Question 15.5: Identification of Acetals and Ketals Which of the following ...
Identification of Acetals and Ketals
Which of the following compounds are acetals and ketals?
(a) \begin{matrix}CH_3\underset{|}{C} HOCH_2CH_3 \\ OCH_2CH_3 \end{matrix} (b) \begin{matrix} O \quad \quad \\ CH_3\overset{||}{C} -OCH_3 \end{matrix} (c) 
(d) 
ANALYSiS As in identifying hemiacetals and hemiketals, look for a carbon atom that has single bonds to two oxygen atoms, but in this case both of them will be —OR groups. Note that the O of the —OR group can be part of a ring. If the two remaining bonded groups are carbons, it is a ketal; if one is a carbon and the other a hydrogen, it is an acetal.
In (a), the central carbon atom is bonded to one —CH_3, one —H, and two —OCH_2CH_3 groups, so the compound is an acetal. Compound (b) does have a carbon atom bonded to two oxygen atoms, but one of the bonds is a double bond rather than a single bond, so this is not an acetal (it is, in fact, an ester; Chapter 17). Compound (c) has an oxygen atom in a ring, making it also part of an —OR group, where R is the ring. Since one of the carbons connected to the O in the ring is also connected to an — OCH_2CH_3 group, compound (c) is an acetal. Compound (d) is a sugar known as mannose; it too has an oxygen atom in a ring, making it part of an —OR group, where R is the ring. Since one of the carbons connected to the O in the ring is also connected to a H and an OH, compound (d) is a hemiacetal.
It should be noted that cyclic systems of these types are the most difficult to recognize, yet they will be the ones you will see most often as you progress through biochemistry, so the more practice you get at recognizing these, the better! Anytime you see an oxygen in a ring, always look at the carbons attached to either side of it to see if one of them has another oxygen attached. If one of them does, you have a cyclic hemiacetal, hemiketal, acetal, or ketal.