Organic Chemistry

338 CHAPTER 9 Reactions of Alkanes • Radicals

9.1 The Low Reactivity of Alkanes

The double and triple bonds of alkenes and alkynes are composed of strong bonds

and relatively weak bonds. We have seen that the reactivity of alkenesand alkynes

is the result of an electrophile being attracted to the cloud of electrons that constitutes

the bond.

Alkaneshave only strong bonds. Because the carbon and hydrogen atoms of an

alkane have approximately the same electronegativity, the electrons in the and

bonds are shared equally by the bonding atoms. Consequently, none of the

atoms in an alkane have any significant charge. This means that neither nucleophiles

nor electrophiles are attracted to them. Because they have only strong bonds and

atoms with no partial charges, alkanes are very unreactive compounds. Their failure to

undergo reactions prompted early organic chemists to call them paraffins, from the

Latin parum affinis,which means “little affinity”(for other compounds).

9.2 Chlorination and Bromination of Alkanes

Alkanes do react with chlorine or bromine to form alkyl chlorides or alkyl

bromides. These halogenation reactionstake place only at high temperatures or in

the presence of light (symbolized by hv). They are the only reactions that alkanes

undergo—with the exception of combustion, a reaction with oxygen that takes place

at high temperatures and converts alkanes to carbon dioxide and water.

The mechanism for the halogenation of an alkane is well understood. The high

temperature (or light) supplies the energy required to break the or

bond homolytically. Homolytic bond cleavageis the initiation stepof the reaction

because it creates the radical that is used in the first propagation step (Section 4.10).

Recall that an arrowhead with one barb signifies the movement of one electron

(Section 3.6).

A radical(often called a free radical) is a species containing an atom with an

unpaired electron. A radical is highly reactive because it wants to acquire an electron to

complete its octet. In the mechanism for the monochlorination of methane, the chlorine

radical formed in the initiation step abstracts a hydrogen atom from methane, forming

HCl and a methyl radical. The methyl radical abstracts a chlorine atom from

forming methyl chloride and another chlorine radical, which can abstract a hydrogen

atom from another molecule of methane. These two steps are called propagation steps

because the radical created in the first propagation step reacts in the second propaga-

tion step to produce a radical that can repeat the first propagation step. Thus, the two

propagation steps are repeated over and over. The first propagation step is the rate-

determining step of the overall reaction. Because the reaction has radical intermediates

and repeating propagation steps, it is called a radical chain reaction.

Cl 2 ,

Cl Cl or 2Cl

h

Br Br or 2Br

h

∆

∆

homolytic

cleavage

Cl¬Cl Br¬Br

methyl chloride

CH 4 + Cl 2 CH 3 Cl + HCl

∆

∆

or

h

ethyl bromide

CH 3 CH 3 + Br (^2) or CH 3 CH 2 Br + HBr
h
(Cl 2 ) (Br 2 )
s
C¬Cs
C¬H
s
p
p
s