Question 14.3: Draw structural formulas for the alkenes formed by the acid-...
Draw structural formulas for the alkenes formed by the acid-catalyzed dehydration of each alcohol. For each part, predict which alkene will be the major product. (a) 3-Methyl-2-butanol (b) 2-Methylcyclopentanol
Strategy
In the acid-catalyzed dehydration of an alcohol, H and OH are removed from adjacent carbon atoms. When dehydration yields isomeric alkenes, the alkene with the greater number of alkyl groups on the carbon atoms of the double bond generally predominates
Solution
(a) Elimination of H_2O from carbons 2–3 gives 2-methyl-2-butene; elimination of H_2O from carbons 1–2 gives 3-methyl-1-butene. 2-Methyl-2-butene has three alkyl groups (three methyl groups) on its double bond and is the major product. 3-Methyl-1-butene has only one alkyl group (an isopropyl group) on its double bond and is the minor product.
(b) The major product, 1-methylcyclopentene, has three alkyl groups on its double bond. The minor product, 3-methylcyclopentene, has only two alkyl groups on its double bond.