Which of the following reagents would not be a good choice for reducing an aryl nitro compound to an amine?

LiAlH₄ in ether

The correct answer is option 2. \(LiAlH_4\) in ether. 

Reducing an aryl nitro compound \((Ar-NO_2)\) to an aryl amine \((Ar-NH_2)\) typically involves converting the nitro group \((-NO_2)\) to an amine group \((-NH_2)\). Several reagents can achieve this reduction, but not all reagents are equally suitable or effective for this transformation. Let's examine each reagent and its suitability for reducing aryl nitro compounds to amines:

1. \(H_2\, \ (excess)/Pt\):

Hydrogen gas in the presence of a platinum catalyst is a powerful reducing agent. This method can effectively reduce nitro groups to amines. It is often used in catalytic hydrogenation reactions. This reagent is suitable for reducing aryl nitro compounds to amines.

2. \(LiAlH_4\) in ether:

Lithium aluminum hydride \((LiAlH_4)\) is a very strong reducing agent, but it is typically used for reducing carboxylic acids, esters, and amides to alcohols or amines. \(LiAlH_4\) is not commonly used for reducing nitro groups directly to amines because it can lead to over-reduction and unwanted side reactions. Moreover, nitro groups are not easily reduced by \(LiAlH_4\). This reagent is not suitable for reducing aryl nitro compounds to amines.

3. \(Fe\) and \(HCl\):

Iron filings \((Fe)\) in the presence of hydrochloric acid \((HCl)\) is a classical method for reducing nitro groups to amines. The iron acts as a reducing agent and the acidic medium helps in the reduction process. This method is effective and widely used for the reduction of aryl nitro compounds to amines.

4. \(Sn\) and \(HCl\):

Tin \((Sn)\) in the presence of hydrochloric acid \((HCl)\) is another classical and effective method for reducing nitro groups to amines. The tin acts as a reducing agent and the acid provides the necessary protons for the reduction. This reagent is suitable for reducing aryl nitro compounds to amines.

Conclusion

Among the given reagents, \(LiAlH_4\) in ether is not a good choice for reducing an aryl nitro compound to an amine. This is because \(LiAlH_4\) is not typically used for this transformation and can lead to over-reduction or unwanted side reactions. The other reagents (\(H_2/Pt\), \(Fe/HCl\), and \(Sn/HCl\)) are effective for reducing nitro groups to amines.