The diol shown is warmed with dilute acid. Water is lost from the carbon that gives the more stable carbocation, and a 1,2-shift then gives a carbonyl compound. Which structure is formed?

Answer: C

The diol is 2-methylpropane-1,2-diol. The tertiary hydroxyl is lost to give the tertiary cation at C-2, and a hydride shifts from the

CH X_{2} OH

carbon, so the oxygen on C-1 becomes a carbonyl. The product is 2-methylpropanal (isobutyraldehyde), an aldehyde.