Preparation and properties of Aromatic Nitrocompounds

Nitroaromatic compounds form an exciting class of organic compounds that find very useful applications in industry and academia. These compounds, characterized by the presence of at least one nitro group (-NO₂) attached to an aromatic ring, are not chemical oddities, but they hold an important place in the synthesis of a large array of products: dyes, medicines, and explosives. For instance, the bright colors of the clothes worn and many life-saving medicines used can be traced back to the chemistry of aromatic nitro compounds.

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This Story also Contains

1. Mechanism of diazotisation
2. Properties of Nitrocompounds and Mulliken-Barker Test
3. Preparation of Aromatic Nitro compounds
4. Basicity of Aromatic Amines
5. Relevance and Applications
6. Some Solved Examples
7. Summary

The dyes used in clothes and food, pharmaceuticals treating infections and chronic diseases—and even some agricultural chemicals—all have their roots in the chemistry of nitro compounds. It is their unique properties that make them indispensable for many chemical processes, so long as they represent high polarity and reactivity. The following paper represents an attempt at a general survey concerning aromatic nitro compounds, from their preparation and properties to the importance of azo-coupling reactions.

Mechanism of diazotisation

For example,

Properties of Nitrocompounds and Mulliken-Barker Test

Typical chemical properties of nitro compounds are functions of the electron-withdrawing character of the nitro group. This forms a very important factor in their reactivity and stability. Nitro compounds are highly polar and hence soluble in polar solvents. They usually have higher boiling points than the non-nitro derivatives since there are strong intermolecular forces.

The Mulliken-Barker test is a qualitative test for nitro groups in compounds. A strong acid will be added to the compound, which aids in the formation of nitro-derivatives. Color change in the resulting product indicates nitro groups. Hence, it becomes a very easy method to establish the existence of nitro groups. Thus, this test becomes very useful in confirming nitro compounds in different samples, facilitating education and industrial applications.

Reduction in acidic medium

Both aliphatic and aromatic nitro compounds are reduced to corresponding primary amines with Sn$\mathrm / \mathrm{HCl}$ or $\mathrm{Fe} / \mathrm{HCl}$ by a combination of some active metals like $t$, inon or zinc and conc. HCl or catalytic reduction with $\mathrm{Ni}, \mathrm{Pt}$ or $\mathrm{Pd} / \mathrm{C}$. The reaction occurs as follows:

$\mathrm{C}_6\mathrm{H}_5\mathrm{NO}_2+6\mathrm{H}\rightarrow \underset{\text { or } \mathrm{Sn} / \mathrm{HCl}}{\mathrm{Fe} / \mathrm{HCl}} \rightarrow\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+2 \mathrm{H}_2 \mathrm{O}$

Reduction in basic medium

Both aliphatic and aromatic nitro compounds are reduced to corresponding hydroxylamines in the neutral medium with zinc dust and $\mathrm{NH}_4 \mathrm{Cl}$ or $\mathrm{Al}-\mathrm{Hg}$ couple.

Reduction in neutral medium

Both aliphatic and aromatic nitro compounds are reduced to corresponding hydroxylamines in the neutral medium with zinc dust and NH₄Cl or Al-Hg couple. The reaction occurs as follows:

$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NO}_2+4 \mathrm{H} \xrightarrow[\Delta]{\mathrm{Zn} / \mathrm{NH}_4 \mathrm{Cl}, \mathrm{H}_2 \mathrm{O}} \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NHOH}+\mathrm{H}_2 \mathrm{O}$

Mulliken barker test

The test is based on the fact that a neutral agent, $-\mathrm{NO}_2$ is reduced to the NHOH group. The formed hydroxylamine reduces the Tollen's reagent and gets oxidised to a nitroso compound. The reaction occurs as follows:

Preparation of Aromatic Nitro compounds

The typical synthesis for the preparation of aromatic nitro compounds is the process of the nitration of aromatic hydrocarbons. This is usually an electrophilic substitution where an aromatic compound reacts with a nitrating agent; commonly, this is a mixture of concentrated nitric acid and sulfuric acid. In the reaction, the nitronium ion acts as an electrophile, which is formed in the course of the reaction.

For example, nitration of benzene results in nitrobenzene, which is an important component of organic chemistry. The reaction conditions can be altered so that substitution on the aromatic ring represents ortho-, meta-, or para-nitro compounds. These nitro compounds can also be further modified and reacted, such as reduction to amines or coupling reactions to form azo dyes, making them very versatile in synthetic chemistry.

By nitration of benzene with a mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature below 330K. The temperature should not increase otherwise m-dinitrobenzene is formed. So, the nitration of benzene depends upon the temperature and nature of the nitrating agent used. The reaction occurs as follows:

1. Nitrobenzene can be prepared by carrying out our diazotization of aniline and then reacting with nitrous acid( $\mathrm{HNO}_2$ ) in the presence of cuprous oxide. The reaction occurs as follows: $\mathrm{C}_6 \mathrm{H}_5 \mathrm{~N}_2 \mathrm{Cl}+\mathrm{HNO}_2 \xrightarrow{\mathrm{Cu}_2 \mathrm{O}} \mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+\mathrm{N}_2+\mathrm{HCl}^{-20}$
2. Nitrobenzene can be prepared from the oxidation of aniline by trifluoroacetic acid. The reaction occurs as follows:
$\mathrm{C}_6\mathrm{H}_5 \mathrm{NH}_2+3[\mathrm{O}]\rightarrow{\mathrm{CF}_3 \mathrm{COOH}}\rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+\mathrm{H}_2 \mathrm{O}$
3. Nitrobenzene can be prepared by the action of acetyl nitrate on benzene.
$\mathrm{C}_6 \mathrm{H}_5 \mathrm{H}+\mathrm{CH}_3 \mathrm{COONO}_2 \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+\mathrm{CH}_3 \mathrm{COOH}$

Basicity of Aromatic Amines

One of the exciting features of aromatic amine chemistry deals with basicity, most especially structure and substituents. While the general definition of an aromatic amine is formed by attaching an amino group (-NH₂) to an aromatic ring, the substituents on the ring result in compounds that have differing extents of basicity due to electron-donating or withdrawing effects.

Generally, aromatic amines are less basic than aliphatic amines due to the resonance stabilization of the lone pair on nitrogen with the aromatic system. Electron-donating groups, like alkyl groups, can place more electron density on nitrogen, thereby increasing the basicity of the aromatic amine. On the other hand, electron-withdrawing groups like nitro groups reduce basicity because of resonance stabilization of the lone pair.

One of the prime interests of aromatic amines is basicity, particularly since they are the intermediates in dye and pharmaceutical syntheses. Basicity knowledge helps define the chemist: how such compounds are going to behave when reacting and, therefore, assist in designing more efficient pathways of synthesis.

Aniline and other aromatic amines are far less basic than ammonia and aliphatic amines. Aniline is a weak base as it forms salts with strong mineral acids. The weaker basic nature of aniline as compared to aliphatic amines can be explained based on resonance. In aliphatic amines, the non-bonding electron pair of N is localized and is fully available for coordination with a proton. On the other hand, in aniline or other aromatic amines, the non-bonding electron pair is delocalized into a benzene ring by resonance and the electron-donating capacity of nitrogen for protonation is considerably decreased to that of NH₃ and aliphatic amines.

Lower stability of anilinium ion than aniline

Anilinium ion formed by aniline on accepting a proton is less resonance stabilized than aniline.

Thus, the electron density is less on the N-atom due to which aniline or other aromatic amines are less basic than aliphatic amines.

Relevance and Applications

The chemistry of aromatic nitro compounds and their derivatives extends into areas as far apart as pharmaceuticals and materials science. In the pharmaceutical industry, nitro compounds act as important starting materials in the synthesis of a wide range of drugs, from antibiotics to anti-inflammatory agents. Here, their reduction properties to amines are very useful, especially since amines can form an important part of drug molecules.

One of the important applications of aromatic nitro compounds in the dye industry is the azo-coupling reaction. This means coupling a diazonium salt with another aromatic compound, usually a nitro compound to produce azo dyes. Azo dyes are colorful and applied in many aspects from the textile and food industries due to their colors to other industries where coloration is required like in clothes and art materials.

The research on aromatic nitro compounds brings one directly into the field of environmental chemistry, where their degradation products are monitored for determination of the extent of pollution. Since nitro compounds in the environment are used to point out industrial contamination, their detection has become very important for the control and protection of the environment.

The application of aromatic nitro compounds in an academic setting allows students to perform a hands-on exercise of organic synthesis, mechanisms of reactions, and analytical techniques. Laboratory experiments on the preparation and characterization of such compounds deepen a student's knowledge about the very basics of chemistry and prepare a career in chemistry or related fields.

Recommended topic video on (Preparation and properties of Aromatic Nitrocompounds )

Some Solved Examples

Example 1
Question: The increasing order of diazotization of the following compounds is:
(a) Aniline
(b) 4-Acetamidoaniline
(c) 4-Methoxyaniline
(d) 3-Acetamidoaniline

Solution:
The diazonium ions of aliphatic amines are very unstable and produce carbocation immediately, which can produce different products.
Diazonium salts of aromatic amines are comparatively more stable and evolve nitrogen only on heating. These diazonium salts can be isolated at low temperatures.
Diazotization of aromatic amine is easier than that of aliphatic amine. Aromatic diazonium salts are more stable than aliphatic diazonium salts due to resonance. Electron-donating substituents increase electron density on the benzene ring, increasing the stability of diazonium salts. Electron-withdrawing substituents decrease electron density on the benzene ring, decreasing the stability of diazonium salts. The COCH₃ group is electron-withdrawing and hence, (d) is less stable than (b). Although -O-COCH₃ is an electron-donating substituent, it is present in the meta position. Hence, it will not have a significant effect on stability.

The increasing order of diazotization is (a)<(d)<(b)<(c).

Hence, the correct answer is Option (2).

Example 2
Question:

The major product formed in the reaction given below will be:

1)

2)

3) (correct)

4)

Solution:

Solution

The diazonium ions of aliphatic amines are very unstable and produce carbocation immediately, which can produce different products. It is to be noted that the carbocation can rearrange and undergo ring expansion

So, the reaction will be -

Hence, the correct answer is Option (3)

Example 3
Question: The reaction of aromatic diazonium ions where 2 N atoms are retained is called:
1) Retention reaction
2) Nitrogen retention reaction
3) (correct) Coupling reaction
4) Nitrogen coupling reaction

Solution:
Benzene diazonium salts react with highly reactive compounds such as phenols and amines to form brightly coloured azo compounds. This reaction is called a coupling reaction. Coupling with phenols occurs in the basic medium (pH 9-10) and that of amines occurs in a fairly acidic medium (pH 4-5) at 273-298K. It is to be noted that the diazo coupling reaction is an example of an Electrophilic aromatic substitution reaction and happens on activated benzene rings.

The reaction of aromatic diazonium ion can be divided into two groups: those which involve the liberation of nitrogen gas by another univalent group and those in which the two N atoms are retained (coupling reactions).

Hence, the answer is Option (3).

Example 4
Question: Coupling reaction involves what mechanism?
1) Nucleophilic substitution
2) Electrophilic addition
3) Nucleophilic addition
4) (correct) Electrophilic substitution

Solution:
Diazonium ions of aromatic amines also undergo a coupling reaction with aromatic rings having a strong activating group to form diazo compounds. N in the diazo group acts as an electrophile and H gets substituted.

Correct option: 4.

Summary

The importance of aromatic nitro compounds does not stop at a laboratory scale; it is useful in finding life-saving medicines, vibrant dyes, and environmental monitoring methods. Comprehending the knowledge of the chemistry of these compounds allows for insight into their application in real-life scenarios and how much contribution they make toward furthering scientific knowledge. It provides an even deeper appreciation of organic chemistry and the robust impact such compounds make in our world.