Question

Write the reactions of (i) aromatic and (ii) aliphatic primary amines with nitrous acid.

Short answer

Aromatic amines form stable diazonium salts; aliphatic amines form alcohols and nitrogen gas.

Step-by-step solution

Understand Aromatic Amines

Aromatic primary amines, such as aniline \(C_6H_5NH_2\), react with nitrous acid to form diazonium salts. This is known as diazotization. The general reaction is: \C_6H_5NH_2 + HNO_2 + 2HCl \rightarrow C_6H_5N_2^+Cl^- + 2H_2O.

Understand Aliphatic Amines

Aliphatic primary amines, such as ethylamine \((CH_3CH_2NH_2)\), react with nitrous acid to produce alcohols, nitrogen gas, and water in a reaction called deamination. The general reaction is: \RNH_2 + HNO_2 \rightarrow ROH + N_2 + H_2O,\ where \R\ is an alkyl group.

Write Aromatic Reaction

For aniline reacting with nitrous acid: \[C_6H_5NH_2 + HNO_2 + 2HCl \rightarrow C_6H_5N_2^+Cl^- + 2H_2O \rightarrow Phenyl\ diazonium\ chloride \ (stable\ in\ cold\ solutions)\\]

Write Aliphatic Reaction

For ethylamine reacting with nitrous acid: \[CH_3CH_2NH_2 + HNO_2 \rightarrow CH_3CH_2OH + N_2 + H_2O \rightarrow Ethanol\ (and\ is\ not\ stable)\\]

Key concepts

Aromatic Amines

Aromatic amines are compounds that contain an amino group attached to an aromatic ring. An example of an aromatic amine is aniline, with the chemical formula \(C_6H_5NH_2\). These amines are characterized by the presence of a benzene ring, which contributes to their unique properties.
One important reaction involving aromatic amines is their interaction with nitrous acid. When aromatic primary amines react with nitrous acid in an acidic solution, diazonium salts are formed through a process known as diazotization. This process replaces the amino group with a diazonium group \(N_2^+\). The stability of the resulting diazonium salt depends on the conditions, such as temperature. It is typically stable in cold solutions and can participate in various reactions, including azo-coupling.

Aliphatic Amines

Aliphatic amines differ from aromatic amines in that they do not contain an aromatic ring. These amines are composed of straight or branched chains of carbon atoms with an attached amino group. A common example is ethylamine \(CH_3CH_2NH_2\).
Reactions of aliphatic amines with nitrous acid lead to deamination. Unlike aromatic amines, they do not form stable diazonium salts. Instead, the reaction produces alcohol, nitrogen gas, and water. This transformation is less controlled due to the instability of the products, especially at room temperature.

• The primary outcome of this reaction is the loss of the amino group as nitrogen gas \(N_2\).
• A secondary alcohol is formed as the rest of the molecule reconfigures.

This process emphasizes the key differences between aliphatic and aromatic amine reactions with nitrous acid.

Diazotization

Diazotization is a crucial chemical reaction where primary amines are converted into diazonium salts under acidic conditions. This process mainly involves aromatic amines due to their ability to stabilize the diazonium ion \(N_2^+\).
During diazotization, the amino group of the aromatic amine combines with nitrous acid, typically in the presence of hydrochloric acid, leading to the formation of diazonium chloride and water.
The reaction can be expressed as: \[C_6H_5NH_2 + HNO_2 + 2HCl \rightarrow C_6H_5N_2^+Cl^- + 2H_2O\] This transformation is a key step in the synthesis of azo compounds, which have applications in dye and pigment industries.
The stability of diazonium salts makes them versatile intermediates for further chemical reactions, enhancing their importance in organic synthesis.

Deamination

Deamination refers to the removal of an amino group from a molecule, and this is evident in the reaction of aliphatic amines with nitrous acid. In the case of aliphatic amines like ethylamine \(CH_3CH_2NH_2\), deamination leads to the formation of alcohol, release of nitrogen gas \(N_2\), and water.
During this process, the original amino group is effectively replaced by a hydroxyl group, resulting in the conversion from an amine to an alcohol.

• The reaction proceeds as follows: \[RNH_2 + HNO_2 \rightarrow ROH + N_2 + H_2O\]
• This reaction is less stable and more prone to decomposition compared to diazotization in aromatic amines.

Deamination plays a pivotal role in both biological processes and synthetic chemistry, particularly in altering the structure of organic compounds.