What is the end product \(\mathrm{C}\) in this series? \(\mathrm{CH}_{3} \mathrm{CN} \stackrel{\mathrm{Na} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow} \mathrm{B}\) \(\stackrel{\mathrm{Cu} / 573 \mathrm{~K}}{\longrightarrow} \mathrm{C}\) (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CONH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Nucleophilic substitution reactions are pivotal in organic chemistry, particularly for NEET aspirants, who must grasp this mechanism to understand a wide range of synthetic processes. Imagine our starting molecule is a simple carbon-nitrogen compound like acetonitrile (CH_3CN). In such reactions, a nucleophile, which is rich in electrons, seeks to replace a leaving group attached to a carbon.

The transformation occurring when acetonitrile reacts with sodium in an ethanolic solution illustrates this process perfectly. Here the negatively charged ethoxide (C_2H_5O^-) ion, formed from ethanol, is the nucleophile that attacks the carbon attached to the cyano group (-CN). It's like a game of molecular tag—ethoxide replaces the cyano, resulting in ethylamine (CH_3CH_2NH_2).

For students, understanding the dance between these molecular entities is crucial. The cyano group leaves, the ethoxide ion attacks, and we get a new molecule, which in this case is compound A, ethylamine. This kind of reaction expands the toolkit for creating various organic compounds, a recurring theme in NEET questions.

Moving ahead to the diazotization reaction, this is where organic chemistry starts feeling like magic. The ethylamine formed in the previous step now plays with nitrous acid (HNO_2). Diazotization involves the conversion of primary amines to diazonium salts. It’s like our ethylamine has put on a fancy dress, becoming something new and reactive.

When we mix ethylamine with nitrous acid, under acidic conditions, a diazonium ion (R-N_2^+) forms momentarily. But this ion doesn’t linger; it’s unstable. It releases nitrogen gas (N_2), waving goodbye as it leaves the scene, and we're left with an alcohol—ethanol (CH_3CH_2OH) in our case, also known as compound B. It's like a sudden plot twist, which is essential for further transformations in organic synthesis.

For NEET examinees, understanding this reaction is not just about making a salt; it's recognizing the transient nature of an intermediate and its role in steering the pathway to a final product. The entire sequence is integral to organic synthesis problems that are sure to pop up in the exam.

Finally, let's turn up the heat with the dehydrogenation reaction. Imagine the ethanol we just made getting cozy with copper (Cu) at a toasty 573 K. Dehydrogenation is essentially the removal of hydrogen from a molecule—think of it as the compound going on a hydrogen diet.

In this high-temperature affair, ethanol is persuaded to relinquish a pair of hydrogen atoms. As these hydrogens leave, they allow the formation of a double bond between the carbon atoms, giving birth to an aldehyde, specifically ethanal (CH_3CHO), known as compound C in our journey.

This step is crucial for NEET candidates to comprehend. It demonstrates the ability to transform a relatively less reactive alcohol into a more reactive aldehyde, which can undergo further reactions, significantly diversifying the chemical repertoire. It's a prime example of how altering conditions can shift the outcome in organic chemistry, a concept that holds true in both the laboratory and multiple-choice questions.