Question

Which of the following is an example of a cyclic ketal?
A.

B.

C.

D.

Answer 1

Hint: In organic chemistry, a ketal refers to a functional group that has been derived from a ketone with the replacement of a carbonyl (\[C = O\]) group by two alkoxy groups. In other words, a ketal can also be referred to as an acetal derived from the ketone. Therefore, a cyclic ketal is actually a ketal in which ketal carbon along with one or both oxygen atoms are members of a ring.

Complete answer:
Cyclic ketals are generally formed when the reaction between the two molecules i.e. a ketone and a diol takes place. This reaction results into the formation of two main products i.e. an ketal and water as shown below:

If you look at the structures given in the options, you will notice that none of the structures except mentioned in Option C comprises the ketal carbon (carbon atom having two oxygen atoms) along with oxygen atoms (one or two) which are part of the ring. The reaction of cyclohexane-1,2-diol with acetone leads to the formation of cyclic ketal as demonstrated below:

Therefore, the correct answer is Option C.

Note:
Cyclic ketals or cyclic acetals are more stable in comparison to the acetals because of the presence of chelate effect that is majorly derived from having both of the\[ - OH\] groups of acetal being connected to each other in diol. Cyclic ketals have their major applications in carbohydrate chemistry.