CHEMCA Free Chemistry Notes for JEE NEET Chemistry MadeEasy: NEET Crash Course Module

Module Focus: The Nitrogen Lone Pair

Amines are organic derivatives of ammonia ($NH_3$) obtained by replacing one, two, or all three hydrogen atoms with alkyl or aryl groups. The chemistry of amines is entirely governed by the unshared lone pair of electrons on the nitrogen atom, making them basic and nucleophilic. Synthesizing primary amines purely requires very specific chemical pathways to avoid over-alkylation into secondary or tertiary forms.

1. General Methods of Preparation

A. Reduction of Nitro Compounds

Nitroalkanes or nitroarenes are heavily reduced to corresponding primary amines.

Reagents: $H_2/Pd$, $Sn/HCl$, or $Fe/HCl$

$R-NO_2 \xrightarrow{Fe/HCl} R-NH_2$

NEET Concept: $Fe/HCl$ is preferred because the $FeCl_2$ formed hydrolyzes to release $HCl$, meaning only a tiny amount of $HCl$ is needed to initiate the reaction.

B. Reduction of Nitriles & Amides

Used for stepping up the carbon chain (adds one $CH_2$ group).

Reagents: $LiAlH_4$ or Catalytic Hydrogenation

$R-C\equiv N \xrightarrow{LiAlH_4} R-CH_2-NH_2$

$R-CONH_2 \xrightarrow{LiAlH_4} R-CH_2-NH_2$

C. Hoffmann's Ammonolysis of Alkyl Halides

Heating an alkyl halide ($RX$) with an ethanolic solution of ammonia yields a primary amine. However, the primary amine formed acts as a nucleophile and attacks another $RX$ molecule.

$NH_3 \xrightarrow{RX} 1^\circ \text{ Amine} \xrightarrow{RX} 2^\circ \text{ Amine} \xrightarrow{RX} 3^\circ \text{ Amine} \xrightarrow{RX} 4^\circ \text{ Ammonium Salt}$

Major Disadvantage: Yields a complex mixture of $1^\circ, 2^\circ, 3^\circ$, and quaternary salts.

How to get only $1^\circ$ Amine? Take Ammonia in large excess to prevent subsequent alkylation.

2. Critical Name Reactions (Prep)

These two methods are strictly used to prepare pure primary ($1^\circ$) amines without the risk of forming secondary or tertiary mixtures.

A. Hoffmann Bromamide Degradation Reaction

Treating a primary amide with Bromine ($Br_2$) in an aqueous or ethanolic solution of Sodium Hydroxide ($NaOH$).

$R-CO-NH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O$

This is a Step-Down reaction: The amine contains exactly ONE carbon less than the parent amide. The carbonyl carbon is lost as carbonate.
The alkyl or aryl group migrates from the carbonyl carbon to the nitrogen atom.

B. Gabriel Phthalimide Synthesis

Uses Phthalimide to synthesize pure aliphatic primary amines.

Phthalimide reacts with KOH to form Potassium phthalimide (nucleophile).
The nucleophile attacks an alkyl halide ($R-X$) via $S_N2$ mechanism.
Alkaline hydrolysis yields the pure primary amine ($R-NH_2$).

NEET Mega Trap: Why can't we synthesize Aniline?

Aryl halides (like chlorobenzene) do not undergo nucleophilic substitution ($S_N2$) because the C-Cl bond has partial double-bond character due to resonance. Therefore, the phthalimide anion cannot displace the halogen, making it impossible to prepare aniline by this method.

3. The Basicity of Amines (Ultimate Trap)

Amines are basic because of the lone pair on Nitrogen. The more easily available this lone pair is for donation to a proton ($H^+$), the stronger the base. The basicity order is fundamentally different depending on whether the reaction happens in a gas or in aqueous solution.

A. Basicity in Gas Phase (or Non-Polar Solvents)

In the gas phase, there is no solvent interaction. Basicity is dictated strictly by the Inductive Effect ($+I$) of the alkyl groups.

Alkyl groups push electron density onto Nitrogen, making the lone pair highly available. More alkyl groups = stronger base.

$3^\circ \ > \ 2^\circ \ > \ 1^\circ \ > \ NH_3$

B. Basicity in Aqueous Phase (The NEET Favorite)

In water, the substituted ammonium cation formed after accepting $H^+$ must be stabilized. The stability depends on a fierce competition between three factors:

$+I$ Effect: Favors $3^\circ > 2^\circ > 1^\circ$
Solvation (H-Bonding with Water): A $1^\circ$ amine gains 3 protons, so it can form 3 H-bonds. A $3^\circ$ amine gains 1 proton, forming only 1 H-bond. Therefore, Solvation favors $1^\circ > 2^\circ > 3^\circ$.
Steric Hindrance: Bulky alkyl groups block water molecules from approaching the cation. Favors $1^\circ > 2^\circ > 3^\circ$.

The Definitive Results to Memorize

If Alkyl Group is Methyl ($-CH_3$)

Steric hindrance is small. Solvation strongly stabilizes the $1^\circ$ over the $3^\circ$.

$2^\circ > 1^\circ > 3^\circ > NH_3$

(The 213 Rule)

If Alkyl Group is Ethyl ($-C_2H_5$)

Ethyl is bulky. Steric hindrance destroys the solvation of $1^\circ$, so $+I$ effect takes over.

$2^\circ > 3^\circ > 1^\circ > NH_3$

(The 231 Rule)

C. Aryl Amines (Aniline)

Aniline ($C_6H_5NH_2$) is much less basic than ammonia or aliphatic amines.

Reason: The lone pair of electrons on the nitrogen atom is in conjugation with the benzene ring. Due to the $+M$ (resonance) effect, the lone pair delocalizes into the ring, making it largely unavailable for protonation. Furthermore, the anilinium ion formed upon protonation lacks resonance stabilization.

Overall Basicity: Alkylamines > Ammonia > Arylamines

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NEET Crash Course Module - 88

Preparation & Basicity of Amines

Module Focus: The Nitrogen Lone Pair

1. General Methods of Preparation

2. Critical Name Reactions (Prep)

A. Hoffmann Bromamide Degradation Reaction

B. Gabriel Phthalimide Synthesis

3. The Basicity of Amines (Ultimate Trap)

A. Basicity in Gas Phase (or Non-Polar Solvents)

B. Basicity in Aqueous Phase (The NEET Favorite)

C. Aryl Amines (Aniline)

NEET Grand Test: Amines Part 1

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