done by distilling off water as it forms. The reverse reaction is favored by treating the acetal with a large excess of aqueous acid to drive the equilibrium to the left.
Acetals are useful because they can serve as protecting groups for aldehydes and ketones in the same way that trimethylsilyl ethers serve as pro tecting groups for alcohols (Section 17.9). As we saw previously, it sometimes happens that one functional group interferes with intended chemistry elsewhere in a complex molecule. For example, if we wanted to reduce only the ester group of ethyl 4-oxopentanoate, the ketone would interfere. Treatment of the starting ketone ester with LiAlH4 would reduce both the ketone and the ester groups to give a diol product.