Question

The compound A in the following reaction is :

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Work backwards: D is acetone (its Ba(OH)₂ aldol condensation gives mesityl oxide), C is benzaldehyde (Cannizzaro), so B = PhCH=C(CH₃)₂, which comes from the Grignard addition to a methyl ketone followed by dehydration.

Answer 1

The Correct Option is D

Step 1: D on Ba(OH)₂/heat gives \((\mathrm{CH_3})_2\mathrm{C{=}CH{-}CO{-}CH_3}\) (mesityl oxide), the aldol condensation product of two acetone molecules. So D = acetone.
Step 2: C with conc. KOH gives benzoate + benzyl alcohol (Cannizzaro reaction), so C = benzaldehyde, \(\mathrm{C_6H_5CHO}\).
Step 3: B on ozonolysis (O₃, Zn/H₂O) gives benzaldehyde + acetone, so B = \(\mathrm{C_6H_5CH{=}C(CH_3)_2}\).
Step 4: B is formed from A by (i) CH₃MgBr/H₂O, (ii) conc. H₂SO₄/heat, i.e. a ketone gives a tertiary alcohol, which is dehydrated. The alcohol must be \(\mathrm{C_6H_5CH_2C(OH)(CH_3)_2}\), so the ketone A is \(\mathrm{C_6H_5CH_2COCH_3}\) (phenylacetone, 1-phenylpropan-2-one).
Check: \(\mathrm{C_6H_5CH_2COCH_3 + CH_3MgBr \rightarrow C_6H_5CH_2C(CH_3)_2OH \xrightarrow{-H_2O} C_6H_5CH{=}C(CH_3)_2}\) (conjugated with the ring, the more stable alkene).
Answer: (4) C₆H₅-CH₂-CO-CH₃