCH 3 CCHCH 2 3 CCH 3 CH 3Br
BrHCH 3 CCH 3+propagation
steps3.- CH 3 C CH 2 H Br Br
CH 3+initiation steps- RO + H Br ROH + Br
RO OR 2 ROsp^2sp^2>RRCR
tertiary radical>RRCH
secondary radical>HRCH
primary radicalHHCH
methyl radicalleast
stablemost
stableCH 3 CH 3CH 3 CHCH
3-methyl-1-butene 1-bromo-3-methylbutaneCH 2 HBr CH 3 CHCH 2 CH 2 Brperoxide
+carbon skeleton is
not rearrangedsp^2Br–. sp^2H+.Radical intermediates do not rearrange.