Effect of Temperature on Elimination and Substitution Reactions

This organic chemistry video tutorial discusses the effect of temperature on elimination and substitution reactions. It explains why E1 reactions are favored over SN1 reactions at high temperatures.

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[akhilsingh]

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[nirmalmishra]

Notes: consider the reaction that we have
with tert-butyl chloride reacting with
iodide dissolved in water but at
different temperatures
in the top example we're reacting it at
a low temperature of 25 degrees Celsius
and in the bottom example at a higher
temperature
at 95 degrees Celsius
what's going to be the major product for
this reaction and what mechanism will it
proceed by
now in both cases we're dealing with a
tertiary alkyl halide
so we're not going to get the sn2
reaction
the substrate is two sterically hinted
for that
now we don't have a strong base
iodide is a good nucleophile very very
very weak base
water is a better base than iodide
so water will be the weak base iodide is
the good nucleophile
because we don't have a strong base
we're not going to get the E2 reaction
so what we're going to get is a mixture
of the sn1 and E1 reaction
we have a tertiary alkyl halide a decent
nucleophile and a weak base which is the
solvent
now what you need to understand is that
as we increase the temperature
we're going to get more of the E1
reaction
so as the temperature goes up
the yield of the E1 reaction increases
the yield of the S1 reaction decreases
as we decrease the temperature we get
more of the S1 product and less of the
E1 product
so high temperatures favor the E1
reaction low temperatures favor the S1
reaction
so this is going to proceed by the S1
mechanism the second example will
proceed via the E1 mechanism
in both cases
in the first example
we're going to replace chloride with
iodide
the water really doesn't need to react
in this example
what it's going to do is going to
solvate the carbocation
but we will get chloride
as a side product
so those are the products for the first
reaction in the second example because
we get an elimination
we're going to form a double bond
chloride will be expelled
and water
is going to abstract a proton
and we're going to get
you could think of it as iodide and h3o
plus
but if you were to subtract water as a
reactant and as a product
you can treat this
as h i
if you take away water from H2O plus
it's basically h plus and I minus
but now with that being said
why does an increase in temperature
favor
the E1 reaction over the sn1 reaction
why does why do high temperatures favor
elimination over substitution
well there's two big reasons
the first one has to do with
thermodynamics
perhaps you recall the equation
Delta G gives free energy is equal to
Delta H the enthalpy minus t Delta s the
change in entropy
now an increase in temperature enhances
the entropy effect
if we look at the reactions for S and
one and E1 that we have here
notice that the S1 reaction we have two
reactants and two products
so in terms of the number of
molecules and ions that we have here
there's no net change it's two for two
but for the E1 reaction
we have two reactants three products
so
there's an increase in entropy for the
E1 reaction
as we increase the temperature it's
going to enhance the entropy effect
because this term T Delta s
will increase as well the absolute value
of T Delta s will increase as
temperature increases now we do have a
negative sign in front of that
and so as this increases
it's going to cause Delta G to decrease
or become more negative
and we know that a negative Delta G
value indicates a spontaneous reaction
so in other words
increase in the temperature
causes this reaction the E1 reaction to
become more spontaneous
so this reaction becomes more
energetically favorable
when the temperature goes up
the position of equilibrium shifts to
the right as we increase the temperature
it becomes more energetically favorable
now let's talk about the kinetics or the
speed of the reaction
at low temperature
the reaction is under kinetic control
the sm1 reaction will occur faster under
low temperatures at high temperatures
it's under thermodynamic control the
energetics favor the E1 reaction
now the E1 reaction
it has a higher
forward activation energy
and that's why at low temperatures
it doesn't proceed
because it is a higher
energy barrier that must be overcome
and so an increase of temperature will
allow us to get over that higher energy
barrier but why is there a higher
activation energy
for the E1 reaction
this has to do with the number of bonds
that must be broken
in order to get the E1 reaction going
if we focus on the S1 reaction mechanism
the only bond that we need to break is
the carbon chlorine bond
that's the slow step
now remember anytime you break a bond
that's an endothermic reaction you need
to put in energy to do that whenever a
bond is formed energy is released that's
an exothermic reaction
so this step requires the input of
energy
in the next example where iodide
combines with the carbocation
energy will be released so in this
mechanism
we only need to bake we only need to
break one bond that is the carbon
chlorine bond
now for the E1 reaction
we also need to break the carbon
chlorine bond to get the carbocation
now this step here
is where's get it gets interesting
because
in the S1 reaction we don't need to
break another Bond but for the E1
reaction we do
in order to get the E1 reaction going we
need to break another bond that is the
carbon hydrogen bond in order to form
that alkene
so for this particular example
in the S Sub 1 reaction we only need to
break one Bond
for the E1 reaction
we have to break
two bonds
and so that requires a greater input of
energy to do that
as a result there's a higher
energy
barrier or a higher activation energy
that we have to overcome to break those
two bonds
so that's why
the sn1 reaction is under kinetic
control
it can occur faster at lower
temperatures because you only need to
break one Bond
for the E1 reaction we need to break two
bonds so we need more energy and that
extra energy is available at higher
temperatures
so now let's review what we've learned
in summary
the reason why an increase in
temperature favors the elimination
reaction
over the substitution reaction has to do
with number one thermodynamics the
elimination reaction is more
energetically favorable
due to an increase in entropy and an
increase in temperature enhances the
entropy effect
causing the reaction to become more
spontaneous at higher temperatures
so that's one reason why an increase in
temperature
favors the elimination reaction is
because
the reaction becomes more spontaneous
due to the entropy effect
as we increase the temperature
the second reason has to do with
kinetics
elimination reactions
have a higher activation energy than
substitution reactions
and as this particular example
demonstrated
we needed to break two bonds to get the
elimination reaction going as opposed to
breaking one bond for the S1 reaction
and breaking two bonds requires a
greater input in energy than break in
one Bond
so bond breaking that's a endothermic
process
so those are the two main reasons why an
increase in temperature favors
elimination over substitution
the entropy effect
which makes the reaction more
spontaneous at higher temperatures
and the fact that elimination reactions
have a higher activation energy due to
the fact that we have to break more
Bonds in it
so that's it for this video for those of
you who want more
example problems on sn1 sn2 E1 E2
reactions
feel free to check the links
in the description section below
especially if you want to take a
practice test as well

[marcochimio]

Excellent breakdown (no pun intended) of the Sn1 vs. E1 competition. Thank you for this wonderful refresher.