Advanced A Level Chemistry Advanced A Level Chemistry - Kinetics-Rates
revision notes Part 7
7.1 The water/alkali hydrolysis of halogenoalkanes
(aqueous hydrolysis of alkyl halides/haloalkanes)
The kinetics of the hydrolysis of halogenoalkanes
Explanation and derivation of orders of
reactants and how to write the rate expression
is also a series of pages entirely devoted to organic reaction mechanisms!
HALOGENOALKANES (HALOALKANES) UNDERGO
SUBSTITUTION BY ONE OF TWO POSSIBLE MECHANISMS AND KINETICS CAN SORT OUT
The possibility of two reaction mechanisms for the hydrolysis of
halogenoalkanes (RX) with sodium hydroxide or water has consequences for
the rate expressions.
mechanism is sometimes described as 'unimolecular' because the rate
only depends on the concentration of the halogenoalkane.
for (i) has three steps
of bimolecular collisions. Here the rate is only
dependent on one reactant, the halogenoalkane, R3C–X, shown in step
(1), (but it still has to collide with the solvent, which never
seems to be shown at AS–A2 level and whose concentration is
produce the overall 1st order rate expression: rate = k1[RX]
kinetics suggests there is a rate determining step involving one of
the reactants, irrespective of the total number of steps, which in
this case is three.
This is because the
activation energy of the 1st step, forming the carbocation
heterolytic bond fission, is so high, that the speed is relatively
low. Therefore step (1)
alone determines the speed of the reaction. This is referred to as
the rate determining step (or rds in shorthand!). Steps
(2)/(3) have much lower
activation energies and are much faster. You would register zero order for the
order of reaction with respect to e.g. any hydroxide ion present or it might even
hydrolyse just in water!
simplicity of the rate expression, despite the complexity of the
diagram (mechanism 10) shows the full reaction mechanism.
diagram (mechanism 42) shows the reaction profile with step (1)
having much the bigger activation energy and hence acting as the rate
determining step. The two 'troughs represent the formation of
the intermediates or transition states whatever their lifetime
mechanism below for reaction (ii), is referred to as 'bimolecular' because the rate depends
on the concentrations of both reactants.
produce the overall 2nd order rate expression: rate = k2[RX][OH],
kinetics suggests there is a rate determining step involving both of
the reactants, irrespective of the total number of steps, though
in this case it is just one step.
This is because it is a
simple single step mechanism involving
a bimolecular collision of the two
reactant molecules/ions. The rate depends on both the
halogenoalkane and hydroxide ion concentrations (1st
order with respect to both reactants).
diagram above (mechanism 2) shows the 'simple' mechanism.
diagram (mechanism 33) shows the 'activated complex' or 'transition
state'* which is the peak of the potential energy of the system
(see diagram 41 below it) where the 'incoming' hydroxide ion is
half–bonded to the carbon and the 'outgoing' chloride ion is still
'half–bonded' to the carbon. No
intermediate is formed.
diagram (mechanism 41) shows the energy changes as a reaction
an 'activated complex' or 'transition state' is not the
same as an intermediate like a carbocation which is a definite
entity in its own right, however short its lifetime.
nucleophilic substitution by