Organic Chemistry

168 CHAPTER 4 Reactions of Alkenes

hydrogens—isfollowed, however, because is the electrophile when HBr adds to an

alkene in the presence of a peroxide.

When a bond breaks such that both of its electrons stay with one of the atoms, the

process is called heterolytic bond cleavageor heterolysis. When a bond breaks such

that each of the atoms retains one of the bonding electrons, the process is called

homolytic bond cleavageor homolysis. Remember that an arrowhead with two barbs

signifies the movement of two electrons, whereas an arrowhead with one barb—

sometimes called a fishhook—signifies the movement of a single electron.

An alkyl peroxide can be used to reverse the order of addition of H and Br to an

alkene. The akyl peroxide contains a weak oxygen–oxygen single bond that is readily

broken homolytically in the presence of light or heat to form radicals. A radical(also

called a free radical) is a species with an unpaired electron.

A radical is highly reactive because it seeks an electron to complete its octet. The

alkoxyl radical completes its octet by removing a hydrogen atom from a molecule of

HBr, forming a bromine radical.

The bromine radical now seeks an electron to complete its octet. Because the dou-

ble bond of an alkene is electron rich, the bromine radical completes its octet by com-

bining with one of the electrons of the bond of the alkene to form a C Br bond.

The second electron of the bond is the unpaired electron in the resulting alkyl radi-

cal. If the bromine radical adds to the carbon of 1-butene that is bonded to the

greater number of hydrogens, a secondary alkyl radical is formed. If the bromine rad-

ical adds to the other carbon, a primary alkyl radical is formed. Like carbocations,

radicals are stabilized by electron-donating alkyl groups, so a tertiary alkyl radicalis

more stable than a secondary alkyl radical, which in turn is more stable than a

primary alkyl radical. The bromine radical, therefore, adds to the carbon that is

bonded to the greater number of hydrogens, thereby forming the more stable (in this

case) secondary radical. The alkyl radical that is formed removes a hydrogen atom

from another molecule of HBr to produce a molecule of the alkyl halide product and

another bromine radical. Because the first species that adds to the alkene is a radical

the addition of HBr in the presence of a peroxide is called a radical addition

reaction.

+ Br

Br CH 2 CHCH 2 CH 3

Br

Br

+

H Br

Br H

CH 2 CHCH 2 CH 3

an alkyl radical

CH 2 CHCH 2 CH 3 + CH 2 CHCH 2 CH 3

(Br–),

sp^2

sp^2

sp^2

p

p ¬

RHO + Br Br

a bromine

radical

RHO +

RO OR

an alkyl peroxide alkoxyl radicals

2 RO

light

or

∆

H Br H+ + Br−

H Br H + Br

heterolytic bond cleavage

homolytic bond cleavage

Br–

Radical stability: tertiary
secondary>primary

>

An arrowhead with two barbs signifies
the movement of two electrons.

An arrowhead with one barb signifies
the movement of one electron.