p-chloroaniline and anilinium hydrochloride can be distinguished by: 1\. Sandmeyer reaction 2\. \(\mathrm{NaHCO}_{3}\) 3\. \(\mathrm{AgNO}_{3}\) 4\. Carbylamine test (a) \(1,2,4\) (b) \(1,2,3\) (c) \(1,3,4\) (d) 1,2

The Sandmeyer reaction is a chemical transformation which is vastly used in the field of organic synthesis. This reaction specifically allows the conversion of primary aromatic amines into a range of aryl halides, cyanides, triflates, or even hydroxyl groups in certain cases. The process begins by treating the aromatic amine with sodium nitrite (\(\text{NaNO}_2\)) and a strong acid, like hydrochloric acid, under cold conditions. This step forms a diazonium salt which is an important intermediate. In the case of a halide conversion, the introduction of a copper(I) halide such as copper(I) chloride \(\text{CuCl}\) or copper(I) bromide \(\text{CuBr}\) will facilitate the replacement of the diazonium group by a halide ion.

While the Sandmeyer reaction is essential for modifying aromatic amines, it cannot distinguish between p-chloroaniline and anilinium hydrochloride because both compounds, through different mechanisms, could form a diazonium salt and undergo subsequent transformations. This means that, when both substances undergo this reaction, they yield similar aryl halides or related products, leaving us without a discernible difference between the two compounds.

The \(\text{NaHCO}_3\) test, which involves reacting a substance with sodium bicarbonate, operates on a simple principle: it tests for the presence of acidic hydrogens. When \(\text{NaHCO}_3\) encounters an acid stronger than carbonic acid, it prompts a chemical reaction that releases carbon dioxide gas \(\text{CO}_2\). This is observable through the bubbling or effervescence typical of \(\text{CO}_2\) release.

When applied to distinguish between p-chloroaniline and anilinium hydrochloride, the test showcases their differing properties. p-Chloroaniline, being a weak base, does not react significantly with \(\text{NaHCO}_3\)—there is no acidic hydrogen capable of triggering the reaction. Conversely, anilinium hydrochloride responds readily to \(\text{NaHCO}_3\) due to the presence of hydrochloride, which is a strong acid component, and consequently, it releases \(\text{CO}_2\). Thus, this simple, yet powerful test effectively distinguishes the two compounds by demonstrating the gas evolution only with anilinium hydrochloride.

The carbylamine test is a compelling technique for identifying primary amines. When a primary amine is heated with chloroform (\(\text{CHCl}_3\)) and an alkali, typically potassium hydroxide (\(\text{KOH}\)), a distinctive and unpleasant-smelling isocyanide (or carbylamine) is formed. This test is highly selective for primary amines; secondary and tertiary amines do not give a positive result, and hence don't form the foul-smelling isocyanide.

In the context of differentiating p-chloroaniline and anilinium hydrochloride, the carbylamine test cannot offer a clear distinction, since both compounds are derivatives of primary amines and would both yield positive results. The process involves a deprotonation step where the amine reacts with the \(\text{KOH}\) to form a potassium amide intermediate, which then attacks the chloroform to eventually result in an isocyanide. In both cases, a foul smell would be detected, indicating the formation of an isocyanide — thus, not serving as a differentiating test between p-chloroaniline and anilinium hydrochloride.