An Introduction
Organic chemistry is a beautiful and magical subject if you happen to understand the core of the subject. Aniline is one such interesting organic compound with versatile materials and diverse industrial applications.
This article is complied to discuss in detail the concept of Anilines. The sole aim is to make the students fall in love with the subject to an extent that scoring high marks will no longer be an unachievable task.
In this article, we shall be discussing -
Aniline - an introduction
What is Aniline?
Anilines Structure
Anilines Physical Properties
Anilines Reactions
Anilines applications
Frequently asked questions
What is Anilines?
Aniline is a type of organic base which is used in the making of several dyes, explosives, plastics, drugs, and rubber, and photographic chemicals. Anilines are the organic compounds that lie in the class of groups coming in the organic chemistry that is referred to as aminobenzene or phenylamine. These compounds are known to be toxic and to be one of the classes of the aromatic amines. They are used in a variety of industrial applications and possess all the characteristics of that of an aromatic compound. The aniline compounds are known to have the formula C6H5NH2 in which the amino group is attached to the phenyl group.
Aniline occurs in the form of a yellowish and slightly brownish oily liquid which has a fishy and a musty odour. It smells like a rotten fish. It is a chemical substance that is a flammable liquid and has a very unpleasant odour. The compound of aniline is soluble in water which is colourless to light brown. Its chemical formula is C6H5NH2 or C6H7N. Since it consists of 6 carbon atoms, 7 hydrogen atoms, and 1 nitrogen atom in its chemical formula, it is an organic compound. Today, we will learn about what is anilines, the phenylamine structure, its physical properties and uses.
Anilines Structure C6H5NH2 Structure
Given below is the aminobenzene structure.
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The structure of benzenamine has a pyramidal shaped molecule and has a nitrogen hybridization which lies in between sp3 and sp2. Due to this, the nitrogen lone pair lies in an spx hybrid orbital and has a high p character. The amino group in aniline is much flatter than that in an aliphatic amine, which is because of the conjugation of the lone pair of electrons along with the aryl substituent.
Anilines Physical Properties
Let us discuss the physical properties of anilines which are given below.
The boiling of aniline is 184.13 °C and its melting point is −6.3 °C.
The compound of aniline is slightly soluble in water and many times freely soluble in the chemicals like alcohol and ether.
This organic compound turns darker when it is exposed to light and air.
It is known to be a weak base and when it reacts with strong acids, it tends to form an anilinium ion C6H5N3+.
It is said to be toxic when it is inhaled via the air or tends to get absorbed into the skin since it produces nitrogen oxides that are harmful to the human body as well as the environment.
Anilines Reactions
Let us now take a look at some of the reactions of anilines and the aniline compounds.
Oxidation Reaction: Oxidation reactions of the anilines compounds tend to lead to the formation of the carbon-nitrogen bonds.
Basicity: Anilines are weaker bases and on reacting with stronger acids, the compounds form anilinium ions.
Acylation: In the acylation reaction, anilines tend to react readily with the carboxylic acids and form amides.
The other reactions of anilines consist of alkylation reaction, hydrogenation, Diazotization, Wohl-Aue reaction, etc.
Anilines Uses
Anilines are used in various fields of science and everyday life. Some of the uses of anilines are given below.
Anilines have their uses in the rubber industry to process the rubber chemicals and products like car tyres, gloves, balloons, etc.
It is also used as a dyeing agent for the manufacturing of clothes like jeans, etc.
It is used for the production of drugs, for example, paracetamol, acetaminophen, and Tylenol.
It is also used in the form of pesticides and fungicides when it comes to the agricultural industry.
It is also used for the manufacturing of polyurethane which is then used for making plastics.
FAQs on Anilines
1. What is aniline and what is its chemical formula?
Aniline is the simplest aromatic amine, consisting of a phenyl group (C₆H₅) attached to an amino group (-NH₂). It is also known by its IUPAC name, benzenamine. Its chemical formula is C₆H₅NH₂. It serves as a parent compound for a vast number of derivatives used in various industries.
2. Why is aniline a much weaker base than aliphatic amines like ethylamine?
Aniline is a significantly weaker base due to resonance. The lone pair of electrons on the nitrogen atom is delocalised into the benzene ring's π-electron system. This delocalisation makes the lone pair less available for donation to a proton. In contrast, in an aliphatic amine like ethylamine (CH₃CH₂NH₂), the ethyl group exerts a positive inductive effect (+I effect), which increases electron density on the nitrogen, making its lone pair more available and rendering it a stronger base.
3. What is the standard laboratory method to prepare aniline from benzene?
The preparation of aniline from benzene is a two-step process as per the NCERT syllabus for the 2025-26 session:
- Step 1: Nitration of Benzene. Benzene is treated with a nitrating mixture (concentrated HNO₃ and concentrated H₂SO₄) to form nitrobenzene.
- Step 2: Reduction of Nitrobenzene. The nitro group (-NO₂) of nitrobenzene is then reduced to an amino group (-NH₂). This is commonly achieved using a reducing agent like tin (Sn) and concentrated hydrochloric acid (HCl), or through catalytic hydrogenation (H₂/Pd).
4. How do the resonance structures of aniline explain its reactivity in electrophilic substitution?
The resonance structures of aniline show that the lone pair of electrons on the nitrogen atom is delocalised into the benzene ring. This process increases the electron density significantly at the ortho (2, 6) and para (4) positions. Consequently, the benzene ring in aniline is highly activated towards electrophilic attack, and incoming electrophiles are directed to these ortho and para positions. For example, aniline reacts with bromine water instantly to form a white precipitate of 2,4,6-tribromoaniline without any catalyst.
5. What are the major industrial uses of aniline?
Aniline is a crucial industrial chemical used as a starting material for many organic compounds. Its primary applications include:
- Dye Industry: It is a precursor for manufacturing a wide range of azo dyes and other pigments for textiles.
- Polymer Industry: It is used to produce methylene diphenyl diisocyanate (MDI), a key component in the manufacture of rigid polyurethane foams.
- Rubber Industry: It is used to make antioxidants and vulcanisation accelerators, which improve the durability of rubber products like tyres.
- Pharmaceuticals: It serves as a building block in the synthesis of various drugs, including paracetamol (acetaminophen).
6. Why is it necessary to acetylate aniline before carrying out its nitration?
Direct nitration of aniline using a nitrating mixture (conc. HNO₃ + conc. H₂SO₄) is not feasible for two main reasons. Firstly, the highly activating -NH₂ group leads to uncontrolled oxidation and charring of the ring. Secondly, in the strongly acidic medium, the -NH₂ group gets protonated to form the anilinium ion (-NH₃⁺), which is a meta-directing and deactivating group. To overcome this, the amino group is first protected by acetylation with acetic anhydride. This forms acetanilide, where the activating nature of the group is reduced, preventing oxidation and ensuring that the major product is p-nitroaniline.
7. What is the diazotization reaction and why is it important in organic chemistry?
Diazotization is the reaction of a primary aromatic amine, like aniline, with nitrous acid (HNO₂, prepared in-situ from NaNO₂ and a strong acid like HCl) at a low temperature of 0-5 °C. This reaction converts the amino group into a diazonium group (-N₂⁺Cl⁻), forming benzene diazonium chloride. This reaction is synthetically vital because the diazonium group is an excellent leaving group and can be easily replaced by a wide variety of nucleophiles (e.g., -Cl, -Br, -CN, -OH, -F, -H) in reactions like the Sandmeyer and Gattermann reactions, making it a versatile intermediate for synthesizing many different aromatic compounds.
8. How can aniline be chemically distinguished from N-methylaniline?
Aniline (a primary amine) can be distinguished from N-methylaniline (a secondary amine) using the carbylamine test (or isocyanide test). When aniline is warmed with chloroform (CHCl₃) and an alcoholic solution of potassium hydroxide (KOH), it produces phenyl isocyanide, a compound with a highly offensive and foul smell. N-methylaniline, being a secondary amine, does not give this test.
9. Why does pure, colourless aniline turn reddish-brown when exposed to air?
Pure aniline is a colourless oily liquid. However, it is highly susceptible to atmospheric oxidation, especially in the presence of light. Over time, it oxidises to form a complex mixture of coloured polymeric impurities and quinone-like compounds. This oxidation process is responsible for the characteristic dark reddish-brown colour that develops in a sample of aniline left exposed to air.
10. Is aniline considered harmful to humans?
Yes, aniline is toxic and can be harmful to humans upon exposure through inhalation, ingestion, or skin absorption. The primary health effect of aniline poisoning is methemoglobinemia, a condition where the iron in hemoglobin is oxidised from the ferrous (Fe²⁺) to the ferric (Fe³⁺) state. This prevents the red blood cells from effectively transporting oxygen, leading to symptoms like cyanosis (bluish skin), headache, dizziness, and shortness of breath. It is classified as a probable human carcinogen.
