Video Transcript
Which of the following substitution
reactions is slowest at room temperature and pressure?
A substitution reaction is a type
of reaction where a part of a molecule is removed and replaced by something
else. We can see that each of the
substitution reactions provided involves methane, a one-carbon alkane. In each reaction, the methane is
reacted with a different diatomic halogen. Over the course of a substitution
reaction between an alkane and a halogen, a carbon–hydrogen bond and the
halogen–halogen bond must break. This allows for the formation of a
new carbon–halogen bond and a new hydrogen–halogen bond, producing a haloalkane and
a hydrogen halide.
The breaking of the halogen–halogen
bond is a critical rate determining step in this reaction. The higher the bond dissociation
energy of this bond, the higher the activation energy of the reaction. In general, a reaction with a
higher activation energy will take longer to occur at room temperature without a
catalyst, such as heat or ultraviolet light. Let’s take a look at the bond
dissociation energy for each of the given halogens.
As we can see, iodine has the
lowest bond dissociation energy and chlorine has the highest. So, based on bond dissociation
energy alone, we might think that the reaction involving chlorine is the slowest and
the reaction involving iodine is the fastest, since breaking the chlorine–chlorine
bond requires the most energy and breaking the iodine–iodine bond requires the
least. But the breaking of the
halogen–halogen bond is just one part of the reaction. The rate of this reaction
significantly depends on the reactivity of the halogen involved.
When we compare the reactivity of
the halogens, we find that iodine is the least reactive and fluorine is the most
reactive. In fact, fluorine is so reactive
that these substitution reactions are often explosive. The reactions involving bromine and
chlorine are less reactive and can be initiated with very little energy input, often
in the form of ultraviolet light. The reaction involving iodine,
however, is not energetically favorable. So, iodine does not generally
undergo substitution reactions with alkanes.
While the trend in bond
dissociation energy suggests that the reaction involving chlorine is the slowest and
iodine is the fastest, the reactivity of the halogens proves to be more important
when determining the rate of reaction for these substitution reactions. So, the reaction involving iodine
is actually the slowest, while the reaction involving fluorine is the fastest.
Therefore, the substitution
reaction that is the slowest at room temperature and pressure is the reaction
between methane and iodine, answer choice (C).
