Question

The reagent required for the following transformation
is

• A. NaBH₄
• B. LiAlH₄
• C. H₃B·THF
• D. Zn(Hg)/HCl

Answer 1

The Correct Option is D

$\text{1. The Transformation}$

The reaction is a specific reduction where a carbonyl group in an alkyl aryl ketone is converted to a methylene group, a process known as deoxygenation.

$$\text{R-C(=O)-R'} \xrightarrow{\text{Reduction}} \text{R-CH}_2\text{-R'}$$

The challenge is to perform this reduction on the $\text{ketone}$ group ($\text{Ph}-\text{C}=\text{O}$) without reducing the $\text{carboxylic acid}$ group ($\text{COOH}$).

$\text{2. Analysis of Reagents}$

$\text{Option 4: } \text{Zn}(\text{Hg})/\text{HCl}$ ($\text{Clemmensen Reduction}$)

Function: This is a classic method for the reduction of ketones and aldehydes to the corresponding alkanes (deoxygenation).

Conditions: It is carried out in strongly acidic conditions (concentrated $\text{HCl}$).

Selectivity: It works well for alkyl aryl ketones and is highly effective when the molecule contains functional groups that are stable to strong acid, such as carboxylic acids ($\text{COOH}$). The $\text{COOH}$ group will be protonated ($\text{COOH}_2^+$) but will not be reduced by this reagent.

Conclusion: This reagent selectively reduces the ketone to a $\text{CH}_2$ group, leaving the carboxylic acid intact, making it the perfect choice.

$\text{Option 1: } \text{NaBH}_4$ ($\text{Sodium Borohydride}$)

Function: A mild reducing agent that reduces aldehydes and ketones to alcohols.

Selectivity: It is not strong enough to reduce the ketone completely to an alkane ($\text{CH}_2$). It would produce the $\mathbf{\text{secondary alcohol}}$, 4-hydroxy-4-phenylbutanoic acid.

$\text{Option 2: } \text{LiAlH}_4$ ($\text{Lithium Aluminium Hydride}$)

Function: A powerful, non-selective reducing agent that reduces almost all carbonyl functional groups.

Selectivity: It reduces both the ketone ($\text{C}=\text{O}$) to a secondary alcohol and the carboxylic acid ($\text{COOH}$) to a primary alcohol ($\text{CH}_2\text{OH}$). The final product would be a diol, not the desired carboxylic acid.

$\text{Option 3: } \text{H}_3\text{B}\cdot\text{THF}$ ($\text{Borane-THF Complex}$)

Function: Borane is a powerful reducing agent, particularly for carboxylic acids.

Selectivity: It primarily reduces carboxylic acids ($\text{COOH}$) to primary alcohols ($\text{CH}_2\text{OH}$) faster than it reduces ketones. It would lead to the undesirable reduction of the $\text{COOH}$ group.

$\text{3. Final Reaction}$

The required transformation is achieved via the Clemmensen reduction:

$$\text{Ph}-\text{C}(=\text{O})-\text{CH}_2\text{CH}_2-\text{COOH} \xrightarrow{\text{Zn}(\text{Hg})/\text{HCl}} \text{Ph}-\text{CH}_2-\text{CH}_2\text{CH}_2-\text{COOH}$$

$$\text{The required reagent is } \mathbf{\text{Zn}(\text{Hg})/\text{HCl}}$$