Which protecting group is formed when an aldehyde reacts with ethylene glycol?

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Multiple Choice

Which protecting group is formed when an aldehyde reacts with ethylene glycol?

Protecting aldehydes with a diol forms an acetal. When ethylene glycol is used, the aldehyde becomes a five‑membered cyclic acetal (a 1,3‑dioxolane ring), often called an ethylene acetal. This protects the carbonyl by converting it into two OR groups on the carbon, while keeping the rest of the molecule intact. The term ketal refers to acetals derived from a ketone (the carbonyl carbon has two carbon substituents), which isn’t the case here since an aldehyde’s carbonyl carbon bears a hydrogen. Hydrate is simply a geminal diol from hydration, not a protective group, and imine formation involves reaction with an amine, not ethylene glycol. The acetal can be reversed later under acidic conditions to regenerate the aldehyde.

Protecting aldehydes with a diol forms an acetal. When ethylene glycol is used, the aldehyde becomes a five‑membered cyclic acetal (a 1,3‑dioxolane ring), often called an ethylene acetal. This protects the carbonyl by converting it into two OR groups on the carbon, while keeping the rest of the molecule intact. The term ketal refers to acetals derived from a ketone (the carbonyl carbon has two carbon substituents), which isn’t the case here since an aldehyde’s carbonyl carbon bears a hydrogen. Hydrate is simply a geminal diol from hydration, not a protective group, and imine formation involves reaction with an amine, not ethylene glycol. The acetal can be reversed later under acidic conditions to regenerate the aldehyde.

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