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CH3-CH2-C(OH)(Ph)-CH3 cannot be prepared by -

1. CH3CH2COCH3 + PhMgX
2. PhCOCH2 + CH3CH2MgX
3. PhCOCH2CH3 + CH3MgX
4. HCHO + PhCH(CH3)CH2MgX

Correct answer: HCHO + PhCH(CH3)CH2MgX

Solution

The target 2-phenylbutan-2-ol is tertiary (C bearing OH, methyl, ethyl, phenyl). A tertiary alcohol needs a ketone + RMgX; HCHO + a Grignard yields only a primary alcohol RCH2OH, so the HCHO route cannot give it. The ketone routes (e.g. ethyl methyl ketone + PhMgX) all work.

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