Exam Master Review Sheet - Amines (JEE & NEET)

CHEMCA

EXAM MASTER REVIEW SHEET

Organic Chemistry: Amines

Comprehensive Notes for JEE Main, Advanced & NEET

1 Basic Strength of Amines

Amines act as Lewis bases due to the lone pair of electrons on the nitrogen atom. The basicity depends heavily on the medium.

Gaseous / Non-Aqueous Phase

In the gas phase, basicity is governed entirely by the $+I$ (inductive) effect of alkyl groups. More alkyl groups = higher electron density on nitrogen.

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

Aqueous Phase (Water)

In water, basicity is a delicate interplay of three factors: Inductive effect ($+I$), Solvation (H-bonding) effect, and Steric hindrance.

Methyl ($R = -CH_3$): $2^\circ > 1^\circ > 3^\circ > NH_3$ (Code: 213)

Ethyl ($R = -C_2H_5$): $2^\circ > 3^\circ > 1^\circ > NH_3$ (Code: 231)

Aromatic Amines & The Ortho Effect

Aniline is much less basic than ammonia or aliphatic amines because the lone pair on nitrogen is delocalized into the benzene ring via resonance.

Substituent Effect

Electron Releasing Groups ($+I, +M$ like $-CH_3, -OCH_3$) increase basicity. Electron Withdrawing Groups ($-I, -M$ like $-NO_2, -X$) decrease basicity.

Ortho Effect (JEE Adv)

Any ortho-substituted aniline (whether the group is EWG or ERG) is less basic than aniline itself due to a combination of steric and electronic factors (Steric Inhibition of Protonation).

2 Methods of Preparation

Gabriel Phthalimide Synthesis

1° Aliphatic Only

Produces pure primary aliphatic amines without contamination from $2^\circ$ or $3^\circ$ amines.

Phthalimide $\xrightarrow{KOH} \text{Salt} \xrightarrow{R-X} \text{N-Alkyl} \xrightarrow{NaOH(aq)}$ $R-NH_2$

Trap: Cannot be used for Aniline (Aryl halides do not undergo $S_N2$ easily).

Hoffmann Bromamide Degradation

Step-Down

Converts an amide to a $1^\circ$ amine with one less carbon atom. Reaction proceeds via an isocyanate intermediate.

$R-CONH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O$

Hoffmann Ammonolysis

Reaction of alkyl halide with ammonia. Yields a mixture of $1^\circ$, $2^\circ$, $3^\circ$ amines and quaternary ammonium salt.

$NH_3 \xrightarrow{RX} R-NH_2 \xrightarrow{RX} R_2NH \xrightarrow{RX} R_3N \xrightarrow{RX} R_4N^+X^-$

Reduction Reactions

Nitro to Amine: $R-NO_2 \xrightarrow{Sn/HCl \text{ or } Fe/HCl} R-NH_2$
($Fe/HCl$ is preferred as $FeCl_2$ formed gets hydrolyzed)
Nitrile to Amine: $R-CN \xrightarrow{H_2/Ni \text{ or } LiAlH_4} R-CH_2NH_2$
Amide to Amine: $R-CONH_2 \xrightarrow{LiAlH_4} R-CH_2NH_2$

3 Important Chemical Reactions

Reaction Name	Reagents	Mechanism & Observations
Carbylamine Test (Isocyanide Test)	$CHCl_3 + \text{Alc. } KOH, \Delta$	Only $1^\circ$ amines (aliphatic & aromatic) react to form Isocyanide ($R-NC$). Observation: Extremely foul, unendurable smell. $2^\circ$ and $3^\circ$ fail.
Hinsberg's Test	$C_6H_5SO_2Cl$ (Benzene sulphonyl chloride)	Used to distinguish $1^\circ, 2^\circ, 3^\circ$ amines: • $1^\circ$ Amine: Forms a ppt that is soluble in alkali (due to acidic H). • $2^\circ$ Amine: Forms a ppt that is insoluble in alkali (no acidic H). • $3^\circ$ Amine: No reaction.
Reaction with $HNO_2$	$NaNO_2 + HCl$ (0-5°C)	• Aliphatic $1^\circ$: Forms Alcohol + copious $N_2$ gas bubbles. • Aromatic $1^\circ$ (Aniline): Forms stable Diazonium Salt (BDC) at 0-5°C. • $2^\circ$ Amines: Form yellow oily Nitrosoamines.
Acylation	$CH_3COCl$ or $(CH_3CO)_2O$ / Pyridine	$1^\circ$ and $2^\circ$ amines react to form amides. Crucial for "protecting" the highly activating $-NH_2$ group during EAS of aniline.

4 Electrophilic Substitution (EAS) in Aniline

The $-NH_2$ group is strongly Activating and Ortho/Para directing. However, because nitrogen is a Lewis base, it interacts with electrophiles (like $H^+$ or Lewis acids) creating unexpected results.

Bromination

Direct reaction with Bromine water ($Br_2/H_2O$) causes poly-substitution due to extreme reactivity.

Aniline $\xrightarrow{Br_2/H_2O}$ 2,4,6-Tribromoaniline (White Ppt)

To get mono-bromo product (p-bromoaniline):

Protect group: Acetylate with $(CH_3CO)_2O$ to form acetanilide.
Brominate: $Br_2/CH_3COOH$ yields p-bromoacetanilide.
Hydrolyze: $H^+ \text{ or } OH^-$ removes acetyl group yielding p-bromoaniline.

Nitration Anomaly

Direct nitration ($Conc. HNO_3 + H_2SO_4$) yields an unexpected mixture:

Para = 51%
Meta = 47% (Trap for JEE!)
Ortho = 2%

Why 47% Meta? In the strongly acidic nitrating mixture, a significant portion of aniline is protonated to form the anilinium ion ($-NH_3^+$), which is strongly deactivating and meta-directing.

5 Benzene Diazonium Chloride (BDC)

BDC ($Ph-N_2^+Cl^-$) is a crucial intermediate for synthesizing a wide variety of aromatic compounds. It is stable only between 0-5°C.

Sandmeyer Reaction

Replacement of diazo group by $Cl, Br, \text{or } CN$ using $Cu(I)$ salts.

BDC $\xrightarrow{CuCl/HCl}$ Chlorobenzene

Gattermann Reaction

Similar to Sandmeyer, but uses Copper powder ($Cu$) instead of $Cu(I)$ salt.

BDC $\xrightarrow{Cu/HCl}$ Chlorobenzene

Replacement by H (Reduction)

Mild reducing agents convert BDC back to Benzene.

BDC $\xrightarrow{H_3PO_2 + H_2O \text{ or } C_2H_5OH}$ Benzene

Coupling Reactions (Azo Dyes)

Electrophilic substitution where the diazonium ion acts as electrophile.

+ Phenol (pH 9-10) $\rightarrow$ p-Hydroxyazobenzene (Orange Dye)
+ Aniline (pH 4-5) $\rightarrow$ p-Aminoazobenzene (Yellow Dye)

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Exam Master Review Sheet - Amines