Similar to water, alcohols can add to the carbonyl carbon of the aldehydes and ketones. The addition of one molecule of alcohol to the carbonyl compound forms the hemiacetal or half acetal. As depicted below, in a hemiacetal, the carbon is directly linked to an OH and OR group.
As alcohols are poor nucleophiles, the formation of hemiacetals is very slow under neutral conditions. The reaction rate is enhanced by using either basic or acidic reaction media.
The acid catalyst, such as sulfuric acid or p-toluenesulfonic acid, interacts with the alcohol by donating a proton. This generates a protonated alcohol that acts as the active species and subsequently protonates the carbonyl oxygen, making the carbonyl carbon strongly electrophilic. Now, an alcohol molecule attacks this carbonyl carbon forming an oxonium cation. The loss of a proton from oxonium cation leads to the formation of hemiacetals.
On the other hand, in the presence of a base, the alcohol undergoes a deprotonation reaction, forming the negatively charged alkoxide anion. This highly basic anion then attacks the carbonyl carbon. The intermediate form will abstract a proton from another alcohol molecule to form the hemiacetal.
The cyclic hemiacetals are formed when the hydroxyl and carbonyl groups are present on the same molecule. The naturally occurring simple carbohydrates generally exist in cyclic hemiacetal form. For instance, the α and β anomeric forms of D-glucose are in the hemiacetal form.
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