Chemistry, Third edition

REACTION MECHANISMS 261

Chemical reactions which (like the chlorination of methane) occur in several

steps are said to be complex.

Reaction intermediates are molecules, radicals or ions which are made (and then

used up) during the reaction mechanism. Although they are often so reactive that

they last for only a short time (or, are present in very small concentrations at any

moment during the reaction) they may often be detected spectroscopically, i.e. by

the characteristic frequencies of light which they absorb.

The use of reaction kinetics in helping to establish the

mechanism of a reaction

The rate expression for a reaction often gives valuable information about the mechan-

ism of a reaction. When we introduced rate expressions (page 250), we made the

point that we could not reliably predict the rate expression from the equation for the

reaction. The reason for this uncertainty is that we cannot usually say whether or not

the chemical equation in question is elementary, i.e. whether or not it occurs in one

or several steps.

Itispossible to predict the rate expression for an elementary reactionfrom its

chemical equation. For example, consider the elementary reaction represented by

the equation

ABC

The rate at which molecules A and B collide is proportional to their concentration.

The rate expression is therefore

ratek[A][B]

In practice, we usually look at the experimentally determined rate equation and see

whether or not it is consistentwith a proposed mechanism. As an example, consider

the reaction of NO 2 and CO gases to produce NO(g) and CO 2. Suppose we propose

that the reaction occurs in one step:

NO 2 (g)CO(g)NO(g)CO 2 (g)

The expected rate expression would then be

ratek[NO 2 (g)][CO(g)]

Experimentally it is found that the rate expression is

ratek[NO 2 (g)]^2

This provides evidence that the reaction of NO 2 and CO does not occur in one stage.

Now consider the reaction of 1-bromopropane with hydroxide ion. Let us pro-

pose that the reaction occurs in one step:

CH 3 CH 2 CH 2 Br(l)OH(aq)CH 3 CH 2 CH 2 OH(l)Br(aq)

Experiments show that the rate expression is:

Ratek[CH 3 CH 2 CH 2 Br(l)][OH(aq)]

Such a rate expression is consistent with (but does not absolutely prove) that the

reaction occurs in one stage. (For more details of the bromopropane/hydroxide ion

reaction, see page 249).

Complicated rate expressions (such as those involving fractional orders of reac-

tion) always indicate that the mechanism of the reaction is complex. An example of

this is the decomposition of ethanal (page 252).