chem 230L dehydration of 2-methylcyclohexanol

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Jan 9, 2024

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CHEM 230L: Organic Chemistry I Lab Chapman University Experiment 9: Dehydration 2-Methylcyclohexanol to Form Methylcyclohexenes Intended Learning Outcomes By completing this lab, students will:  Understand the theory of how to use the dehydration of alcohols to generate alkene based compounds.  Learn how to set up a typical acid catalyzed dehydration reaction.  Understand how to analyze basic alkene compounds for the presence of unsaturated carbon-carbon bonds and how to analyze mixtures of isomers via gas chromatography (GC).  Synthesize a mixture of alkenes from the dehydration of 2-methylcyclohexanol and analyze the product(s). Introduction This week in lab you use one of the reactions you will about in lecture (Dehydration of alcohols) to synthesize a mixture of 1-methylcyclohexene and 3-methylcyclohexene. As with alkyl halides, alcohols can undergo elimination reactions to form alkenes. However, the alcohol (hydroxy) group by itself is a poor leaving group and must be protonated to form water as a good leaving group. Typically, treatment of an alcohol with acid under heating conditions will allow this reaction to occur. First, acidic conditions allow for protonation of the hydroxy group to generate a good leaving group (water) and heat will then promote an elimination reaction (E1 or E2). The reaction that you will be performing in lab is shown below.

CHEM 230L: Organic Chemistry I Lab Chapman University As this reaction is in equilibrium with the reverse acid catalyzed hydration, the desired dehydration can be favored if the desired product is removed during the reaction. Thus, the reaction is performed using a distillation set-up where the desired product is distilled away as it is formed. After the synthesis and isolation of methylcyclohexes, some simple test will be run to look for the presence of alkenes by seeing if bromine and permanganate can perform addition reactions across the double bond in the desired products. Finally, students will try to determine the ratio of the two isomers formed in the reaction by using Gas Chromatography (GC). Pre-Lab Reading The reading below needs to be completed before the start of lab. 1. Lecture textbook (Klein, 3 trd edition): Sections 12.9 (E1 and E2 Reactions with alcohols section only), pages 532-533. 2. Lecture textbook (Klein, 3 trd edition): Sections 8.9 (halogenation sections only), pages 373-375. 3. Lab textbook (Pavia, 6 th edition): Experiment 24: 4-Methylcyclohexene (introduction only), page 207. 4. Carefully read the procedure for the lab experiment below to ensure that you understand the purpose of each step. Pre-Lab Assignment (15 points) Answer the following questions: 1. Draw a reasonable mechanism for the reaction and formation of both products. (5 pts.)

CHEM 230L: Organic Chemistry I Lab Chapman University 2. Determine how many moles of 2-methylcyclohexanol (MW = 114.2 g/mol) are used in the reaction and then determine the theoretical yield for the methylcyclohexene products (MW = 96.2 g/mol). (5 pts.) 2-methylcyclohexanol (MM = 114.2g/mol; density = 0.93 g/mL) = 5 mL 5 mL * 0.93g/mL = 4.65 g 4.65 g * mol/114.2 g = 4.07*10^-2 mol 96.2g/mol * 4.07*10^-2 mol = 3.92 g of methylcyclohexane products 3. Draw the product(s) that should form in the unsaturation test using Br 2. Explain why the disappearance of the orange color of the solution (due to the presence of Br 2 ) is confirmation that there are unsaturated C-C bonds (alkene or alkynes) present. (5 pts.) Since bromine has a strong attraction for electrons and can undergo addition reactions with alkenes and alkynes (alkenes in this case), the bromine molecule is added across the double bond. Adding bromine to the unsaturated C-C bonds makes the orange color of the bromine disappear, as the bromine is now part of a new compound. Thus, when we see the disappearance of the orange color, it ensures that unsaturated C-C bonds are present and participating in the stated reaction. Procedure Safety: Sulfuric acid is corrosive and harmful and causes skin burns/irritation, serious eye irritation, and respiratory irritation if fumes are inhaled. Wear eye protection, lab coat, and gloves, and work in a hood at all times. 1. In a pre-weighed graduated cylinder (63.860 g), pour 5 mL 2-methylcyclohexanol (mixture of cis-and trans isomers) and determine the mass of the alcohol. Into a 25-mL round bottomed flask equipped with a stir bar, add the 5 mL of 2-methylcyclohexanol, followed by 3 mL of 9M sulfuric acid. Gently swirl the flask to thoroughly mix the liquids. 69.039 g – 63.860 g = 5.179 g

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CHEM 230L: Organic Chemistry I Lab Chapman University 2. Attach a fractionating column, three-way adapter, and condensing tube to the round bottomed flask to set the reaction up for fractional distillation. Use a 10 mL graduated cylinder as the receiving flask. 3. Turn on the water for the condensing tube, begin stirring, and set the reaction to 150 o on the hotplate. Wrap the reaction flask and fractionating column with aluminum foil. After 20 minutes at 150 o C, increase the temperature to 180 o and continue for another 20-40 minutes. 4. The distillation temperature at which the distillate will collect should be between 80-90 o C and should be at a rate of about one drop every two to four seconds. Collect the distillate until 3-4 mL of the alkene is collected. The distillation temperature should begin to drop once the alkene product has been distilled. 5. Once enough of the organic product has been collected, transfer the distillate to a small separatory funnel. Wash the organic product successively with 10 mL of water, followed by 10 mL of 3N aqueous sodium hydroxide solution, and then 10 mL of saturated sodium chloride solution. 6. Drain the organic layer into a 25 mL Erlenmeyer flask and add a small amount of anhydrous calcium chloride. After swirling for a few minutes, check to see if the solution is clear. If the solution does not appear clear, add more of the drying agent and swirl again until clear. 2.073 g 6. Remove the organic solution away from the CaCl 2 using a Pasteur pipet and transfer the solution to a tared vial and determine the mass of the products to calculate the percent yield. 8. To test the product mixture for unsaturation, perform the tests below. Continue working in the hood for these tests. (A) The Bromine test: • Put 1-2 mL of 0.1M bromine in dichloromethane solution in a test tube. • Add 1-2 drops of the alkene product under to the test tube. • The rapid disappearance of the orange bromine color to give a colorless solution, is a positive test for unsaturation. • Repeat the above experiment with cyclohexane as a control test (B) The Baeyer test with potassium permanganate: • Put 0.5 mL of the product into a test tube. • Add 2-3 drops of potassium permanganate solution to the test tube and shake vigorously. • The formation of a heavy brown precipitate (MnO 2 ) is a positive test for unsaturation. • Repeat the above experiment with cyclohexane as a control test.

CHEM 230L: Organic Chemistry I Lab Chapman University 9. Analyze the products by gas chromatography to determine the percentage composition of each product in the mixture. Post – Lab Assignment (30 pts.) Data Analysis: 1. Enter the mass of your final 2-methylcyclohexanol that was used (2 pts.) __ 5.179 g ________. 2. Determine the theoretical yield based on the mass from 1 (show your calculations). (4 pts). 2-methylcyclohexanol (MM = 114.2g/mol; density = 0.93 g/mL) = 5.179 g 5.179 g * mol/114.2 g = 4.54*10^-2 mol 96.2g/mol * 4.54*10^-2 mol = 4.36 g of methylcyclohexane products 3.Enter the mass of your final alkene product(s) was collected (2 pts.) __ 2.073 g ________. 4. Determine % yield of the reaction (show your calculations) (4 pts.) (2.073g/4.36g)*100%= 47.55% 5. What are the results (from your product and cyclohexane control) from the Br 2 and Baeyer tests for unsaturation? What do these results tell you regarding if your dehydration of an alcohol to form alkenes was successful? (6 pts.) Our results from the Br2 tests for unsaturation was successful because we noticed a rapid change of the orange bromine color to colorless, indicating that our sample was a positive test. The Baeyer test was also successful since it went from dark purple to a heavy brown precipitate, meaning it was a positive test. Tests are positive only in the presence of an alkene, and since the product of dehydration reactions of an alcohol are alkenes, our positive tests in both the Br2 and Baeyer tests indicate that the dehydration reaction did indeed occur in our experiment.

CHEM 230L: Organic Chemistry I Lab Chapman University 4. Using the data collected from the GC (gas chromatography) of the products and their known boiling points, which of the methyl cyclohexene isomers is the major product and which is the minor product? Roughly, what is the ratio formed between the isomers? (5 pts.) The first peak has an area of 1310 mV*min, and the second peak has an area of 1313 mV*min. This means that the second peak is the major product since the area under the peaks indicate the quantity of each isomer. Given that 1-methylcyclohexane is a tri-substituted alkene and 3-methylcyclohexane is a di- substituted alkene, 1-methylcyclohexane is thus more stable. With 1-methylcyclohexane being more stable, it would then be the major product of the reaction. In the GC graph, it would be the area under the second peak. The ratio for the isomers is approximately 1.00 (1313mV*min/1310mV*min=1.0022) Conceptual Questions: 1. Explain why the observed major isomer of the methylcyclohexenes formed if favored over other isomers. (3 pts.) The observed major isomer of the methylcyclohexanes formed over the other isomers because 1-methylcyclohexane has a tri-substituted alkene that is more stable than the di-substituted alkene in 3-methylcyclohexane. 2. Determine and draw the alkene products that will be produced, and which isomer would be favored if 2-methyl-2-pentanol (shown below) was used in the dehydration reaction. (4 pts.) The two isomers on top the circled isomer are equal in stability with both being di-substituted alkenes, thus both will be minor products in the dehydration reaction. The major product is the

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CHEM 230L: Organic Chemistry I Lab Chapman University circled isomer, because it is more stable than the two isomers above (tri-substituted more stable > di-substituted)