Reaction mechanisms

Many reactions proceed via a series of steps called a mechanism.

A + B → C + D

C + D → E + F

E + F → G

It is highly unlikely that these steps have the same rate.

Fast

Slow

Fast

Rate determining step

• No reaction can be faster than its slowest step.

• This is called the “rate determining step”

• Eg; E + F -slow→ G• Anything that appears in the rate determining

step will appear in the rate equation according to its stochiometry.

• Eg; Rate = k [E]1[F]1

• Everything else will have zero order.

• Eg; 2N2O5 → 4NO2 + O2

• From stochiometry the rate equation might be predicted as;

• Rate = k [N2O5]2

• In fact experiments demonstrate that the rate equation is;

• Rate = k [N2O5]1

The actual mechanism is;

N2O5 → NO2 + NO3

NO2 +NO3 → NO + NO2 + O2

NO + NO3 → 2NO2

Slow

Fast

Fast

Only one molecule of N2O5 participates in the rate determining step.

Giving a rate equation; Rate = k [N2O5]1

Writing equations from mechanisms.

Add all the reactants of the different steps together and place them on the LHS.

Then do the same with the products and place them on the RHS.

Ignore any intermediates and catalysts.

Eg Decomposition of N2O5

N2O5 → NO2 + NO3

NO2 +NO3 → NO + NO2 + O2

NO + NO3 → 2NO2

LHS = 2N2O5 (everything else is an intermediate)

RHS = NO2 +NO2 +O2 + 2NO2 = 4NO2 + O2

Giving; 2N2O5 → 4NO2 + O2

N2O5 → NO2 + NO3

Eg; Iodination of propanone

CH3COCH3 + I2 → CH2ICOCH3 + HI

From stochiometry the rate equation might be; Rate = k [CH3COCH3]1[I2]1

But experiments show it is;Rate = k [CH3COCH3]1[H+]1

The reaction is zero order with respect to iodine, which cannot be part of the rate determing step.

The actual mechanism is;

CH3COCH3 + H+ → CH3C(OH+)CH3

CH3C(OH+)CH3 → CH2C(OH+)CH3 + H+

CH2C(OH+)CH3 + I2 → CH2ICOCH3 + HI

The proton catalyst is involved in the rate determining step and so appears in the rate equation.

slow

fast

fast

But iodine does not and so will not appear in the rate equation.

SN2 reactions

SN2 reactions have only one step.

The C/halogen bond breaksAs the C/nucleophile

bond forms.

Rate equations for SN2 reactions.

Both the haloalkane and the nucleophile take part in the rate determining step.

Rate = k [haloalkane] [nucleophile]

The reaction is first order with respect to both reactants, so it is second order overall.

SN1Reactions have two steps;

Step 1 The C/halogen bond breaks heterolytically forming a carbocation;

+

XX-

→

+

Step 2

The carbocation reacts with the nucleophile.

+

XX

+

→

Rate equations for SN1 reactions

Experiments reveal SN1 reactions are first order with respect to haloalkanes but zero order with respect to nucleophiles, so the first step is rate determining.

The first step only involves the haloalkane. If it was rate determining the rate equation would be;

Rate = k [haloalkane]

But the second step involves both nucleophile and haloalkane. If it was rate determining the rate equation would be;

Rate = k [haloalkane] [nucleophile]