The diol shown is treated with dilute acid. Take the convention that hydride migrates in preference to methyl, and that water is lost from the more substituted carbinol carbon. Which structure is the major product?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: D

The diol is 2-methylbutane-2,3-diol. The tertiary hydroxyl leaves, giving the tertiary cation at C-2. A hydride moves from C-3 to C-2, and C-3 becomes the carbonyl of 3-methylbutan-2-one. Methyl migration would give 2,2-dimethylpropanal instead, which is excluded by the stated convention.

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