A hemiacetal hydroxyl group refers to the fact that in polyhydroxy aldehydes or poly methyl ketones, some hydrogen atoms on certain methyl groups can spontaneously undergo addition reactions with the ketone group (carbonyl group), thereby forming a methyl group, which is called a hemiacetal hydroxyl group. In polyhydroxy aldehydes or poly methyl ketones, some hydrogen atoms on certain methyl groups can undergo addition reactions with the ketone group (carbonyl group), thereby forming a methyl group, which is known as the hemiacetal hydroxyl group.

A hemiacetal hydroxyl group refers to the fact that in polyhydroxy aldehydes or poly methyl ketones, some hydrogen atoms on certain methyl groups can spontaneously undergo addition reactions with the ketone group (carbonyl group), thereby forming a methyl group, which is called a hemiacetal hydroxyl group. The way to determine whether a molecule contains a hemiacetal hydroxyl group is to connect a hydroxy group and a methyl group to the same carbon atom.

Taking glucose as an example, many properties of glucose in solution indicate that the glucose molecular structure generally exists in a ring-shaped structure. The aldehyde or ketone groups within glucose can spontaneously and rapidly react with another methyl group on the same molecular structure, another oxygen atom, to form a reversible reaction, converting into a hemiacetal, and then forming a ring structure. In this structure, the methyl group on the first oxygen atom is called a hemiacetal hydroxyl group, which has very open physical properties.

When the 1st carbon aldehyde group and the 5th carbon methyl group undergo condensation reaction, the 1st carbon and the 5th carbon are connected by an oxygen bridge, forming a cyclic structure. Such a ring composed of 5 carbon atoms and 1 oxygen atom is called a pyran ring. During the formation of the ring, the aldehyde group on the 1st carbon becomes a methyl group, which still exhibits part of the original aldehyde’s oxidizing property, known as a hemiacetal hydroxyl group.

3-Methyl-OH, also known as hydroxyl, is a common optically active functional group. The methyl group is crucial, including alcohols and phenols. It is a monovalent group (—OH) composed of one hydrogen atom and one oxygen atom. Methyl has some similar properties to water, being a typical optically active functional group, which can form covalent bonds with water, existing in the form of a positively charged cation (OH-1) in the solution of inorganic compounds, known as the hydroxide ion.