Dehydration of 2-Methylcyclohexanol Formal Report
IntroductionA dehydration reaction of an alcohol results in an alkene. This type of reaction requires an alcohol, an acid catalyst and heat. Generally strong concentrated acids, like sulfuric acid and phosphoric acid, are used as the acid catalyst.The acid catalyst protonates the alcohol, to make a much better leaving group. Weakest bases make the best leaving groups, so once the alcohol is protonated the leaving group leaves and produces in a carbocation and water. In order to form the double bond, one of the beta hydrogens, hydrogens on a carbon adjacent to the carbocation, must be removed or eliminated. Therefore, another name for a dehydration reaction is beta elimination. Water acts as a nucleophile and attacks the carbocation, removing a beta hydrogen; the electrons from the C-H bond move to make a C-C pi-bond. The general mechanism is displayed by equation (1.0)Equation (1.0) shows a reaction with a primary alcohol. Usually primary alcohols form primary carbocation. However, primary carbocations are too unstable to form as intermediates, so they can undergo a rearrangement or E2 mechanism. A rearrangement occurs when an alkyl group or a hydrogen on the neighboring carbon of the carbocation shift to delocalize the positive charge to result in a more stable carbocation. A rearrangement almost always occurs whenever a carbocation can form a more stable molecule. An E2 mechanism means that the reaction is an elimination and the rate-determining step is bimolecular, two species are involved in one step of the mechanism. If the alcohol in equation (1.0) were to undergo an E2 mechanism then the beta hydrogen would be eliminated (the nucleophile would attack the electrophilic carbocation), C-C pi bond would be formed, and the leaving group would leave all in the rate determining step.
Additionally, equation (1.0) only shows one product, which is not true for all alcohols. Certain alcohols can produce more than one product when they undergo dehydration, such as
Additionally, equation (1.0) only shows one product, which is not true for all alcohols. Certain alcohols can produce more than one product when they undergo dehydration, such as
References: "Introduction to IR Spectrum Interpretation," Elizabethtown College, Department of Chemistry and Biochemistry, Chem 113, 2009. •"Dehydration of 2-methylcyclohexanol," Elizabethtown College, Department of Chemistry and Biochemistry, Chem 113, 2009.