11 Dehydration.docx - Lab#11 Dehydration of...

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Lab #11: Dehydration of 2-methylcyclohexanol Yook Teng (Tiffany) Lam April 17, 2015 TA: Lukas Klees CHEM 335, Section B74 Department of Chemistry, Binghamton University, Binghamton, NY 11362
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Introduction A simple way to make alkenes is by dehydration of alcohols. The reaction involves an alcohol, acid catalyst, and heat. In the process, a hydroxyl group leaves and forms bond with hydrogen leading to the formation of water byproduct. Different types of alcohols dehydrate through different mechanism pathway. Primary alcohols undergo reaction through the E2 mechanism, secondary and tertiary alcohols go through the E1 mechanism. The main contrast in the two pathways is the formation of the carbocation in E1. The hydroxyl group on the alcohol is a poor leaving group. The acid catalyzes the reaction by donating its proton to the hydroxyl group such that it forms a water molecule. Water is a better leaving group and afterwards a proton is removed the adjacent carbon thus forming a double bond. Additionally, the acid acts a drying agent to remove the water byproduct. The products are of the reaction either are Hoffman products or Zaitsev products. There is always a small chance that the double bond is formed on the least substituted carbon, but major products follow the Zaitsev rule. Double bonds are formed on the most substituted carbon. When double bonds are formed, cis or trans products are possible. Zaitsev products with trans configuration is most energetically favorable. The objective of the lab is to dehydrate 2-methylcyclohexanol using phosphoric acid as the catalyst. Major product is 1-methylcyclohexene constituting about 85% of the total products.
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