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.