Animated Solution for Chemistry - Organic Chemistry: The total number of reagents from those given below, that can convert nitrobenzene into aniline is ........ (Integer answer) I. $\text{Sn}-\text{HCl}$ II. $\text{Sn}-{\text{NH}}_4\text{OH}$ III. $\text{Fe}-\text{HCl}$ IV. $\text{Zn}-\text{HCl}$ V. ${\text{H}}_2-\text{Pd}$ VI. ${\text{H}}_2\text{-Raney nickel}$

A Reagent's Tale

Welcome to a classic exploration in organic chemistry! In this problem, we are presented with a lineup of six different reagents and tasked with identifying the ones capable of reducing nitrobenzene to aniline. This is a fundamental transformation, and understanding the nuances of the reaction medium is key to mastering it.

Let's first examine the acidic metal reducers. When active metals like tin ($\text{Sn}$), iron ($\text{Fe}$), or zinc ($\text{Zn}$) are placed in an acidic environment, such as hydrochloric acid ($\text{HCl}$), a vigorous reaction occurs. The metal reacts with the acid to release nascent hydrogen ($[\text{H}]$).

This nascent hydrogen is an incredibly potent reducing agent. It aggressively attacks the nitro group ($-{\text{NO}}_2$) on the benzene ring, stripping away the oxygen atoms and replacing them with hydrogen atoms, ultimately yielding the amino group ($-{\text{NH}}_2$). Therefore, the reagents $\text{Sn}-\text{HCl}$, $\text{Fe}-\text{HCl}$, and $\text{Zn}-\text{HCl}$ are all highly effective for this conversion.

Next, we turn our attention to catalytic hydrogenation. This method involves passing hydrogen gas (${\text{H}}_2$) over a finely divided metal catalyst. The catalyst provides a surface for the hydrogen molecules to dissociate and react with the nitrobenzene.

Common and highly efficient catalysts for this purpose include palladium ($\text{Pd}$) and Raney nickel (a specially prepared, highly porous form of nickel). This approach is often preferred in laboratory settings because it is clean and avoids the messy metal salts produced in acidic reductions. Thus, ${\text{H}}_2-\text{Pd}$ and ${\text{H}}_2-\text{Raney Ni}$ are also correct choices.

Now, we must address the outlier: $\text{Sn}-{\text{NH}}_4\text{OH}$. While tin is an active metal, ammonium hydroxide creates a basic medium. This is a crucial distinction.

In a basic or alkaline environment, the reduction of nitrobenzene does not proceed all the way to aniline. Instead, the intermediate reduction products (like nitrosobenzene and phenylhydroxylamine) tend to react with one another. This bimolecular coupling leads to the formation of compounds containing nitrogen-nitrogen bonds, such as azoxybenzene, azobenzene, or hydrazobenzene. Because it fails to produce aniline, this reagent is incorrect for our specific objective.

By carefully analyzing the reaction conditions, we have identified three acidic metal reducers and two catalytic hydrogenators that successfully convert nitrobenzene to aniline.

Counting them up, we have a total of 5 valid reagents. This problem beautifully illustrates how the choice of medium—acidic versus basic—can completely alter the course of an organic reaction!