OCHEM The Synthesis of Alkenes 7

OCHEM The Synthesis of Alkenes 7 - The Synthesis of Alkenes...

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The Synthesis of Alkenes: The Dehydration of Cyclohexanol Lindy Hilpert Brad Hutnick Organic Chemistry 330 October 31, 2007
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Introduction: The purpose of this experiment is to synthesize cyclohexene from cyclohexanol through dehydration and then test for double bonds by adding bromine. Also, an IR analysis will be done to characterize the product. Theory, Reactions, and Mechanisms: Dehydration is a type of elimination reaction in which water is eliminated. There are two types of elimination reactions, E1 and E2. Many factors affect which type of elimination will occur including the type of substrate, reaction conditions, nature of the base, and nature of the leaving group. E1 reactions prefer tertiary substrates over secondary substrates. Primary and methyl substrates do not go through the E1 pathway. E1 prefers a polar protic solvent because it helps to stabilize the carbocation through hydrogen bonding. E1 prefers a weak base. E1 prefers a good leaving group. E1 reactions do not need a good nucleophile because it is not included in the rate, so it doesn’t affect the reaction. E2 reactions prefer methyl substrates over primary substrates over secondary substrates over tertiary substrates. E2 prefers a polar aprotic solvent because it doesn’t want hydrogen bonding. E2 prefers a strong base. E2 prefers a good leaving group. E2 reactions prefer good substrates because they are included in the rate, so they affect the reaction. The rate of an E1 reaction is rate = k[substrate]. This means that the reaction is unimolecular. First, the leaving group leaves and creates a carbocation. A carbocation is a carbon with a positive charge. The second step is that the nucleophile pulls of an alpha hydrogen and a double bond is formed. The E1 reaction does not need a good nucleophile because it is not included in the rate. Polar protic solvents are favored because the hydrogens stabilize the positive carbocation through hydrogen bonding. E1 prefers a tertiary substrate the most because it makes it easier for the leaving group to leave and makes a more stable carbocation. Carbocations are more stable when they are attached to more carbons, so tertiary is the best. The rate of an E2 reaction is rate = k[substrate][base]. This means that the reaction is bimolecular and the reaction is concerted. The leaving group leaves and the nucleophile pulls off the alpha hydrogen at the same time. In this process, a double bond is formed. E2 reactions do need strong bases because they are included in the rate, so they affect the reaction. Polar aprotic solvents are favored because if protic solvents are added, they create a caging effect around the nucleophile. The caging affect is when the protic solvent surrounds the nucleophile using hydrogen bonds. This makes it much harder for the nucleophile to attack the substrate. E2 reactions prefer methyl or primary substrates because there is less steric hindrance, and the base can come in and attack the substrate more easily. Le Chatelier’s Principle states that if a system at equilibrium is disturbed by a
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This note was uploaded on 04/07/2008 for the course CHEM 0330 taught by Professor Forget during the Fall '07 term at Pittsburgh.

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OCHEM The Synthesis of Alkenes 7 - The Synthesis of Alkenes...

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