Question

Please help with the question that I have circled in red. This is from a lab I did and I have included the entire handout in case it helps you to answer the question.

2015 Chem 263 2-Credit Org Lab 6T: Dehydration of Cyclohexanol to Cyclohexene Lab 6T w lecture you recently learned that alkenes undergo acid-catalyzed addition of H20 (hydration of the alkene) to yield alcohols. This lab with illustrate the same chemistry in reverse; the microscopic reverse of alkene hydration is alcohol dehydration. The alkene hydration is an equilibrium process driven to the right by having lots of water represent. The reverse reaction (elimination of water) is driven by removing ill build on your experience with preparative chemistry and microscale distillation. In Chem 261 ater as it is formed by azeotrophic distillation. (For a discussion of azeotropes, see for example http://chemwiki.ucdavis.edu/Physical Chemistry/Physical Properties of Matter/Solutions and Mixtur es/Nonideal Solutions/Azeotropes ). For future reference, this lab illustrates a simple E1 elimination reaction of the sort you will discuss later in the semester in Chem 261 lecture. To date, you explored just one type of organic transformation: a substitution reaction, in particular a free radical halogenation reaction. Another broad class of organic transformations is represented by elimination reactions, and within that class, the E1 elimination constitutes another of the cornerstone reactions of organic chemistry. Mechanistically related to the SN1 reaction, the E1 often competes with attempted substitution. Through proper choice of reaction conditions, it can usually be made the predominant or exclusive pathway. The acid catalyzed dehydration of an alcohol is one of the principal ways to prepare alkenes. (Note the arrows indicating an equilibrium process.) H+ catalyst R(H)C=C(H)CH-R + H2O > RCH-CH(OH)-CH2R + H+ The mechanism of the E1 reaction is actually quite simple, and as illustrated below, involves a series of equilibria. Treatment of an alcohol with a strong acid catalyst reversibly forms a small amount of protonated alcohol which can undergo reversible loss of water to yield a very small amount of a highly reactive carbocation intermediate. Loss of an adjacent proton (H+) forms an alkene and regenerates the acid catalyst. The successful E1 reaction usually requires: (a) removal of water during the reaction to drive the equilibria to the right (i.e., toward product); and (b) the absence of good nucleophiles (otherwise Sw1 chemistry would compete). The rate determining step is formation of the carbocation. Consequently, substrates that can form relatively stable carbocations (tertiary, allylic or benzylic, secondary) react faster than substrates that do not. Alcohols are the most common substrate, but appropriate ethers, esters, tosylates or alides can also be employed. When mixtures of stereoisomers (E/Z) or regioisomeric alkenes are (as in the case illustrated above), the product distribution will reflect thermodynamic possible preferences. In this lab experiment, you will carry out the acid catalyzed dehydration of cyclohexanol. No ereoisomers are possible; cyclohexene is the only alkene that can be formed under these conditions. Water is generated in the reaction and removed via azeotropic distillation to drive the

Answer 1

(i) Reaction mechanism for the conversion of cyclohexanol to cyclohexene is as follows.