Anilines (C6H5NH2) - Formula, Preparation, Structure, Properties, FAQs

Aniline

Aniline is the simplest organic compound that is an aromatic benzene with the chemical formula- $\mathrm{C}_6 \mathrm{H}_5-\mathrm{NH}_2$. Aniline is an aromatic amine in which an amine group (-NH₂) is attached to a benzene group. Therefore, The Functional group of Aniline is amino group.

Nomenclature of Aniline

• The General common name for all aromatic amines is aryl amine.
• IUPAC name for aromatic amine is benzenamine. For example, the IUPAC name of aniline is benzenamine because aniline is the simplest aromatic amine.
• Aniline and benzylamine are two different compounds. Aniline is an aromatic amine; however, benzylamine is an aliphatic amine, which means the amine is not directly attached to an aromatic system in benzylamine.

Physical Properties of Aromatic Amines

The physical properties of aromatic amines depend on the presence of the amino group (–NH₂) attached directly to an aromatic ring. Some important physical properties are:

1. State and Appearance: Lower aromatic amines are generally colorless liquids, while higher members may be solids. On exposure to air, they often become yellow or brown due to oxidation.
2. Odour: Aromatic amines possess a characteristic unpleasant fishy or amine-like smell.
3. Boiling Points: Aromatic amines have relatively high boiling points because of intermolecular hydrogen bonding. Their boiling points are generally higher than those of hydrocarbons having similar molecular masses.
4. Solubility in Water: Lower aromatic amines are only slightly soluble in water. Their solubility is lower than that of aliphatic amines because the large hydrophobic aromatic ring reduces interaction with water molecules.
5. Solubility in Organic Solvents: Aromatic amines are readily soluble in organic solvents such as alcohol, ether, benzene, and chloroform.
6. Density: Most aromatic amines are denser than aliphatic amines but are generally less dense than water.
7. Hydrogen Bonding: Primary and secondary aromatic amines can form intermolecular hydrogen bonds due to the presence of N–H bonds, which influences their boiling points and physical state.

Isomerism in Aniline

• Aromatic amine can show position isomerism when another group such as methyl or nitro group are introduced on the benzene ring. The positions of these groups can change in position with respect to amine group.
• Ortho, para and meta are three positions for amine group with respect to methyl. All these three structures will give position isomers of aniline.

Basicity of Aniline

Aniline $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2\right)$ is an aromatic amine in which the amino group $\left(-\mathrm{NH}_2\right)$ is directly attached to a benzene ring. Aniline behaves as a weak base because the lone pair of electrons on the nitrogen atom can accept a proton $\left(\mathrm{H}^{+}\right)$. In aniline, the lone pair of electrons on the nitrogen atom is involved in resonance with the benzene ring. Due to delocalisation, the electron density on nitrogen decreases, making the lone pair less available for donation to a proton.

Resonance in Aniline

The lone pair on nitrogen participates in conjugation with the π-electrons of the benzene ring:

As a result, the lone pair is partially shared with the ring, reducing the tendency of nitrogen to donate electrons.

Comparison with Ammonia

Aniline is less basic than ammonia because:

• In ammonia (NH₃), the lone pair is localized on nitrogen and is readily available for protonation.
• In aniline, the lone pair is delocalized into the benzene ring through resonance.

Therefore,
Basicity: $N H_3>C_6 H_5 N H_2$

Effect of Substituents on Basicity

Electron-Donating Groups (+I or +M Effect)

Groups such as –CH₃, –OCH₃, –NH₂ increase electron density on nitrogen and increase the basic strength of aniline.Example:

• p-Toluidine > Aniline

Electron-Withdrawing Groups (–I or –M Effect)

Groups such as –NO₂, –CN, –COOH withdraw electron density and decrease the basic strength of aniline. Example:

• Aniline > p-Nitroaniline
• Anilinium ion- This ion is formed after the Amine group attached to benzene group gets protonated and nitrogen acquires a positive charge. The resonating structures for this ion are only two and we know the more is the number of resonating structures more is the stability of that compound.

Preparation of Aniline

A variety of substrates of are used to synthesize aniline such as alkyl halides, nitro groups, nitriles, amides, benzamides, and benzoic acid.

1. From Alkyl halides

Nucleophilic substitution reaction of ammonia on alkyl halides takes place at 373 K in a sealed tube. An ammonium salt is formed which reacts with a base to form primary amine.

$\mathrm{RX}+\mathrm{NH}_3 \rightarrow \mathrm{RNH}_3{ }^{+} \mathrm{X}^{-}$

$\mathrm{RNH}_3{ }^{+} \mathrm{X}^{-}+\mathrm{NaOH} \rightarrow \mathrm{RNH}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{NaX}$

2. From Amides

Aniline can be prepared from benzamide by treating it with bromine and aqueous potassium hydroxide. This reaction is known as the Hofmann Bromamide Reaction or Hofmann Rearrangement.

$\mathrm{C}_6 \mathrm{H}_5 \mathrm{CONH}_2+\mathrm{Br}_2+4 \mathrm{KOH} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+2 \mathrm{KBr}+\mathrm{K}_2 \mathrm{CO}_3+2 \mathrm{H}_2 \mathrm{O}$

3. From Carboxylic acid

Carboxylic acids are used to synthesize primary amines in the presence of hydrazoic acid.

4. From Nitro Compounds

Nitro compounds get reduced in the presence of acid and active metals such as zinc, iron, and tin.

• However, on an industrial scale the best method for the production of aniline is the catalytic hydrogenation of nitrobenzene by hydrogen in the presence of platinum.
• Few other known methods for the synthesis of aniline are by the treatment of chlorobenzene with ammonia at 473 k and a pressure of 60 atm pressure.

Also read :

• NCERT solutions for Class 12 Chemistry Chapter 13 Amines
• NCERT Exemplar Class 12 Chemistry solutions Chapter 13 Amines
• NCERT notes Class 12 Chemistry Chapter 13 Amines

Reactions of Aniline

1. Reaction with Acid

Due to the presence of lone pair on nitrogen aniline behaves as basic in nature. Anilinium hydrochloride is the product that is formed. If a strong base is added, then aniline can be liberated.

2. Acylation

Benzenamine reacts with acetyl chloride in the presence of base such as pyridine to form analide. A hydrogen group of -NH₂ is replaced with –RCO group. The presence of base is necessary for the removal of Cl of acid chloride. Base should be stronger than ammonia to shift the equilibrium to the right side of the reaction.

Aniline reacts with acetyl chloride or acetic anhydride to form acetanilide.
With Acetyl Chloride:

$\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+\mathrm{CH}_3 \mathrm{COCl} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NHCOCH}_3+\mathrm{HCl}$

With Acetic Anhydride:

$\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+\left(\mathrm{CH}_3 \mathrm{CO}\right)_2 \mathrm{O} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NHCOCH}_3+\mathrm{CH}_3 \mathrm{COOH}$

Analide formula is N- Phenyl ethanamide or acetanilide.

Analide structure is shown below-

3. Alkylation of Aniline

It is possible to introduce a methyl group onto the nitrogen atom of -NH₂ group by the removal of Hydrogen. An alkyl halide is introduced for the nucleophilic substitution reaction. Here the amine will act as nucleophile and alkyl halide performs the attack.

Also check-

Some Solved Examples

Question 1: Arrange the following amines in the decreasing order of basicity:

1) $\mathrm{I}>\mathrm{II}>\mathrm{III}$

2) $\mathrm{III}>\mathrm{I}>\mathrm{II}$

3) $\mathrm{III}>\mathrm{II}>\mathrm{I}$

4) $\mathrm{I}>\mathrm{III}>\mathrm{II}$

Solution:

The general order of basicity -

Aliphatic Amines > Ammonia > Aromatic Amines

Pyridine is more basic than pyrrole as the lone pairs over N in pyrrole is involved in aromaticity

Hence, the correct answer is option (2)

Question 2: The increasing order of basicity of the following compounds is :

1) (A) < (B) < (C) < (D)

2) (B) < (A) < (D) < (C)

3) (D) < (A) < (B) < (C)

4) (B) < (A) < (C) < (D)

Solution:

Therefore, the correct order of Basic strength is (B) < (A) < (D) < (C).

Hence, the correct answer is option (2).

Question 3:

Considering the above reaction, X and Y respectively are :

1)

2)

3)

4)

Solution:

The reaction will be-

Hence, the correct answer is option (4)