Question

What is a cyclic acetal?

Answer 1

Hint : We know that an acetal forms when two ether groups are attached to the same carbon. It is also prepared from a hemiacetal, which is derived from an aldehyde that contains one ether group and one alcohol group.

Complete Step By Step Answer:
When aldehyde and ketones react with alcohols under an acidic environment, they form acetals. These are functional groups with tetrahedral geometry, in which two alkoxy $(-OR)$ groups are bonded to the central carbon atom. The mechanism starts with protonation of carbonyl oxygen that makes the $C=O$ bond highly electrophilic, which is then attacked by alcohol that forms oxonium intermediate. This intermediate gets deprotonated and forms a hemiacetal. Hemiacetal has only one alkoxy group compared to acetals which has two alkoxy groups. Further, the \[OH\] group of hemiacetal is protonated and changed into a good leaving group. As carbon becomes sterically hindered. An acetal is a compound that contains two ether groups on the same carbon atom.
Hemiacetal are formed from an aldehyde or ketone and an alcohol, by forming an ether bond and \[OH\]bond on the same carbon. It is a tetrahedral intermediate in the formation of acetal. When it is catalyzed by an acid, it becomes faster but acid can precede it to decomposition also, back to the starting material. If the two \[OH\] groups are part of a \[1,2-\] or a \[1,3-diol\], the product is a cyclic acetal. The purple \[O\] atom in the hemiacetal intermediate is held close to the hemiacetal carbon, so the intermolecular ring-closing reaction is fast.

Note :
Remember that a hemiacetal structure acts as a reducing sugar and thus it is unstable in nature. On the contrary, acetals are non-reducing sugars and are stable as they are not converted back into aldehyde or hemiacetal.