Primary Aliphatic Amines

• Amines function as Bronsted-Lowry bases because the lone pair on the nitrogen atom can accept a proton ($H^+$) to form a dative covalent bond.

• The strength of the base is determined by the availability of this lone pair; the more electron-dense the nitrogen, the more readily it attracts protons.

• Positive Inductive Effect: Alkyl groups are electron-releasing, meaning they push electron density toward the nitrogen atom, increasing the availability of the lone pair.

• Consequently, primary aliphatic amines are stronger bases than ammonia, as the alkyl group enhances the nitrogen's electron density compared to the hydrogen atoms in ammonia.

1. Nucleophilic Substitution of Halogenoalkanes

• Reagents: Excess hot ethanolic ammonia under pressure.
• Process: The ammonia acts as a nucleophile, attacking the electron-deficient carbon in the halogenoalkane and displacing the halide ion.
• Condition: Using excess ammonia is critical to minimize further substitution reactions that would otherwise produce secondary or tertiary amines.

2. Reduction of Nitriles

• Reagents: $LiAlH_4$ in dry ether, or hydrogen gas ($H_2$) with a Nickel catalyst.
• Process: The $-C\equiv N$ group is reduced to a $-CH_2NH_2$ group, effectively adding two hydrogen atoms to both the carbon and nitrogen of the triple bond.

• Reaction with Water: Amines dissolve in water to form alkaline solutions by accepting a proton from water, leaving hydroxide ions: $RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-$.

• Reaction with Acids: They react with strong acids to form ionic ammonium salts (e.g., methylammonium chloride), which are typically soluble crystalline solids.

• Reaction with Ethanoyl Chloride: This is an addition-elimination reaction that produces an N-substituted amide and hydrogen chloride gas ($HCl$).

• Reaction with Copper(II) Ions: Amines act as ligands. Initially, they form a pale blue precipitate of $[Cu(H_2O)_4(OH)_2]$. With excess amine, this dissolves to form a deep blue solution containing the complex ion $[Cu(RNH_2)_4(H_2O)_2]^{2+}$.

• Solubility Trend: While short-chain amines are miscible with water due to hydrogen bonding, solubility drops sharply as the hydrophobic hydrocarbon chain lengthens, outweighing the polar $-NH_2$ group's effect.

• Identify the Reagents: When asked for the preparation of a primary amine from a halogenoalkane, always specify excess ammonia to prevent further substitution.

• Basicity Comparisons: If asked to rank basicity, look for the number of alkyl groups. More alkyl groups generally increase basicity (up to secondary amines) due to the cumulative inductive effect.

• Observation Questions: For reactions with $Cu^{2+}$, remember the two-step observation: first a pale blue precipitate, then a deep blue solution upon adding excess amine.

• Formula Precision: Ensure you distinguish between the amine ($RNH_2$) and the ammonium salt ($RNH_3^+X^-$) when writing equations for reactions with acids.