Friday, May 6, 2011

Dehydration of An Alcohol: Cyclohexanol and Cyclohexene

 

Objectives:

1. To produce cyclohexene through the acid catalyzed elimination of water from cyclohexanol

2. To understand mechanism involved in the reaction

3. To learn the technique of distillation

Introduction:

Dehydration is defined as a process of removing water from a substance. The loss of water from a molecule is called dehydration which is exactly opposite with the process of hydrolysis. Dehydration is an elimination reaction of an alcohol involves the loss of an OH from one carbon and an H from an adjacent carbon. Overall, this amounts to the elimination of a molecule of water, resulting in a pi-bond formation of an alkene or alkyne. In most of the dehydration of alcohol, heat and catalyze are needed in the reaction. Sulphuric acid (H2SO4) and phosphoric acid (H3PO4) are the most commonly used acid catalysts.

The dehydration process can be carried out by in two ways. The first way is heating a mixture of alcohol and dehydrating agent in a distilling flask and collecting the olefin (also known as alkene) from the mixture. The second way is passing the alcohol vapour through a heated tube packed with the dehydrating agent at 350°C. The collecting the olefin as it emerges from the tube. The chemical equation for dehydration of alcohol to form alkene and water is shown as below diagram:

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For the dehydration of alcohol, the alkene is formed in the reaction. At the same time, the side products are produced from the reaction such as dicyclohexyl ether, polymer, mono and dicyclohexyl sulphate abd degradation products (carbon and carbon dioxide). In many cases, the phosphoric acid is used in the dehydration of alcohol instead of using sulphuric acid due to two reasons. The one of the reason is the lost of organic compound can be minimize through oxidation of phosphoric acid. In addition, the product is being produced without contamination with volatile decomposition products such as sulphurous acid.

Apparatus: Round-bottomed flask(50ml), take off distillation adapter, condenser, thermometer, electric flask heater

Materials: boiling chips, cyclohexanol, cyclohexene, 85% conc. Phosphoric acid, anhydrous magnesium sulpahte

Procedure:

1. Cyclohexanol and conc.(85%) phosphoric acid are added in a round bottomed flask and is mixed together by swirling.

2. Several boiling chips are added, the flask is clamped to a ring stand at electric flask heater height attached with a take off distillation adapter, a thermometer, a condenser and a small receiving flask as shown in the diagram below.

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3. The mixture is heated and distillate boiling in the range 85°C-90°Cis obtained.

4. When the distillate is exhausted, the heat is increased gradually. Using the same receiver, the distillate boiling in the range of 90°C - 100°C is collected.

5. The two layers in the receiving flask are tested by adding the distilled water. With the aid of a 9-disposable pipette, the aqueous layer is drawn off and is being discarded.

6. The organic layer is dried up with anhydrous magnesium sulphate.

7. The drying agent is removed by filtering the mixture through a cotton wool plug wedged into the constricted part of a small funnel.

8. The filtrate is collected round bottom flask or small distilling flask. Boiling chip is added to the dried product and distil it through a take off distillation adapter packed with a few small wads of coarse steel wool.

9. The product boiling is collected in the range 3 below and 2 above the boiling point of cyclohexene (83°C) in a tarred bottle.

10. The yield in grams is calculated and the product is submitted to instructor.

Result and calculation:

Weight of cyclohexanol = 10.0060g

Weight of dry conical flask = 51.2460g

Weight of dry conical flask + weight of cyclohexene = 56.2281g

Experimental weight of cyclohexene = 4.9821g

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Number of mole of C6H11OH =

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= 0.09996 mole

1 mole of C6H11OH produces 1 mole of C6H12

0.09996 mole of C6H11OH produces 0.09996 mole of C6H12

Theoretical weight of C6H12 = 0.09996 mole X 84.096 g/mol

= 8.4066g

Yield percentage =

 clip_image012

= 59.26%

Discussion:

In this experiment, the cyclohexanol solution is being used in the dehydration process. The cyclohexanol is a six carbon aromatic hydrocarbon which one of the hydrogen atoms, H is substituted by one hydroxyl group, OH-. Due to the low melting point, the cyclohexanol appear in liquid form at room temperature. The dehydration process of alcohol will convert cyclohexanol which the hydroxyl group, OH- will be removed to become cyclohexene. Cyclohexene is a six carbon aromatic hydrocarbon with a single double bond in the molecule.

The solution is added with concentrated phosphoric acid in a round bottomed flask and is mixed together by swirling. The phosphoric acid is added as catalyze as such increase the rate of reaction in dehydration without affects the particular chemical reaction. The boiling chips are added into the solution in order to prevent over boiling of the solution. The boiling chips are small, insoluble, and porous stones made of calcium carbonate or silicon carbide. There are a lot of pores inside the boiling chips which provide cavities both to trap air and to provide spaces to allow bubbles of solvent can be form. When boiling chips are heated, it will release tiny bubbles which can prevent boiling over. Boiling over of solvent will cause lost of solution which may lead to inaccurate result to be obtained.

The heating of mixture is carried out in a fractional distillation apparatus. As the mixture is heated, the alkene and water are produced as the products in the reaction. Besides, the side product and impurities of the reaction will be produced at the same time. The temperature of mixture when heating is fluctuated. During the temperature 80°C, the temperature of mixture drops suddenly by 2°C and the temperature remained at 78°C constantly for few seconds. This is because some impurities or side products are being produced in the mixture which may have the boiling point of 78°C. The temperature of mixture drops suddenly because the heat being is absorbed which used to break down the bonding of the side products. When the boiling point is reached, then temperature of mixture remains constant as the state of the side products are converted from liquid form to gaseous form. The temperature of mixture is increases until 107°C after all the side products are being converted.

The temperature of mixture is reached to maximum at 107°C. This temperature is known as the activation temperature which the cyclohexanol start to be dehydrated. The temperature of mixture drops to 83°C and remains constant. This phenomenon takes place because the cyclohexene with lowest boiling point will tends to be distilled first before the higher one. The temperature remains unchanged because the heat is being absorbed to break down the bond between cyclohexene molecules. In all the distillation process, some of the product will be lost since it is hold up in the apparatus which reduce the product yield. In order to maximize the yield, the mixture is continued to be heated at higher temperature range which more than the boiling point of cyclohexene. When the mixture is heated at 90°C-100°C, the water in the mixture will push over the products into the receiving flask along the condenser. The products produced are collected in the same receiving flask.

Then, the receiving flask containing cyclohexene, water and small amount of the impurities. Two layers liquid are present in the receiving flask, one drop of distilled water is added into flask in order to determine the location of aqueous layer. Since the water droplet mix with the lower layer, so the upper layer is determined as cloudy solution while the lower layer is aqueous layer. The upper cloudy layer is cyclohexene with some impurities and water inside. The lower aqueous layer is removed and discarded. But, that is not easy to remove all the water in the receiving flask. So, anhydrous magnesium sulphate is being added. The purpose of adding of anhydrous magnesium sulphate is used to remove residual water in the organic solvent. The magnesium sulphate in the granular form will be preferable. It is known as drying agent in the organic solvent which are not dissolves in the solvent but drying the solvent. The magnesium sulpahte clump together with the water droplets as it solidified them. In another words, it is reacts with water to form hydrates which is their preferred form when water is available. An excess drying agent should be used to ensure that all the water in solvent is removed. If the water remains in the materials collected, it could interfere with the analysis.

Water has been successfully removed from the organic compound mixture, so it is very important to not reintroduce water into the mixture. The final distillation of unpurified cyclohexene must be done very carefully in order to obtain purified products. The temperature of mixture is heated until approximate 85.5°C since this temperature is the boiling point of cyclohexene and hence the pure cyclohexene could be obtained rapidly. The weight of the cyclohenexe is 4.23g.

Error of source includes the condenser and Liebig tubes are not rinsed with a little of distilled water, a little of ethanol and acetone to speed up drying process. So that, the water in the desired products would not presents. This is because some of the water droplets is hold up and stick on the wall of condenser and the second distillation will produce the contaminated cyclohexene.

 

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