Final answer:
Using a diol in acetal formation leads to the creation of a cyclic acetal or ketal, wherein both -OH groups of the diol react with a carbonyl compound to form a stable cyclic structure after water is eliminated.
Step-by-step explanation:
When a diol is used in place of two equivalents of ROH in acetal formation, a cyclic acetal or ketal can be formed, depending on whether the starting compound is an aldehyde or a ketone, respectively. In this reaction, the diol acts as a diglycol, with each of its two -OH groups reacting with the carbonyl compound.
In the acetal formation from a diol, the two -OH groups become bonded to the same carbon atom once they have each reacted with the carbonyl group, leading to the elimination of a water molecule and the formation of a cyclic structure.
This is distinct from using two equivalents of a monohydric alcohol, where two separate -O-R groups would be attached to the carbonyl carbon, forming a regular (non-cyclic) acetal. The cyclic acetals and ketals often have greater stability than their acyclic counterparts and are useful as protective groups in organic synthesis.
When a diol is used in place of two equivalents of ROH in acetal formation, an HOH (water) molecule will be eliminated and an alkene will be formed. It does not matter which adjacent carbon loses the H atom; in either case, the product will be an alkene. This is because diols have two -OH groups attached to the same carbon, which makes them more reactive than alcohols.