Account for the following observations (i) Acetonitrile is preferred as a solvent for many organic reactions. (ii) Benzamide is not soluble in hydrochloric acid, whereas aniline is soluble, though both contain the \(-\mathrm{NH}_{2}\) group.

Acetonitrile (\(\mathrm{CH}_{3}\mathrm{CN}\)) is a widely used polar aprotic solvent. Polar aprotic solvents have a significant dipole moment and lack acidic protons available for hydrogen bonding. Acetonitrile has low toxicity, low miscibility with water, and the ability to dissolve a wide range of organic solutes.

Due to its polar aprotic nature, acetonitrile can dissolve a wide variety of organic compounds. Furthermore, it can stabilize the transition states of reactions, reducing the activation energy required for the reaction to occur. Its low boiling point makes it easy to remove after the reaction has taken place. Overall, its properties make it an excellent solvent choice for many organic reactions.

Benzamide is an amide with the chemical formula \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CONH}_{2}\), while aniline is an amine with the chemical formula \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\). Despite both containing the \(-\mathrm{NH}_{2}\) group, benzamide has a carbonyl group (\(-\mathrm{C=O}\)) in its structure, which plays a crucial role in its difference in solubility compared to aniline.

When placed in hydrochloric acid (\(\mathrm{HCl}\)), aniline reacts by donating a pair of electrons from the nitrogen in the \(-\mathrm{NH}_{2}\) group, forming a salt, which makes it soluble in the acidic solution: $$\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2} + \mathrm{HCl} \rightarrow \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{3}^{+}\mathrm{Cl}^{-}$$ However, benzamide does not react with hydrochloric acid due to its amide functionality. The carbonyl group (\(-\mathrm{C=O}\)) has a resonance structure that delocalizes the lone pair of electrons on the nitrogen, making it less available for reacting with \(\mathrm{HCl}\). Without any ionic interaction or formation, benzamide remains insoluble in hydrochloric acid.

In conclusion, acetonitrile is preferred as a solvent for many organic reactions due to its polar aprotic nature and ability to dissolve a wide range of organic compounds while stabilizing the reaction's transition states. Benzamide is not soluble in hydrochloric acid because its amide functionality prevents it from reacting with the acid, while aniline forms a soluble salt with the acid due to its primary amine functionality.