(a) Which kind of mechanism can better account for the product mixture obtained
ID: 780645 • Letter: #
Question
(a) Which kind of mechanism can better account for the product mixture obtained from the dehydration of cis- and trans-4-methylcyclohexanol: E1, E2, or a combination of the two? (b) Based on your answer, write detailed mechanisms explaining the formation of all of the observed products.Explanation / Answer
C-CH3-CH=HC=HO-C=HC=HC -------------> C6H5CH3 4-methylcyclohexanol H3PO4 MW=114.2 MW=96.2 b.p.=171-173 b.p.=101-102 (cis/trans mixture) Mechanism for dehydration of cis- and trans-4-methylcyclohexanol are given in URLs below: http://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Mechanism_for_the_dehydration_of_4-methylcyclohexanol.jpeg.png/800px-Mechanism_for_the_dehydration_of_4-methylcyclohexanol.jpeg.png http://upload.wikimedia.org/wikipedia/commons/a/a0/Dehydration_of_4-methylcyclohexanol.jpeg.png In step 1, the OH group is protonated. In step 2, the OH2 leaves as a water molecule and a carbocation is left behind. Then a proton leaves, and the double bond is formed. The products of this elimination reaction can include the following isomers: 1-methyl cyclohexene;3-methyl cyclohexene,4-methyl cyclohexene. Elimination reactions such as this can proceed by different mechanisms. Direct elimination of water from 4-methylcyclohexanol would produce 4-methylcyclohexene, Elimination reactions can occur via the formation of a carbocation and subsequently, rearrangements can take place. This sort of mechanism would result in the formation of 1- and 3-methylcyclohexene, in addition to 4- methylcyclohexene, from the dehydration of 4-methylcyclohexanol.
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