Problem 45 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Nitrobenzene can be reduced to aniline by (i) \(\mathrm{H}_{2} / \mathrm{Ni}\) (ii) \(\mathrm{Sn} / \mathrm{HCl}\) (iii) \(\mathrm{Zn} / \mathrm{NaOH}\) (iv) \(\mathrm{LiAlH}_{4}\) (a) i, ii and iii (b) \(\mathrm{i}\) and ii (c) i, ii and iv (d) only ii

Step-by-Step Solution

Answer

The correct answer is (c) i, ii, and iv.

Step 1: Identify the Reaction Type

The reduction of nitrobenzene to aniline is a type of reduction reaction where the nitro group (-NO2) in nitrobenzene is reduced to an amino group (-NH2) in aniline.

Step 2: Evaluate the Reducers

Analyze each potential reducing agent: 1. \( \mathrm{H}_2 / \mathrm{Ni} \)2. \( \mathrm{Sn} / \mathrm{HCl} \)3. \( \mathrm{Zn} / \mathrm{NaOH} \)4. \( \mathrm{LiAlH}_4 \)However, \( \mathrm{Zn} / \mathrm{NaOH} \) is not typically used for the reduction of nitro groups in nitrobenzene to aniline.

Step 3: Match with Options

Reduce nitrobenzene to aniline:- \( \mathrm{H}_{2} / \mathrm{Ni} \)- \( \mathrm{Sn} / \mathrm{HCl} \)- \( \mathrm{LiAlH}_{4} \)Identify which list of reagents (\( i, ii, iii, iv \)) matches one of the provided options.

Step 4: Select Correct Answer

Based on the evaluation, the suitable reducing agents are:- \( \mathrm{H}_{2} / \mathrm{Ni}, \mathrm{Sn} / \mathrm{HCl}, \mathrm{LiAlH}_{4} \).Thus, option (c) matches these reagents.

Key Concepts

Nitrobenzene to Aniline

The transformation of nitrobenzene to aniline is an important reaction in organic chemistry. Nitrobenzene has a nitro group \((-NO_2)\) which is converted to an amino group \((-NH_2)\) in aniline during this process. This reaction is crucial in producing aniline, which is a valuable industrial chemical used in dyes, pharmaceuticals, and other important products.

This reaction is a reduction reaction where we aim to increase the hydrogen content in the compound. It's essential to understand that the nitro group's oxygen atoms are replaced by hydrogen atoms. This conversion allows nitrobenzene, an aromatic compound with relatively low reactivity, to become aniline, which is more reactive and can be used in various applications.

Reduction Reactions

Reduction reactions are processes in chemistry where a molecule, atom, or ion gains electrons, leading to a decrease in oxidation state. In the context of organic chemistry, reduction often involves adding hydrogen or removing oxygen from a molecule. When we discuss reducing nitrobenzene to aniline, this process specifically involves reducing the nitro group \((-NO_2)\) to an amino group \((-NH_2)\).

The key points in reduction reactions include:

• Identifying the group to be reduced.
• Selecting a suitable reducing agent or catalysts.
• Ensuring the reaction conditions favor reduction.

In our case, understanding how the nitro group is reduced with the right reagents and conditions helps demystify this important transformation.

Reducing Agents in Organic Chemistry

Reducing agents are substances that facilitate reduction reactions. In organic chemistry, they are crucial for converting various functional groups, including \(NO_2\) to \(NH_2\), as seen in the reduction of nitrobenzene to aniline. Choosing the right reducing agent depends on several factors, such as the specific reaction conditions and the desired product.

Common reducing agents include:

• Hydrogen gas \((H_2)\) with a nickel catalyst, which offers a direct method of hydrogenation.
• Metal acid combinations like tin \((Sn)\) with hydrochloric acid \((HCl)\), often used for reducing nitro compounds.
• Lithium aluminum hydride \((LiAlH_4)\), a powerful reducing agent used for more demanding reductions.

Each of these agents has its benefits and limitations depending on the context of the reaction. When dealing with nitrobenzene, understanding these options helps you choose the most efficient and practical method for its reduction to aniline.