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An ester A with the molecular formula C5H10O2 reacts with excess CH3MgBr. The resulting intermediate is treated with H2SO4, producing an alkene B. When B undergoes ozonolysis, a ketone with molecular formula C4H8O is obtained, which gives a positive iodoform test. Identify the structure of A.

1. CH3CH2OCOCH3
2. CH3OCOCH2CH3
3. HCOOCH(CH3)2
4. CH3CH2CH2COOCH3

Correct answer: CH3CH2OCOCH3

Solution

Excess CH3MgBr adds twice to an ester, and after acid workup a tertiary alcohol is formed, which can dehydrate to an alkene. Ozonolysis of that alkene gives a C4 ketone that responds to the iodoform test, identifying the ester as CH3CH2OCOCH3.

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