Preparation Of 4-Methylcyclohexene

ORGANIC CHEMISTRY

OBJECTIVE

1) To learn the distillation technique

2) To understand the mechanism involved in the reaction

3) To produce 4-methylcyclohexene through the acid-catalyzed elimination of water from 4-methylcyclohexanol

INTRODUCTION

In this experiment, 4-methylcyclohexanol undergoes acid-catalyzed dehydration to give 4-methylcyclohexene. The product is distilled from the reaction flask along with the water generated. The distillate is washed with salt solution, dried and distilled.

The hydration of alcohols using strong mineral acids, such as sulfuric acid or phosphoric acids, as a catalyst is a general laboratory method for preparing alkenes. The reaction is conducted in a distillation apparatus. As the reaction mixture is heated, the lower boiling products (cyclohexene with boiling point= 83^oC and water boiling point= 100^oC distilled out and are collected out in the receiving flask. Any unreacted cyclohexanol (the starting material) and phosphoric acid (the catalyst) are left in the distilling flask because of their high boiling points (161^oC and 213^oC). However, a small amount of phosphoric acid still appears in the receiving flask, the product is washed with aqueous sodium carbonate to neutralize the acid. As we know, cyclohexene is insoluble in water and it is not lost during the crude cyclohexene by drying the liquid over anhydrous sodium sulfate. Sodium sulfate is a salt which forms a hydrate. Final purification is achieved by simple distillation.

This week experiment, 4-methylcyclohexanol undergoes acid-catalyzed dehydration to give 4-methylcyclohexene. The product is distilled from the reaction flask along with water generated. Then, the distillate is washed with salt solution, dried and distilled.

MATERIALS

7.5 mL of 4-methylcyclohexanol (molecular weight=114.2; boiling point= 171-173^oC), 2.0 mL of 85% phosphoric acid, 30 drops (0.4 mL) of concentrated sulfuric acid, saturated sodium chloride solution, granular anhydrous sodium sulfate, bromine test reagent, potassium permanganate test reagent

APPARATUS

50 mL round bottom flask, 25 mL round bottom flask, Erlenmeyer flask, glass stirring rod, Pasteur pipets, boiling stones, distillation apparatus, ice-water bath, heating mantle, weighing balance

PROCEDURE

Safety Consideration

Phosphoric and sulfuric acids are very corrosive. We are not allowed to touch our skin. The experiment have strong odors because have many of compound so that we was used caution when smelling them. In addition, the sample was kept away from flames unless testing small amounts for behavior when heated. We were reminded all the times to place

all discarded in the waste bottle in fume hood.

· Apparatus assembly

The apparatus for simple distillation was set up first. 50 mL round bottom flask was used as distilling flask and a 25 mL round bottom flask as a receiver. An ice bath was placed around the receiver to reduce the loss of product by evaporation. An empty 50 mL round-bottom flask was weighed. 7.5 mL of 4-methylcyclohexanol was placed in that round bottom-bottom flask using graduated pipets.

· Dehydration

The distillation temperature was maintained below 100^oC. We was continued until only 0.5-0.6 mL of the residue remains in the distillation flask or the mixture begins to smoking or bumping.

· Isolation and drying of the product

The distillate was transferred to a separatory funnel with the aid of 3ml saturated sodium chloride solution. (We don’t have to use centrifuge tube and Pasteur pipette)

· Distillation

Drying agent was prevented before starting with distillation process. The 25ml round-bottom flask was used as a receiver that should be pre-weighed. The temperature range which is most of the product distills is the boiling point of our 4-methylcyclohexene.

· Spectroscopy

We are reminded by our supervisor to not do this part

· Unsaturation test

This test must be done and our result was recorded and explained in discussion

DATA/ RESULT:

Weight of empty round-bottom flask= 46.3255 g

Weight of empty round-bottom flask + 4-methylcyclehexanol= 56.2991 g

Weight of 4-methylcyclohexanol= 47.2991 g - 46.3255 g = 9.9736 g

Weight of empty Erlenmeyer flask= 51.1172 g

Weight of empty Erlenmeyer flask= 53.8272 g

Weight of 4-methylcyclohexene= 2.71 g

Melting point range for experimental= 174^oC

Melting point range for literature= 102^OC

Percent Yield = % ?

From The Reaction,

1 mol of 4-methylcyclohexanol = 1 mol of 4-methylcyclohexene

Molecular Mass 4-methylcyclohexanol= 114.2 g mol^-1

Molecular Mass 4-methylcyclohexene= 96.2 g mol^-1

n(mol) of 4-methylcyclohexanol l= 9.9736 g/ 114.2 g mol^-1= 0.0873 mol

Thus, 0.0873 mol of 4-methylcyclohexanol was produced.

Mass of cyclohexene = 0.0873 mol X 96.2 g mol^-1 = 8.39826 g

= 8.3983 g

Percent Yield(%) = (Experimental Yield/ Theorertical Yield) x 100

= 2.71 g x 100 = 32.2684 %

8.3983 g

DISCUSSION

Elimination reactions involve the loss of a small molecule (H-X) from adjacent carbon atoms, resulting in pi-bond formation. Elimination reactions are good synthetic methods for producing alkene. These reactions occur through a process called heterolytic bond cleavage. Heterolytic bond cleavage occurs when one atom leaves a compound with both electrons of the original bond, resulting in the formation of ions. The leaving group departs with both electrons from the original C-X bond. The electrons in the adjacent C-H bond form the new pi bond of the alkene, with the loss of the proton.

The elimination of water (H-OH) from 4-methylcyclohexanol in this experiment is called a dehydration reaction. In many cases, alcohol dehydration is an acid-catalyzed reaction that proceeds by an elimination mechanism.

The mechanism is a cyclohexyl cation, which can undergo substitution as well as elimination. In this experiment, the substitution reaction are the use of strong acids with anions that are relatively poor nucleophiles and a high reaction temperature, which favors elimination. The anion of phosphoric acids in this experiment is a poor nucleophile, while substitution reactions are not favored. The first step of dehydration is a proton transfer from the acid catalyst to the oxygen atom of the alcohol. This protonation forms a oxonium ion, the conjugate acid of the alcohol. Weak base are good leaving groups, so changing the leaving group from hydroxide to water favours the reaction. The second step of the dehydration reaction is loss of water from the oxonium ion forming a positively charged secondary carbocation. This step of the mechanism is rate determining. The third and final step, a molecule of water deprotonates the carbocation at either of the adjacent carbons. The remaining electrons flow towards the positive charge producing a sigma–bond between the carbons and forming a double bond.

Precaution steps that must we cared are phosphoric acids and 4-methylcyclohexene. Phosphoric acids are strong, corrosive acids. If any acid is splashed on your skin or clothing, wash immediately with copious amounts of water. 4-methylcyclohexene are not particularly dangerous but are highly flammable. Both are quite painful if splashed in the eyes and must be removed by extensive eye washing. Last but not least, remaining 4-methylcyclohexene should be disposed of in the fume-hood sink because 4-methylcyclohexene vapors are heavier than air where they will accumulate in the sink.

CONCLUSION

32.2684 % percent of theoretical yield was calculated, which 2.71 g of 4-methylcyclohexene was produced experimentally. We has produce 4-methylcyclohexene through the acid-catalyzed elimination of water well. And mechanism that involved in the reaction was understood in mind.

REFERENCE

1) Martin S. Silberberg, Patricia G. Amateis, “CHEMISTRY: THE MOLECULAR NATURE OF MATTER AND CHANGE “, 7^th edition, McGraw-Hill Education, Inc. N.Y 2015, page 617-658

QUESTIONS

1) The preparation of cyclohexene from cyclohexanol, what is basic type of reaction?

= Elimination Reaction

2) Why is the receiving flask supposed to be kept on ice during the preparation of cyclohexene?

=In order to lower the temperature and condense it. Furthermore, it can minimize losses because the product is semisolid material at room temperature and solidifies completely when kept in a refrigerator.

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