Amine Properties

Amines are organic derivatives of ammonia in which one or more hydrogen atoms have been replaced by alkyl or aryl groups. They play a vital role in organic synthesis, biochemistry, and the manufacturing of dyes and pharmaceuticals.

Classification of Amines

Amines are classified as primary, secondary, or tertiary depending on the number of carbon atoms directly attached to the nitrogen atom. This is different from the classification of alcohols or halogenoalkanes, which depends on the carbon atom holding the functional group.

If a fourth group bonds to the nitrogen atom via a dative covalent bond using the nitrogen lone pair, a quaternary ammonium salt is formed (general formula: \\( \text{R}_4\text{N}^+ \text{X}^- \\)). In this case, the nitrogen atom carries a permanent positive charge, and the compound behaves as an ionic salt.

Basicity of Amines

According to the Brønsted-Lowry theory, a base is a proton acceptor. Amines act as weak bases because the nitrogen atom possesses a lone pair of electrons that can accept a proton (\( \text{H}^+ \)) by forming a dative covalent bond.

In aqueous solution, amines establish a dynamic equilibrium by accepting a proton from water: \[ \text{RNH}_2\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{RNH}_3^+\text{(aq)} + \text{OH}^-\text{(aq)} \]

The Basicity Trend

You must be able to compare and explain the relative basic strengths of ammonia, primary aliphatic amines (such as methylamine), and aromatic amines (such as phenylamine):

Let us look at why this trend exists by comparing the electron density on the nitrogen atom in each class of compound:

• Primary Aliphatic Amines: Alkyl groups are electron releasing. Through the inductive effect (specifically, the positive inductive effect, denoted +I), the alkyl group pushes electron density along the single covalent bond towards the nitrogen atom. This increases the electron density on the nitrogen atom, making its lone pair of electrons more attractive and available to accept a proton.
• Ammonia: Lacks any alkyl groups or aromatic rings. Its basicity represents the reference point where there is no electron releasing or electron withdrawing influence on the nitrogen atom.
• Aromatic Amines: The lone pair of electrons on the nitrogen atom is adjacent to the delocalised pi system of the benzene ring. Sideways overlap occurs between the orbital containing the lone pair and the p-orbitals of the carbon atoms in the ring. As a result, the lone pair becomes delocalised into the benzene ring. This significantly decreases the electron density on the nitrogen atom, making the lone pair far less available to accept a proton.

Preparation of Primary Aliphatic Amines

There are two primary methods for preparing aliphatic amines. You must understand the reagents, conditions, and chemical trade offs of each route.

Route 1: Halogenoalkanes with Excess Ammonia

Halogenoalkanes undergo nucleophilic substitution when reacted with ammonia. The ammonia acts as a nucleophile, attacking the electron deficient carbon atom bonded to the halogen: \[ \text{R-X} + 2\text{NH}_3 \rightarrow \text{R-NH}_2 + \text{NH}_4\text{X} \]

• Reagents: Halogenoalkane and excess concentrated ammonia.
• Conditions: Heated in ethanol solvent under pressure in a sealed tube (to prevent volatile ammonia gas from escaping).

Route 2: Reduction of Nitriles

Unlike the halogenoalkane route, reduction of nitriles provides a cleaner synthetic pathway that yields only the primary amine as a product.

Method A: Laboratory Reduction with LiAlH₄
Lithium tetrahydridoaluminate (LiAlH₄) is a powerful reducing agent, represented in equations as 4[H]: \[ \text{R-C}\equiv\text{N} + 4[\text{H}] \rightarrow \text{R-CH}_2\text{NH}_2 \] This reaction is carried out in dry ether solvent, followed by the addition of dilute acid to hydrolyse the intermediate complex.

Method B: Industrial Catalytic Hydrogenation
Industrially, using LiAlH₄ is too expensive. Instead, nitriles are reduced using hydrogen gas in the presence of a transition metal catalyst: \[ \text{R-C}\equiv\text{N} + 2\text{H}_2 \rightarrow \text{R-CH}_2\text{NH}_2 \] Typical catalysts include nickel (Ni), palladium (Pd), or platinum (Pt) at elevated temperatures and pressures.

Preparation of Aromatic Amines

Aromatic amines like phenylamine are prepared by the reduction of nitro compounds, such as nitrobenzene. This is a multi step process:

\( \text{Nitrobenzene } (\text{C}_6\text{H}_5\text{NO}_2) \xrightarrow{\text{Sn / conc. HCl}} \text{Phenylammonium chloride} \xrightarrow{\text{NaOH}} \text{Phenylamine } (\text{C}_6\text{H}_5\text{NH}_2) \)

Step 1: Reduction of Nitrobenzene

Nitrobenzene is heated under reflux with tin (Sn) and concentrated hydrochloric acid (HCl). The tin and acid act as the reducing agent (represented as 6[H]): \[ \text{C}_6\text{H}_5\text{NO}_2 + 6[\text{H}] \rightarrow \text{C}_6\text{H}_5\text{NH}_2 + 2\text{H}_2\text{O} \]

Because the reaction mixture is highly acidic, the basic phenylamine product immediately reacts with the excess HCl to form a soluble salt: phenylammonium chloride (\( \text{C}_6\text{H}_5\text{NH}_3^+ \text{Cl}^- \)).

Step 2: Liberating the Phenylamine

To obtain the free organic amine, sodium hydroxide (NaOH) solution is added to neutralise the acid and deprotonate the phenylammonium ion: \[ \text{C}_6\text{H}_5\text{NH}_3^+ + \text{OH}^- \rightarrow \text{C}_6\text{H}_5\text{NH}_2 + \text{H}_2\text{O} \] The phenylamine can then be separated from the mixture using steam distillation and solvent extraction.