Animated Solution for Chemistry - Organic Chemistry: Which one of the products of the following reactions does not react with Hinsberg reagent to form sulphonamide?

A Chemical Detective

Imagine you are a chemical detective tasked with identifying different types of amines. Your most trusted tool is the Hinsberg Reagent, chemically known as benzene sulphonyl chloride (${\text{Ph-SO}}_2\text{Cl}$).

This reagent is highly selective. It reacts beautifully with primary ($1^{\circ }$) and secondary ($2^{\circ }$) amines to form stable sulphonamides. However, it completely ignores tertiary amines and other nitrogen-containing groups like imines ($-\text{C=NH}$). Our mission in this problem is to find out which of the given reactions produces a compound that will not react with this detective reagent.

Let's look closely at options (a), (c), and (d). In all these cases, we are treating a nitrile (a cyano group, $-\text{C}\equiv \text{N}$) with incredibly strong reducing agents:

Option (a): Sodium amalgam in ethanol ($\text{Na/Hg, EtOH}$) is a classic method known as the Mendius reduction. Option (c): Lithium aluminium hydride (${\text{LiAlH}}_4$) is one of the most powerful reducing agents in organic chemistry. Option (d):* Hydrogen gas with a nickel catalyst (${\text{H}}_2\text{/Ni}$) performs a complete catalytic hydrogenation.

All these reagents have one thing in common: they don't stop halfway. They completely reduce the triple bond of the nitrile, converting the $-\text{C}\equiv \text{N}$ group all the way down to a primary amine group ($-{\text{CH}}_2{\text{NH}}_2$).

Because they all form primary amines, they will definitely react with the Hinsberg reagent. So, none of these can be our answer.

Now let's turn our attention to option (b). Here, cyanobenzene is reacting with stannous chloride and hydrochloric acid (${\text{SnCl}}_2+\text{HCl}$). This specific combination of reagents should immediately ring a bell—it's the famous Stephen reduction!

The Stephen reduction is a two-step process. First, the nitrile is reduced to an intermediate imine hydrochloride. Second, this intermediate is hydrolyzed with water (${\text{H}}_3{\text{O}}^{+}$) to form an aldehyde.

But there is a massive catch here! Look at the reaction arrow for option (b) again. There is no water provided for hydrolysis.

Without water, this reaction stops halfway. The cyanobenzene is reduced, but it gets stuck at the imine stage, forming benzaldimine ($\text{Ph-CH=NH}$).

As we established at the very beginning, our chemical detective, the Hinsberg reagent, only reacts with primary and secondary amines. It does not react with imines.

Since the product of reaction (b) is an imine and not a primary or secondary amine, it will fail the Hinsberg test. Therefore, option (b) is the correct answer. It's a beautiful example of how missing a single reagent (like water) completely changes the outcome of a chemical story!