Tip:
Highlight text to annotate it
X
Throughout this lesson,
we've seen that energy differences
are the key to understanding reactivity.
For instance, we noted that faster reactions occur
when there is a small energy difference
between the starting materials and transition state.
Thus, on the reaction coordinate diagram
we're looking at here,
we would expect the left-hand reaction to P1
to be favored kinetically.
That's because the ΔG‡, or the activation energy
to get past transition state 1
is smaller than the activation energy required
to get past transition state 2,
and let's label these ΔG‡1 and ΔG‡2.
We also noted that thermodynamically
more stable products
tend to be formed in greater amounts.
Looking again at P1 and P2, we would expect that P2
is the thermodynamically favored product
because the ΔGo
from the starting materials to P2
is more negative than the ΔGo
from the starting materials to P1.
And let's again label these ΔGo1 and ΔGo2.
The problem here is that one product is favored kinetically
and the other product is favored thermodynamically.
You should immediately have the question,
how do we know which factor, kinetics or thermodynamics
is controlling the reaction?
And the answer is
it's a function of the reaction conditions,
which is nice for us
because we can bias the reaction conditions
to favor either the kinetic or thermodynamic products
if they're different.
In this case, they are different,
as I've labeled on the reaction coordinate diagram,
and the way the distribution of products
is most often controlled
is to mess with either reaction temperature
or reaction time.
As you might expect,
the kinetic product is favored for short reaction times.
This is because at short reaction times,
more molecules of starting material will tend to follow
the path to the lower energy transition state.
Stopping the reaction early
gives very few molecules that have reacted to become P1
the chance to get back over TS1
and convert all the way over
to the more thermodynamically stable P2.
Alternatively, low temperatures
will favor the kinetic product
because low temperatures offer little energy to P1
to return over TS1 and eventually convert to P2.
So the trick to accessing the kinetic product
is to use either short times,
in which case we trap out the kinetic product,
or low temperatures,
in which case we trap the product
by providing very little energy for P1 to get back over TS1.
In other words, we make the reverse reaction
from P1 back to the starting material
essentially negligible.
To favor thermodynamic products,
we do the opposite.
So at long times
or at high temperatures,
we favor the thermodynamic product.
And the ideas are similar here.
At high temperatures,
we're providing a great deal of energy
to all of the stable species here.
So P1, the starting materials, and P2
all have a great deal of energy available to them.
As a result, molecules will very easily surmount
the transition states in either direction.
Thus, starting material molecules that have become P1
are able to revert to the starting material
and eventually surmount TS2 to access
the more thermodynamically stable product, P2.
This overall equilibration over both transition states
lead ultimately moreso
to the more thermodynamically stable product,
given enough time.
A classic example of a kinetic versus
thermodynamic control situation
is shown in the MarvinSketch window, here.
The hydrobromination of dienes
can actually form two different products
depending on the relative positions
of the added H and bromine atoms.
If the H and the bromine add in a [1,2] fashion,
we form the product you can see here.
If the H and the bromine instead add in a [1,4] fashion,
then we form the product shown here.
This is an issue of thermodynamic
versus kinetic control
because the top product is favored kinetically.
After protonation by HBr,
the bromide anion has to travel
very little distance to reach the cationic carbon
at this position and bond with it.
This is why this is the kinetically favored product.
However, we can see that the bottom product
is thermodynamically favored
because the double bond in it is more stable
than the terminal double bond of the top product.
Being more thermodynamically stable,
we would expect the bottom product to be the one favored
thermodynamically.
Thus, at low temperatures,
we would expect to favor the product on top,
that's the kinetically favored product.
And at high temperatures,
we would expect to favor the more thermodynamically stable
[1,4] hydrobromo compound.
If you come up against a situation
where different reaction conditions,
particularly temperature and time,
lead to different products,
you know you're dealing with this situation
of kinetic versus thermodynamic control.
Keep in mind that at low temperatures
and short reaction times, kinetic control is operative,
while at high temperatures and long reaction times,
thermodynamic control is operative.