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Q8.17(i):

Complete each synthesis by giving missing starting material, reagent or products.

Solution :

Step 1: Identifying the Substrate and Reaction Conditions

The provided reaction sequence is as follows:

$\text{C}_6\text{H}_5\text{CHO} \xrightarrow[2. \text{H}_3\text{O}^+]{1. \text{CH}_3\text{MgBr}} \text{Product (A)} \xrightarrow{\text{CrO}_3} \text{Product (B)}$

Step 2: Analysis of the Grignard Reaction (Formation of A)

The reaction of benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$) with methylmagnesium bromide ($\text{CH}_3\text{MgBr}$) is a nucleophilic addition to the carbonyl group [Per the Mechanism of Nucleophilic Addition to Aldehydes].

  • The nucleophilic methyl group ($\text{CH}_3^-$) from the Grignard reagent attacks the electrophilic carbonyl carbon of the aldehyde.
  • This results in an alkoxide intermediate: $\text{C}_6\text{H}_5-\text{CH}(\text{O}^-)-\text{CH}_3$.
  • Upon acidic workup ($\text{H}_3\text{O}^+$), the alkoxide is protonated to form a secondary alcohol.

Product (A): $1\text{-phenylethanol}$ ($\text{C}_6\text{H}_5\text{CH}(\text{OH})\text{CH}_3$)

Step 3: Analysis of the Oxidation Reaction (Formation of B)

The subsequent reaction involves treating the secondary alcohol, $1\text{-phenylethanol}$, with chromium trioxide ($\text{CrO}_3$).

  • $\text{CrO}_3$ is a strong oxidizing agent [Per Jones Oxidation conditions].
  • Secondary alcohols are oxidized to ketones under these conditions [Per the General Oxidation Principle of Alcohols].
  • The hydroxyl group is removed along with the alpha-hydrogen, establishing a double bond between the carbon and oxygen.

Reaction Scheme: $\text{C}_6\text{H}_5\text{CH}(\text{OH})\text{CH}_3 + [\text{O}] \rightarrow \text{C}_6\text{H}_5\text{COCH}_3 + \text{H}_2\text{O}$

Product (B): Acetophenone ($\text{C}_6\text{H}_5\text{COCH}_3$)

Visual Representation of the Transformation

C6H5CHO 1. CH3MgBr 2. H3O+ C6H5CH(OH)CH3 CrO3 C6H5COCH3

Summary of Chemical Conversions

Step Reagent Process Result
1 $\text{CH}_3\text{MgBr}$ Nucleophilic Addition $1\text{-phenylethanol}$
2 $\text{CrO}_3$ Oxidation $\text{Acetophenone}$

Final Solution: Product (A) is $1\text{-phenylethanol}$ ($\text{C}_6\text{H}_5\text{CH}(\text{OH})\text{CH}_3$) and Product (B) is Acetophenone ($\text{C}_6\text{H}_5\text{COCH}_3$).


More Questions from Class 12 Chemistry Aldehydes Exercises


CBSE Solutions for Class 12 Chemistry Aldehydes


Chapters in CBSE - Class 12 Chemistry


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