Question 14.2: Draw the structures of (a) 2,3-dimethylbutan-2-ol and (b) 3-...
Draw the structures of (a) 2,3-dimethylbutan-2-ol and (b) 3-ethylcyclopentanol. Classify each as primary, secondary, or tertiary.
ANALYSIS For both, begin by determining the longest carbon chain; number the carbon atoms and put groups on appropriate atoms. If no index number is given for the —OH group, it is assumed to be on the first carbon.
(a) This alcohol is a butanol, so it has a longest chain of four carbons:
\underset{1}{C} —\underset{2}{C}—\underset{3}{C}—\underset{4}{C}
Since it is a butan-2-ol, the ¬OH group is bonded to carbon 2; the methyl groups are bonded to carbons 2 and 3:
\begin{matrix} OH\,\,\,\, \\ \underset{1}{C} —\underset{2|}{\overset{|}{C}}—\underset{3|}{C}—\underset{4}{C} \\ \,\,\,\,\,\,\,\,CH_3\,CH_3 \end{matrix}
Filling out the remaining Hs gives us the following (in both condensed and line structure formats):
Since the —OH group is bonded to a carbon atom that has three other carbons bonded to it, this is a tertiary alcohol.
(b) The name tells us that the —OH group is bonded to a cyclopentane ring; since no position number is given, the —OH group is on carbon 1. Putting in the ethyl group on carbon 3 gives us the following:
Since the —OH group is bonded to a carbon atom that has two other carbons bonded to it, this is a secondary alcohol.