23.2    Reaction Mechanisms

Rate-determining step

• In any chemical change, some bonds are broken and new ones are made. Quite often, these changes are too complicated to happen in one simple stage. Instead, the reaction may involve a series of small changes one after the other

• A reaction mechanism describes the one or more steps involved in the reaction in a way which makes it clear exactly how the various bonds are broken and made.

• The overall rate of reaction is governed by the rate of the slowest step. The slow step is therefore called the rate-determining step.

• For example, the reaction between 2-bromo-2-methylpropane(a tertiary halogenoalkane) and hydroxide ion happens in two stagesThe overall equation is:
• In this example, Step 1 is the rate-determining step because the rate of reaction depends entirely on the rate of this step. Increasing the rate of the already fast step 2 will not increase the overall rate of reaction

• By doing experiments, it can be found that the the rate equation is: rate = [(CH3)3CBr]
• The rate therefore does not depend on the hydroxide ion, which takes part in the second stage of  the reaction.
• Another example is the reaction between 1-bromoethane(a primary halogenoalkane) and hydroxide This reaction happens in a single step.
  
• The rate equation for this reaction is rate = [CH3CH2Br][OH⁻] because the rate of reaction depends on these two reactants

Molecularity of reaction

• Molecularity of a reaction is the number of species taking part in a (stage of) reaction.

• In the first example above, Stage 1 has a molecularity of one because only one species is involved in this stage. Therefore Stage 1 is said to be unimolecular.
• Stage 2 involves two molecules colliding and therefore it has a molecularity of two. Stage 2 is said to be bimolecular.

• In the second example, two species are involved, therefore it is a bimolecular reaction having a molecularity of two

Order and mechanism

• Given the rate equation and the orders of reaction, it is possible to deduce the reaction mechanism and vice versa
• 1. For example, given:
     rate = k[A][B]
     And given two possible reaction mechanisms. It can be deduced that mechanism 2 might be correct. This is because the rate equation tells us both A and B are involved in the rate-determining step(slow step) of  the reaction.
  2. If we know that mechanism 2 is correct. We can deduce that the above rate equation might be the correct one, since two species are involved in the slow step.
• 1. If the rate-determining step is not the first step, for example:
  2. It can be deduced that the rate equation might be
     rate = k[A][X]
  3. However, X is only an intermediate. Its concentration can be expressed in terms of concentrations of A and B, using the fact that the first step is in equilibrium.
  4. The steps are: