Question

A carbonyl compound (A) on reaction with Grignard reagent (B) followed by hydrolysis gives an alcohol. This alcohol on reaction with (i) \(PBr_3\) and (ii) \(Zn/dil.~H^+\) gives an alkane. Photochemical chlorination of the alkane gives only one monochloro derivative. A and B respectively are:

• A. Acetone, Ethyl magnesium bromide
• B. Methanal, Isopropyl magnesium bromide
• C. Propanal, Ethyl magnesium bromide
• D. Methanal, tert-Butyl magnesium bromide

Hint:

Whenever a question states that an alkane gives only one monochloro derivative, immediately think of highly symmetrical alkanes such as methane, ethane or neopentane. Then work backwards through the reaction sequence to identify the required alcohol and Grignard reagent.

Answer 1

The Correct Option is D

Concept: Grignard reagents react with carbonyl compounds to produce alcohols. \[ R-MgX + HCHO \xrightarrow{H_3O^+} RCH_2OH \] The alcohol can be converted into an alkyl bromide using \(PBr_3\), and subsequent reduction with \(Zn/dil.~H^+\) gives the corresponding alkane. The alkane formed must satisfy the condition that it produces only one monochloro derivative on photochemical chlorination.

Step 1: Analyze the condition regarding monochlorination.
Only those alkanes produce a single monochloro product in which all hydrogen atoms are equivalent. Examples: \[ CH_4,\quad C_2H_6,\quad (CH_3)_4C \] Among higher alkanes, neopentane \[ (CH_3)_4C \] gives only one monochloro derivative because all twelve hydrogen atoms are equivalent. Hence the alkane obtained after reduction must be \[ \boxed{\text{Neopentane}} \]

Step 2: Determine the alcohol required.
The corresponding alcohol is \[ (CH_3)_3CCH_2OH \] (neopentyl alcohol). Reaction sequence: \[ (CH_3)_3CCH_2OH \xrightarrow{PBr_3} (CH_3)_3CCH_2Br \] \[ (CH_3)_3CCH_2Br \xrightarrow{Zn/dil.H^+} (CH_3)_3CCH_3 \] which is neopentane.

Step 3: Find the Grignard reagent and carbonyl compound producing neopentyl alcohol.
Methanal reacts with tert-butyl magnesium bromide as: \[ HCHO + (CH_3)_3CMgBr \] followed by hydrolysis gives \[ (CH_3)_3CCH_2OH \] (neopentyl alcohol). Thus \[ A = HCHO \] and \[ B = (CH_3)_3CMgBr \]

Step 4: Verify the complete sequence.
\[ HCHO \xrightarrow[(H_3O^+)]{(CH_3)_3CMgBr} (CH_3)_3CCH_2OH \] \[ (CH_3)_3CCH_2OH \xrightarrow{PBr_3} (CH_3)_3CCH_2Br \] \[ (CH_3)_3CCH_2Br \xrightarrow{Zn/dil.H^+} (CH_3)_3CCH_3 \] (neopentane) \[ (CH_3)_3CCH_3 \xrightarrow{Cl_2/h\nu} \text{only one monochloro derivative} \] Hence all conditions are satisfied. Therefore, \[ \boxed{A=HCHO,\qquad B=(CH_3)_3CMgBr} \] \[ \boxed{\text{Correct Option = (4)}} \]