Formation of an Alkene by Alcohol Dehydration
The acid-catalyzed dehydration of secondary and tertiary alcohols involves non-isolable carbocation intermediaries. In the first step of the of the reaction mechanism below, a phosphoric acid catalyst adds a proton to the oxygen atom of the alcohol to form an oxonium ion. The OH is converted to a better leaving group as the positive charge on the oxygen weakens the carbon-oxygen bond. The carbon-oxygen bond breaks during heating giving water an unstable carbocation.
Without a nucleophile, the carbocation intermediate loses a proton from a carbon and forms a carbon-carbon double bond from the carbon-hydrogen electrons. The formation of a carbocation intermediate from the loss of a leaving group classifies this as an Elimination Reaction. The phosphoric acid in this lab is a corrosive acid. Caution must be used to not get the acid in contact with skin or clothes. Gloves must be worn while pouring it. If there is a spill, wash the area with large amounts of water.
Cyclohexanol, and especially cyclohexene, are very flammable liquids. Do not use an open flame as a source of heat with these substances. Outline of the Procedure Add 20 mL of cyclohexanol and 5 mL of 85% phosphoric acid to a 50-mL round-bottom flask along with 1 or two boiling chips Swirl the flask gently to thoroughly mix the layers and incorporate this into a simple distillation column Heat the flask until distillation begins and slowly distills the mixture, controlling the heating so that the temperature of the distillation vapor does not go above 100-105 degrees C. Cool the receiver in an ice bath to reduce the loss of distillate to evaporation Continue the distillation until only 5-10 mL of high-boiling residue is left in the flask Allow the flask to cool and disconnect the apparatus
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Transfer the distillate to a separatory funnel and shake it with 10 mL of saturated sodium chloride solution, then drain off lower aqueous layer Add 5 mL of 10% sodium carbonate to solution to neutralize any acid, gently shake the mixture, and drain off the lower aqueous layer again Pour the upper layer into a dry, clean Erlenmeyer flask Add 1-2 g of anhydrous calcium chloride, and allow the flask to stand for 15-20 minutes with occasional stirring Clean and dry the condenser, distillation flask and adapters for the final distillation Decant or filter the dry cyclohexene from the calcium chloride into a dry distillation flask and set up a simple distillation apparatus Add 1-2 boiling chips to the flask and distill the cyclohexene, collecting the liquid that boils between 79 and 85 degrees C in a small graduated cylinder Data and Observations The purpose of this lab is to synthesize cyclohexene from cyclohexanol using phosphoric acid. This student had difficulties maintaining the temperature of the distillation vapor between 100-105 degrees C. the distillate recovered that was between 79 and 85 degrees C was discarded. The receiver was placed in an ice bath to reduce evaporation. A total of 21 mL was collected.
Results and Discussion
Volume of cyclohexene: 2. 8 mL Weight from density of cyclohexene: (0. 810 g/mL) / (2. 8 mL) = *0. 29 g cyclohexe*ne Percentage yield of cyclohexene: 20 mL x 0. 960 g/mL = 19. 2 g = 0. 192 mol 0. 192 mol x 82 g/mol = 15. g x 1mL/0. 81 g = 19. 4 mL (Theoretical Yield) 2. 8 mL/19. 4 mL x 100 = 14. 4% Yield (but of excellent quality) In a small test tube, add 5 drops of cyclohexene to 1 mL of 0. 5% aqueous potassium permanganate solution The solution turned brown immediately and then formed a dark, thick precipitate within 1 minute 1 mL of a 2% solution of bromine in carbon tetrachloride The solution immediately turned clear, and remained that way throughout the lab Conclusions The dehydration of alcohols is one of several methods to produce alkenes. This dehydration may be accomplished using a strong mineral acid such as sulfuric and phosphoric acid.
The first step in this transformation is protonation of the hydroxyl group by the acid yielding an excellent leaving group. Next water is eliminated from the molecule resulting in a carbocation intermediate, which then eliminates a proton to form an alkene. Name and draw the structure of the alkenes that might be obtained by acid-catalyzed dehydration of 2-methylcyclohexanol 3-methyl-3-hexanol. What would be the best alcohol to use in an acid-catalyzed dehydration to prepare 1, 3-dimethylcyclopentene? The most suitable alcohol to use is 2, 5 dimethylcyclopentanol. Write equations to show the products that form in the reactions of cyclohexene with potassium permanganate and bromine.
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Formation of an Alkene by Alcohol Dehydration. (2018, Feb 11). Retrieved from https://phdessay.com/formation-of-an-alkene-by-alcohol-dehydration/
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