Question 15.4: identification of hemiacetals and hemiketals Which of the fo...
identification of hemiacetals and hemiketals
Which of the following compounds are hemiacetals and which are hemiketals?
(a) 
(b) 
(c) \begin{matrix}\,\,\,\,\,\, OH \\ CH_3—\overset{|}{\underset{|}{C}} —CH_3 \\ \,\,\,\,\,\,\,\,\,\,\,\,OCH_3 \end{matrix}
ANALYSiS To identify a hemiacetal or a hemiketal, look for a carbon atom with single bonds to two oxygen atoms, with one being an —OH group and one an —OR group. Note that the O of the —OR group can be part of a ring. If the two remaining groups are carbons, it is a hemiketal; if one is a carbon and the other a hydrogen, it is a hemiacetal.
Compound (a) contains two O atoms, but they are bonded to different C atoms; it is not a hemiacetal; rather it is a diol. Compound (b) has one ring C atom bonded to two oxygen atoms, one in the substituent —OH group and one bonded to the rest of the ring, which is the R group; the other two groups bonded to that carbon are a H and another C; it is a cyclic hemiacetal. Compound (c) also contains a C atom bonded to one —OH group and one —OR group, but here the other two bonded groups are carbons, so (c) is a hemiketal.