Kinetic versus Thermodynamic Control in Competing
Reactions
Introduction:
The purpose of this experiment is to test kinetic theory by running several reactions and
determine which product are formed under kinetic conditions and which products are formed
under thermodynamic conditions.
Kinetically controlled products are those which have the lowest transition states
between competitive reactions; this would be represented by TS1 in the above diagram. At low
temperatures, this makes it easier for the reaction to overcome the activation barrier ensuring
that the kinetic product is formed faster.
Thermodynamically controlled products are those which have a more stable product (P2)
due to the product energy level being lowest between competitive reactions. Therefore at high
temperatures, the thermodynamic product is dominant because it is possible to reach the higher
transition state. It is in essence easier to reach the activation energy barrier.
In Part A of the experiment, two reactions are carried out to synthesize semicarbazones
to be used in Part C of the experiment.
The first reaction being the preparation of cyclohexanone semicarbazone;
The second being the preparation of furfural semicarbazone;
These reactions will be further analyzed under thermo conditions and kinetic conditions
in part B of this experiment.
To test this theory experimentally, in Part B of the experiment, two competing reactions
are carried out;
Cyclohexanone, furfural and semicarbazide are reacted, initially without heat and then
again this reaction is carried out with heat. The significance of this process is to determine
which product is formed under kinetic conditions (forms faster at low temperatures) and which
product is formed under thermodynamic conditions (forms slower but is the major product at
high temperatures).
Experimental:
The purpose of Part A of the experiment is to furfural and cyclohexanone
semicarbazones for part C of the experiment.
50 mmol of semicarbazide hydrochloride along with 10.0g of sodium bicarbonate
are mixed in 125 ml of deionized water in an Erlenmeyer flask. Once effervescence ceases,
50 mmol of cyclohexanone is added and swirled in the flask. Once a precipitate forms, the
mixture is then vacuum filtered and the product isolated. The product is then recrystallized from
deionized water. The melting and yield is then recorded
This procedure is then repeated for preparation of the furfural semicarbazone (50 mmol
of furfuraldehyde is used instead of 50 mmol of cyclohexanone).
Part B of the experiment is to truly demonstrate the fundamental theory behind
competitive reactions and thermo vs kinetic control.
10 mmol of semicarbazide hydrochloride is mixed with 2.0g of sodium bicarbonate
and 25 ml of deionized water in a 125 ml Erlenmeyer flask. Once effervescence has ceased,
10 mmol cyclohexanone and 10 mmol of furfuraldehyde are added to the solution. Once all
reactants are properly mixed and a precipitate forms, the mixture is then vacuum filtered to
isolate a product. This product is the recrystallized from deionized water. The product is then
verified by measuring the melting point and mixed melting point.
This procedure is then repeated under heating for about 1 hour.
Part C of the experiment is to verify kinetic vs thermo control of part B through interconversion
reactions.
10 mmol of cyclohexanone semicarbazone (obtained from Part A) is mixed with 10
mmol of furfuraldehyde in 20 ml of deionized water in a 125 ml Erlenmeyer flask. This mixture
is placed in a heat bath for roughly 1 hour. The mixture is then cooled to room temperature and
the precipitate is then vacuum filtered and recrystallized from deionized water. The product is
then identified using melting point and mixed melting point.
This procedure is then repeated using 10 mmol of furfuraldehyde semicarbazone also
obtain from Part A and cyclohexanone instead of 10 mmol of cyclohexanone semicarbazone
and furfuraldehyde.
Results:
Competitive Reaction
Yield A-1
Theoretical:7.76g
Actual: 6.047g
Yield A-2
Theoretical: 7.66g
Actual : 5.254g
Discussion:
The purpose of this experiment was to test the theory of kinetic vs thermo control by
carrying out reactions under specific conditions to yield specific products.
Part A of the experiment consisted exclusively of preparing the semicarbazones to
be used for the experiment. In A-1, 5.794 g of semicarbazide hydrochloride was mixed with
9.990 g of sodium bicarbonate in 125 ml of deionized water. It is important to note that the
semicarbazide is chlorinated because the amino group would otherwise be easily protonated,
so it is easier to handle when chlorinated. Upon addition of water, an intense amount of
effervescence was observed. Once effervescence has ceased, 5.185 ml of cyclohexanone
was added to the solution. A foamy solution followed after the addition of cyclohexanone and
the mixture was then vacuum filtered. Once isolated, the product is then recrystallized using
deionized water to remove further impurities and a melting point range of (168-170) °C was
recorded. The literature value for the melting point of cyclohexanone semicarbazone is 166°C
which would indicate that the product obtained is in fact cyclohexanone semicarbazone. The
reason for which the melting point of the actual product obtained is slightly higher is due to trace
amounts of impurities that were not filtered out or still remained in the reaction flask during and
after recrystallization.
Part A-2 followed in a similar procedure to A-1 however, instead of using cyclohexanone,
4.147 ml of furfuraldehyde was added along with 10.0 g of sodium bicarbonate, 5.765 g of
semicarbazide hydrochloride and 125 ml of deionized water. This mixture however did not show
effervescence as in the first part and the solution displayed a yellow colour. Once filtered and
recrystallized, a melting point of 199.2°C was recorded. The literature value for the melting point
of furfural semicarbazone is 202 which would indicate that the product obtained is in fact furfural
semicarbazone. It is speculated that the reason for the melting point being slightly lower than
literature value is due to trace amounts of impurities in the final product.
Part B of the experiment exclusively deals with competitive reactions where the
formations of different semicarbazones are attained through different reaction conditions.
Altering reaction conditions can give rise to specific products predominating given the specific
reaction condition.
The first part of B was performed under conditions with no heat. 1.115 g of
semicarbazide hydrochloride was mixed with 2.0g of sodium bicarbonate in 25 ml of water. Not
much effervescence was observed and the solution remained clear. 1.036 ml of Cyclohexanone
was then added along with 0.829 ml of furfuraldehyde. With the addition of these two reagents,
the competitive reaction then begins. One of two products can predominate; cyclohexanone
semicarbazone or furfural semicarbazone. Since the reaction condition is at low temperature,
whichever product forms the fastest and is the major product will in fact be the kinetic product.
Once the reaction was complete and the product was isolated through vacuum filtration and
recrystallized from deionized water. A melting point range of (156-158) °C was recorded which
is closest to the literature value of cyclohexanone semicarbazone (166°C) which indicates that
this semicarbazone is the product.
The second part of B is performed in much the same way however, the reaction
conditions are different and heat is introduced. 1.117 g of semicarbazide was mixed with
1.998g of sodium bicarbonate in 25 ml of deionized water. Again there was little effervescence,
then 1.036 ml of cyclohexanone and 0.829 ml of furfuraldehyde were added and the reaction
commenced under heating for approximately 1 hour. Note that heating for this period of time
is to ensure that a specific product predominates. Since the variable of heat was introduced,
reaching the higher transition state is easier to accomplish (as in TS2) and the thermodynamic
product should predominate;
Once the reaction had completed and the product had been isolated through vacuum
filtration and recrystallized from deionized water, a melting point range of (196-198°C) was
recorded. This value is approximately the literature value of furfural semicarbazone (202°C)
which indicates that this particular semicarbazone is the most stable product, is favoured at high
temperatures, and is the thermodynamic product.
Part C of the experiment is done to further prove the results obtained in Part B of the
experiment through interconversion reactions.
In C-1, 1.025 g of cyclohexanone semicarbazone (kinetic product) was used along
with 0.829 ml of furfuraldehyde in 20 ml of deionized water. This mixture was then heated
for roughly 1 hour, isolated through vacuum filtration and then recrystallized from deionized
water. A melting point range of (186-189) °C which is close the literature value of furfural
semicarbazone implies that the kinetic product interconverts to the thermo product (the most
stable) under heating and in the presence of furfuraldehyde. The lower melting point range
obtained is due to impurities present in the product.
In C-2, 1.254 g of the thermo product was used along with 1.036 ml of cyclohexanone
and 20 ml of deionized water. As in C-1, the mixture was then heated for roughly 1 hour,
isolated through vacuum filtration and recrystallized from deionized water. A melting point range
of (
) was observed which implies that again, the thermo product (furfural semicarbazone)
predominates under these conditions involving higher temperatures.
Conclusion:
In conclusion, cyclohexanone forms faster under kinetic conditions (lower temperatures)
due to a lower transition state than that of furfural semicarbazone. Furfural semicarbazone is the
most stable product and is favoured at high temperatures, it is the thermodynamic product.
Things For Your Consideration
(1) The transition state of product B in figure (i) is higher than that of Product A, therefore
product B cannot be kinetically controlled. Product B is also overall less stable than that of A
due to a higher final product energy level, therefore it neither thermodynamically controlled.
(2) In part B-1, cyclohexanone semicarbazone was formed faster therefore it is kinetically
controlled. In part B-2, furfural semicarbazone was the major product therefore it is
thermodynamically controlled.
In both parts of C, the thermo product predominates (due to heating), which is furfural
semicarbazone.
(3) (a) An increase in the amount of semicarbazide will not alter the data for either product
(cyclohexanone or furfuraldehyde would be limiting).
(b) Doubling the volumes of the aldehyde or ketone will not affect the yield (semicarbazide
would be limiting)