Reaction Mechanisms

1
Reaction Mechanisms
Most reactions occur as a sequence of elementary
steps.
e.g. 2 C8H18 25 O2 ? 16 CO2 18 H2O
octane
In order for this reaction to occur in one step,
two molecules of octane must simultaneously
collide with twenty-five molecules of oxygen
(with enough energy at the right angle) . . .
A HIGHLY UNLIKELY event!
2
The slowest step in any reaction mechanism is
called the rate-determining step.
If the rate law equation for a reaction is
r k X m Y n
then the rate-determining step must be
mX nY ? reaction intermediates
3
Reaction Intermediate
A product of one step in a reaction mechanism
that later becomes a reactant in another.
e.g.
Krebs cycle
4
rate-determining step (highest Ea)
A
A
A
I
I
A activated complex
I reaction intermediate
5
Which step in the reaction mechanism shown below
is the rate-determining step?