(a) When n-heptaneburns in a gasoline engine, the combustion process takes place too quickly. The explosive detonation makes a noise called knocking. When 2,2,4 trimethylpentane (isooctane) is burned, combustion takes place in a slower, more controlled manner. Combustion is a free-radical chain reaction, and its rate depends on the reactivity of the free-radical intermediates. Explain why isooctane has less tendency to knock than doesn-heptane

(b) Alkoxy radicals (R-O)are generally more stable than alkyl (R) radicals. Write an equation showing an alkyl free radical (from burning gasoline) abstracting a hydrogen atom from tert-butyl alcohol,(CH₃)₃COH. Explain whytert-butyl alcohol works as an antiknock additive for gasoline.

(c) Use the information in Table 4-2 (page 203) to explain why toluene (PhCH₃) has a very high-octane rating of 111. Write an equation to show how toluene reacts with an alkyl free radical to give a relatively stable radical.

An atom or group of atoms containing odd or unpaired electrons is known as free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

The general formula of alkyl free radical is (R) , whereas the general formula of alkoxy free radical is, (R-O) .

Internediate is a species that exists for a finite amount of time, and has some stability, although its lifespan is short.

(a)

n-heptane

2,2,4 trimethylpentane

The formation of a secondary and less stable primary radical is possible in n-heptane, and the formation of an extremely stable tertiary radical is possible in 2,2,4 trimethylpentane. Due to its higher stability, tertiary radical reacts more slowly.

(b)

(alkyl free-radical) (tert-butyl radical) ( tert-butoxy radical)

The rate of the reaction drops down due to the formation of a stable tert-butoxy radical.

(c)

(alkyl free-radical) (toluene) (benzylic radical)

The rate of the reaction drops down due to the formation of stable benzylic radical. Benzyl radical is stabilized by resonance.