Final answer:
The cyclic structures in sugars are formed when a carbonyl group (aldehyde or ketone) reacts with an alcohol (OH) group to form a hemiacetal, creating an additional asymmetric or chiral center. This process gives rise to two stereoisomers known as anomers. Five or six-membered rings like pyranose and furanose are the most stable configurations for these cyclic sugar structures.
Step-by-step explanation:
The formation of cyclic structures in sugars involves the reaction of a carbonyl group (aldehydic or ketonic) with an alcohol group (OH). Specifically, in straight-chain monosaccharides, the carbonyl group on the first carbon atom can react with an OH group on another carbon atom, typically the fourth or fifth carbon atom. This reaction leads to the formation of a hemiacetal. During this process, an additional asymmetric center, or chiral center, known as the anomeric carbon, is created when the aldehydic (aldehyde group) or ketonic (ketone group) carbon reacts with the OH group, thus forming two different stereoisomers referred to as anomers.
For instance, in hexoses like glucose, the sixth carbon's OH group can react with the aldehydic group at the first carbon to form a six-membered ring known as a pyranose ring which resembles the organic compound, pyran. In a similar manner, pentoses can form five-membered rings called furanose rings, resembling furan. The stability of these sugar cyclic structures is higher if the ring contains five or six atoms, mirroring the stability preference in cyclic alkanes.
Through this reaction, sugars gain an additional chiral center and therefore the ability to form different isomers. These isomers, or anomers, differ in their structure only around the newly formed anomeric carbon. Monosaccharides primarily exist in these cyclic hemiacetal forms due to their greater stability compared to the open-chain form.