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Tuesday, 20 November 2012

Title: Dehydration Of An Alcohol: Cyclohexene From Cyclohexanol

Title: Dehydration Of An Alcohol: Cyclohexene From Cyclohexanol Objective: To produce cyclohexene through the acid catalyzed elimination of water from cyclohexanol. To understand mechanism involved in the reaction. To learn the technique of distillation. Introduction: A secondary alcohol, such as cyclohexanol, undergoes dehydration by an E1 mechanism. The key intermediate in the mechanism is a cyclohexyl cation, which can undergo substitution as well as elimination. To prepare a cyclohexene (olefin) in good yield, it is necessary to suppress the substitution reaction. In this experiment, the substitution reaction is suppressed by: (1) the use of strong acids with anions that are relatively poor nucleophiles ; (2) a high reaction temperature, which favors elimination; and (3) distillation of cyclohexene from the reaction mixture as it is formed. The dehydration reaction is of paramount importance in the preparation of olefins, which are the raw materials of much of the plastics industry. From the historical point of view it is no less importance, because it has been used time and again in the laboratory in the preparation of important compounds. The first complete synthesis of the alkaloid morphine, for example, involved the use of an olefin intermediate, which was prepared by the dehydration methods. Side Reactions The side products of the dehydration reaction are virtually identical with those encountered in the preparation of n-amyl bromide, the only difference being that the olefin is no longer a side product but is now the desired product. Specifically, the side products are dicyclohexyl ether, polymer, mono and dicyclohexyl sulphate, and degradation products such as carbon, sulphur dioxide and carbon dioxide. The dehydration of cyclohexanol is carried out in such a way that the product, cyclohexene, distils from the reaction mixture as it is formed, the distillation technique serves to remove the olefin from contact with the sulphuric acid before polymerization can set in and it also serves as a first stage in the eventual purification of the olefin. The products and side products fall three categories: (a) gases, composed of sulphur dioxide and carbon dioxide and carbon dioxide, (b) distillate, composed of cyclohexene, un-reacted cyclohexanol, water and traces of sulphurous acid; and (c) residue, composed of high-boiling or non-volatile substances such as dicyclohexyl ether, mono- and dicyclohexyl sulphate, polymer and carbon. Pure cyclohexene is obtained from the crude distillate by the following procedure: Treatment with aqueous sodium carbonate solution to remove sulphurous acid; Addition of calcium chloride, to remove all of the water and part of the cyclohexanol; and Distillation to separate the remainder of the cyclohexanol. The dehydration of an alcohol with phosphoric acid instead of sulphuric acid has two distinct advantages: Very little organic material is lost through oxidation by the acid and The product is not contaminated with volatile decomposition products (e.g. sulphurous acid) Both advantages are attributable to the fact that phosphoric acid, unlike sulphuric acid is not an oxidizing agent. As a result, the yield of olefin is usually higher with phosphoric acid, the workup is simplified, and important from the point of view of the experimenter the labour required to clean the reaction flask is greatly reduced. (Sulphuric acid produces an intractable black tar which adheres tenaciously to the walls of the reaction flask.) Apparatus and Materials: Round-bottomed flask (50 mL), boiling chips, bunsen burner, take-off distillation adapter, condenser, thermometer, cyclohexanol, concentrated (85%) phosphoric acid, anhydrous magnesium sulphate. Experimental Procedure: 10.0 g of cyclohexanol and 2 mL of conc.(85%) phosphoric acid were placed in a 50 mL ST round bottomed flask and the two were mixed by swirling. Several carborundum porcelain or anthracite boiling chips (do not use marble chips) were added, the flask was clamped to a ring stand at Bunsen burner height, and a take-off distillation adapter was attached, a thermometer, a condenser, and a small receiving flask. The reaction mixture was heated so that it boils gently and distillate boiling in the range 85-90 ℃ was obtained. When the distillate was exhausted, the heat was increasing gradually. The same receiver was using; the distillate boiling was collected in the range of 90-100℃. The two liquid layers were tested in the receiving flask to see which the aqueous layer was. With the aid of a 9-in disposable pipette, the aqueous layer was drawn off and discarded the aqueous layer. The organic layer remaining in the receiving flask was dried by adding to it 0.1-0.2g of anhydrous magnesium sulphate. The resulting mixture was swirled for a minute or two, and then the drying agent was removed by filtering a mixture through a cotton wool plug wedged into the constricted part of a small funnel. The filtrate was collected in a 50-mL ST round-bottom flask or a small distilling flask. A boiling chip was added to the dried product and it was distilled through a take-off distillation adapter packed with a few small wads of coarse steel wool. The product boiling in the range 3 below to 2 above the boiling point of cyclohexene(83℃) was collected in a tarred bottle. Results and Calculations Weight of round-bottomed flask + beaker 86.15g Weight of round-bottomed flask + beaker + cyclohexene 96.05g Weight of cyclohexanol started with 9.90 g Weight of conical flask 43.93g Weight of conical flask + cyclohexene 46.38 g Weight of cyclohexene obtained 2.55 g Percent yield: 31.41% From the reaction, 1 mol of cyclohexanol produce 1 mol of cyclohexene. Molecular mass of cyclohexanol is 100 g mol-1. Mole of cyclohexanol = 9.90 g / 100 g mol-1 = 0.099mol Thus, 0.099 mol of cyclohexene was produced. Molecular mass of cyclohexene is 82 g mol-1. Mass of cyclohexene = 0.099 mol X 82 g mol-1 = 8.118g (Theoretical mass) Experimental mass = 2.55g Percentage of yield = experimental yield x 100 % Theoretical yield Percentage of yield = 2.55g x 100 % 8.118g Percentage of yield = 31.41% Question: Dehydration of cyclohexanol gives cyclohexene. Draw mechanism for the reaction. What alkene will be produced when each of the following alcohols is dehydrated? a) t-butyl alcohol CH3 CH3 CH3 – C –OH CH3 – C ═ CH2 + H2O CH3 2-methyl-1-propene b) 3-methylcyclohexanol 80% = 4-methylcyclohexene and 3-methylcyclohexene 20% = 1-methylcyclohexene The dehydration of 3,3-dimethyl-2-butanol yields three different products. Write equations to show how carbonation rearrangements explain two of the products. Elimination step 1 (Secondary carbocation): Product yield is (CH3)3CCH=CH2 (3,3 Dimethyl-1-butene). It is a normal elimination product and the least from the amount. Rearrangement of carbocation: Elimination step 2 (Tertiary carbocation): Product yield is 2,3-dimethyl-2-butene. It is the major product. Elimination step 3 (Tertiary carbocation): Product yield is 2,3-dimethyl-1-butene. It is the minor product. Discussion: Elimination reactions involve the loss of a small molecule (H-X) from adjacent carbon atoms, resulting in pi-bond formation. Consequently, elimination reactions are good synthetic methods for producing alkenes or alkynes. 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. For example, elimination of H-X from an organic molecule involves the loss of a proton (H+) and a leaving group (X-). 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 alcohols in this experiment is called a dehydration reaction. In many cases, alcohol dehydration is an acid-catalyzed reaction that proceeds by an elimination mechanism called E1. The key intermediate in the mechanism is a cyclohexyl cation, which can undergo substitution as well as elimination. To prepare an alkene in good yield, it is necessary to suppress the substitution reaction. In this experiment, the substitution reaction is suppressed by: (1) the use of strong acids with anions that are relatively poor nucleophiles; (2) a high reaction temperature, which favors elimination. The anion of phosphoric acids in this experiment is a poor nucleophile, and thus 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 ease of alcohol dehydration follows the trend 3° > 2° > 1°. 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 –bond between the carbons and forming a double bond. From the experiment, only 2.55g of cyclohexene was produced, which is 31.41 % from the theoretical mass. This is due to a significant amount of product left and lost during distillation. Since the connection of the distillation set has been closed fitly, thus it can be sure that some products were left in the flask and in the column. Hence, for recovery of otherwise lost reaction product, a “chaser” solvent e.g. toluene, should be added after the distillation and carry on distillation for second time. Once the toluene distils up the column and reaches the thermometer, most of the cyclohexene and water has been pushed over into the collection vial and maximum yield is ready to be collected. The anhydrous MgSO4 was added due to it is an inorganic drying agent that binds strongly with water and thus removes any traces of water from the solution. Besides that, our group put wrong magnesium sulphate heptahydate to remove the water, this affect the yield that we got. Precaution steps: Phosphoric acids are strong, corrosive acids. If any acid is splashed on your skin or clothing, wash immediately with copious amounts of water. Cyclohexene and toluene 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. Remaining cyclohexene should be disposed of in the fume-hood sink because cyclohexene vapors are heavier than air, they will accumulate in the sink. Conclusion: 2.55 g of cyclohexene was produced, which is 31.41% from the theoretical mass. The loss of water from a cyclohexanol to give a cyclohexene does not occur in just one step; a series of steps are involved in the mechanism of dehydration of alcohols. References: Reference books: T.W.G. Solomons and C. Fryhle, Organic Chemistry, Chapter 7.7, Dehydration of Alcohols. K. L. Williamson, Macroscale and Microscale Organic Experiments, 2nd Ed. 1994, Houghton Mifflin, Boston d. p268 McMurry, J. (2008). Organic Chemistry 7th ed. Brooks/Cole: Thomson Learning. P619-621 Webpages: Preparation of Cyclohexene from Cyclohexanol: an Elimination Reaction. Synthesis of Cyclohexene The Dehydration of Cyclohexanol. Synthesis of Cyclohexene from Cyclohexanol by ( E1 ) Elimination.


  1. Thank You so much for this explanation it really helps!!

  2. Can you show the calculations for theoretical yield please?

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