A cyclic acetal made from a ketone and ethylene glycol is used as a protecting group. It is stable towards

A $H X_{3} O X^{+}$ in excess water
B $CH X_{3} MgBr$ in ether
Cdilute aqueous $HCl$ on warming
Daqueous $H X_{2} SO X_{4}$ (1 M)

Answer: B

Acetals have no electrophilic carbonyl and contain no acidic hydrogen, so they survive Grignard reagents, hydrides and bases. They are hydrolysed readily by aqueous acid, which is how the carbonyl group is deprotected.

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