Post on 18-Jul-2016
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Kinetic Versus Thermodynamic Control in Chemical Reactions
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Chemistry Laboratory
Instructor
Date
Abstract
The point of this experiment was to figure out which chemical reaction was kinetic and which
one was thermodynamic. After yielding different products from each test kinetic control was
proven to create cyclohexanone semicarbasone and be able to reverse the formation of the
product. Thermodynamic control was proven to create 2-furaldehyde semicarbazone and was not
able to be reversed.
Introduction
Thermodynamic and kinetic control are important to tell the differences in the chemical
reactions. Kinetic control is where the products have a low transition state between reactions.
Low temperatures make it easier for the reaction to overcome activation barriers which forms the
kinetic product faster. Thermodynamic control creates more stable products because the energy
level is the lowest between reactions. At high temperatures the product is dominant because it
reaches a higher transition state which makes it easier to reach the activation barrier (Figure 1).
Figure 1- Kinetic versus Thermodynamic Table
Discussion
In the experiment, first Cyclohexanone semicarbazone was made by combining semicarbazide
and cyclohexanone (Scheme 1) . After drying for a week, weighing the product, and finding the
melting point, the resulted melting point range for cyclohexanone semicarbazide was 167°C-
174.3°C (Table 1). Compared to the actual melting point, 166°C, the melting point was only 1°C higher
than the actual. After calculating the % yield, the amount of cyclohexanone semicarbazide that was
produced was 16.7% from our starting material. 2-furaldehyde semicarbazone was then made for the
second step of the experiment by combining semicarbazide and 2-furaldehyde (Scheme 2). The melting
point of this product should have been 202°C. After creating and drying the product and measuring the
melting point range the final temperature was 203.8°C- 207.5°C(Table 1). Then by weighing the product
formed the % yield was then calculated to get a 68% yield.
Scheme 1- Creation of cyclohexanone semicarbazone
Scheme 2- Creation of 2-furaldehyde semicarbazone
After generating the first two products, then the next step started. The point of this procedure was to
create a competition study to see each reaction with semicarbazide at 0°C and 2-furaldahyde at 80°C.
Equal portions of semicarbazide hydrochloride, 2-furaldehyde, and cyclohexanone, were made into
aqueous solutions and then were divided in half to create different products. One portion was cooled in an
ice bath for 5 minutes and then dull, yellow crystals were collected. After collecting and drying these
crystals they were then used to find the melting point range. For the Product 2 the melting point range
was a low number of 88.1°C- 91.5°C (Table 1). Unable to find out why it was that low the next step of
the experiment was then started. The second portion that was used in the next step of the experiment was
heated at 80°C in a hot water bath for 15 minutes. After collecting and drying the shiny, gold crystals they
were then used to find the melting point range of 201.8°C-204.2°C(Table 1). After completing the
test semicarbazide was kinetic and 2-furaldehyde was thermodynamic (Scheme 3&4).
Scheme 3- Formation of cyclohexanone semicarbazide by Kinetic Control
Scheme 4- Formation of 2-furaldehyde semicarbazide by Thermodynamic Control
Testing the reversibility of the semicarbazone formation was the next step. The kinetic control
should reverse whereas the thermodynamic control should not. Taking cyclohexanone
semicarbazide and 2-furaldehyde and having created a mixture, it was then warmed to 80°C for 5
minutes. After cooling the solution to room temperature and collecting the dull, yellow crystals the
melting point was then measured, making the range of 197.1°C-200.1°C (Table 1). 2-furaldhyde
semicarbazone and cyclohexanone were then put into a solution and heated at 80°C for five
minutes. After the product had formed it was then dried. The melting point for the last set of shiny,
yellow crystals was 201.9°C-203.9°C (Table 1). The test was confirmed from the data
obtained(Scheme 5&6).
Scheme 5- Test of Reversibility from cyclohexanone semicarbazide
Scheme 6- Test of Reversibility from 2-furaldehyde semicarbazone
Table 1- Melting Point Ranges of Products
Experiment/Product Number Measured M.P. °C Products
A 167°-174.3° Cyclohexanone semicarbazide
B 203.8°-207.5° 2-furaldehyde semicarbazide
Product 1 88.1°-91.5° ------
Product 2 201.8°-204.2° ------
Product 3 197.1°-200.1° ------
Product 4 201.9°-203.9° ------
Conclusion
In the experiment thermodynamic and kinetic control were to be proven.. After yielding different
products from each test kinetic control was proven to create cyclohexanone semicarbazone and
to reverse it. Thermodynamic control was proven to create 2-furaldehyde semicarbazone and was
not reversible.
Experimental
This experiment was Kinetic versus Thermodynamic Control in Chemical Reactions1. In the
experiment semicarbazide hydrochloride, dibasic potassium phosphate, cyclohexanone, 95%
ethanol, 2-furaldehyde, and water were used for the reactions. The equipment used for the
experiments was Erlenmeyer flasks, test tubes, vacuum filtration system, hot plate, thermometer,
melting point reader, and a scale.
1 Landgrede, J.A., Theory and Practice in the Organic Laboratory with Microscale and Standard Scale Experiments, Thompson Wadsworth: United States, 2005, 414.
Part A- Preparation of cyclohexanone semicarbazone
In an Erlenmeyer flask 0.5 g of semicarbazide hydrochloride, 1.3 g dibasic potassium phosphate,
and 6 mL of water were all dissolved. In a test tube, 0.5 mL of cyclohexanone and 2.5 mL of
95% ethanol were mixed and then poured and swirled into the semicarbazide solution and then
allowed 10 minutes for crystallization. After crystallization the crystals were then filtered and
dried for a week. The melting point range was 167°C-174.3°C and the % yield was 16.7%
(Table 1).
Part B- Preparation of 2-furaldehyde semicarbazone
In an Erlenmeyer flask 0.5 g of 2-furaldehyde, 1.3 g dibasic potassium phosphate, and 6 mL of
water were dissolved. In a test tube, 0.5 mL of cyclohexanone and 2.5 mL of 95% ethanol were
mixed and then poured and swirled into the 2-furaldehyde solution and then allowed 10 minutes
for crystallization. After crystallization the crystals were then filtered and dried for a week. The
melting point range was 203.8°-207.5° and the % yield was 68.0% (Table 1).
Part C- Reaction of semicarbazide with a mixture of cyclohexanone and 2-furaldehyde
In 50 mL of water 2.0 g of semicarbazide hydrochloride and 5.2 g of dibasic potassium
phosphate were dissolved. A solution of 2.0 mL of cyclohexanone and 1.6 mL of 2-furaldehyde
in 10 mL of 95% ethanol was prepared. It was then divided into two portions. One portion of
each aqueous and ethanolic solution were cooled in an ice bath. After cooling, the solutions were
mixed and stood in an ice bath for 5 minutes. Crystals were collected by vacuum filtration and
washed with cold water. The crystals created were dull and yellow and yielded semicarbazone
cycloehexanone by kinetic control. The melting point range was 88.1°C-91.5°C, a little lower
than expected(Table 1).
The remaining portions of aqueous and ethanolic solution were heated to 80-85°C and mixed
together in an Erlenmeyer flask. They were then heated for 15 minutes and then cooled to room
temperature and placed in ice. Crystals were then collected by vacuum filtration and washed in
cold water. The crystals were shiny and yellow and were then used to find the melting point
range of 201.8°C-204.2°C (Table 1). 2-furaldehyde cyclohexanone was created by
thermodynamic control.
Part D- Test of reversibility of semicarbazone formation
In a Erlenmeyer flask 0.3 g of cyclohexanone semicarbazone, 0.3 mL of cyclohexanone, 2 mL of
95% ethanol, and 10 mL of water were placed and then heated to 80°-85° C for 5 minutes. The
solution was cooled to room temperature and then isolated the crystals by suction filtration and
dried. The melting point range for this dull, yellow product was 197.1°C-200.1°C. The
cyclohexanone semicarbazone kinetic product was reversed.
In a Erlenmeyer flask 0.3 g of 2-furaldehyde semicarbazone, 0.3 mL of cyclohexanone, 2 mL of
95% ethanol, and 10 mL of water were placed and then heated to 80°-85° C for 5 minutes. The
solution was cooled to room temperature and then isolated the crystals by suction filtration and
dried. The melting point range for the shiny, yellow product was 201.9°C-203.9°C. The 2-
furaldehyde thermodynamic product was not reversed.