An unknown aromatic carbonyl compound 'X' is subjected to the following sequence of reactions: (i) NaCN/HCl (ii) H₃O^+ , heat (iii) EtOH/H^+ (iv) Excess CH₃MgBr , followed by H₂O If the final major product is 2-methyl-3-phenylbutane-2,3-diol, the starting compound 'X' is:

Step-by-step solution

Let us trace the reaction sequence backwards from the final product, 2-methyl-3-phenylbutane-2,3-diol, which has the structure CH₃-C(OH)(Ph)-C(OH)(CH₃)₂ . Step (iv): The -C(OH)(CH₃)₂ group is a tertiary alcohol formed by the reaction of an ester with excess CH₃MgBr . Thus, the precursor before step (iv) was an ethyl ester: CH₃-C(OH)(Ph)-COOCH₂CH₃ . Step (iii): The ester was formed by the esterification of a carboxylic acid with EtOH/H^+ . The precursor was the -hydroxy acid: CH₃-C(OH)(Ph)-COOH . Step (ii): The carb