Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the end product in the following sequence of operations? \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{PCl}_{3}}{\longrightarrow} \mathrm{B} \stackrel{\mathrm{NH}_{3}}{\longrightarrow} \mathrm{C}\) (a) ethyl cyanide (b) methylamine (c) ethylamine (d) acetamide

Step-by-Step Solution

Answer

The end product is ethylamine (option c).

Step 1: Initial Reaction with Nitrous Acid

The compound C2H5NH2 is ethylamine. When it reacts with nitrous acid (HNO2), this results in the formation of a diazonium salt, but due to the instability of the primary aliphatic diazonium salts, the compound is instead converted to ethanol (C2H5OH). Thus, compound A is C2H5OH.

Step 2: Reaction of A with Phosphorus Trichloride

Next, ethanol (C2H5OH), which is compound A, reacts with phosphorus trichloride (PCl3). In this step, the hydroxyl group in ethanol is replaced by a chlorine atom, yielding ethyl chloride (C2H5Cl) as compound B.

Step 3: Treatment of B with Ammonia

Finally, when ethyl chloride (C2H5Cl) reacts with ammonia (NH3), a substitution reaction occurs where the chlorine atom is replaced by an amino group to form ethylamine (C2H5NH2). Thus, compound C is C2H5NH2.

Key Concepts

Reactions of Amines

Amines are compounds derived from ammonia and are characterized by the presence of one or more alkyl or aryl groups replacing hydrogen atoms. When amines react with various reagents, such as nitrous acid, they undergo interesting transformations due to their structure and functional groups.
The reaction of primary amines like ethylamine (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\)) with nitrous acid (\(\mathrm{HNO}_{2}\)) typically results in the formation of diazonium salts. However, for primary aliphatic amines, this diazonium salt is highly unstable. Instead, a rapid decomposition into more stable compounds like alcohols occurs.
Understanding these transformations is crucial, as it provides insight into controlling chemical reactions for desired synthesis pathways. Primary amines' ability to form various products under specific reactions is a key area in organic chemistry research and applications.

Diazonium Salts

Diazonium salts are unique compounds formed by the reaction of primary aromatic amines with nitrous acid. Despite the fleeting nature of aliphatic diazonium salts, aromatic diazonium salts remain quite stable, allowing for versatile chemical manipulations.
In our reaction sequence, the initial intention is to create a diazonium salt from ethylamine, but due to its instability, the aliphatic diazonium salt quickly decomposes into ethanol, not allowing for isolation as a salt.

• Aliphatic diazonium compounds often decompose into alcohols.
• Aromatic versions play a crucial role in multiple synthesis routes due to their stability.

Grasping the instability of different diazonium salts based on their parent amine helps chemists predict the outcomes of reactions and choose the right conditions for desired chemical paths.

Substitution Reactions

Substitution reactions are a fundamental type of chemical reaction in organic chemistry, where an atom or group of atoms in a molecule is replaced by another atom or group. These reactions are classified mainly based on the type of reagents and conditions used.
In the context of our problem set, ethanol (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)) undergoes a substitution reaction with phosphorus trichloride (\(\mathrm{PCl}_{3}\)) where the chlorine atom replaces the hydroxyl group (OH). This transforms the alcohol into an alkyl halide, specifically ethyl chloride (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\)).
Such transformations are essential for synthetic organic chemistry, as they allow the conversion of more reactive intermediates, which can further undergo another transformation, such as reaction with ammonia to yield the original amine.

Alcohol to Alkyl Halide Conversion

Converting an alcohol to an alkyl halide is a common reaction in organic synthesis that allows for further modifications of the carbon skeleton. In these reactions, the hydroxyl group is replaced by a halogen via reagents such as phosphorus trichloride.
For the ethanol formed after the instability of diazonium salts, reacting with phosphorus trichloride results in ethyl chloride, essentially changing the functional group from OH to Cl. This chlorination is an essential step for generating reactive intermediates in synthesis.

• Alkyl halides serve as pivotal intermediates in creating more complex molecules.
• Their formation can involve mechanisms like nucleophilic substitution that enable various transformation pathways.

Learning this conversion method provides an excellent tool for chemists, as alkyl halides often serve as stepping stones in multi-step synthesis procedures.