Amines & Nitrogen Compounds | Chemistry Made Easy

1. Amine Classification & Nomenclature

Amines are organic derivatives of ammonia (NH3) in which one or more hydrogen atoms have been replaced by alkyl or aryl hydrocarbon groups:

Primary (1 degree) Amine: Nitrogen bonded to ONE carbon group: R-NH2 (e.g. ethylamine, CH3CH2NH2).
Secondary (2 degree) Amine: Nitrogen bonded to TWO carbon groups: R2NH (e.g. dimethylamine, (CH3)2NH).
Tertiary (3 degree) Amine: Nitrogen bonded to THREE carbon groups: R3N (e.g. trimethylamine, (CH3)3N).
Quaternary (4 degree) Ammonium Salt: Nitrogen bonded to FOUR carbon groups, bearing a permanent positive formal charge: R4N+ X- (e.g. tetramethylammonium chloride).

2. Comparative Base Strength of Amines

Amines act as Bronsted-Lowry bases because the nitrogen atom possesses a lone pair of electrons that can accept a proton (H+) via dative covalent bonding:

R-NH2 + H+ <=> R-NH3+

The relative strength of an amine as a base depends entirely on the availability of the nitrogen lone pair to accept a proton.

The Absolute Base Strength Hierarchy Primary Aliphatic Amine > Ammonia > Aromatic Amine (Phenylamine)

1. Primary Aliphatic Amines (e.g. Ethylamine) - Strongest Bases: The alkyl group has an electron-releasing positive inductive effect (+I). This pushes electron density towards the nitrogen atom, increasing the charge density of the nitrogen lone pair and making it more available to accept a proton than in ammonia.
2. Ammonia (NH3) - Intermediate Base: Lacks alkyl groups (no inductive electron release) and lacks an aromatic ring (no delocalisation).
3. Aromatic Amines (e.g. Phenylamine) - Weakest Bases: The lone pair of electrons on nitrogen overlaps sideways and delocalises into the benzene ring pi electron system. This significantly reduces the electron density on the nitrogen atom, making the lone pair much less available to accept a proton.

3. Base Strength & Lone Pair Availability Diagram

4. Synthetic Preparation of Amines

Method 1: Nucleophilic Substitution of Haloalkanes

Heating a haloalkane with excess ethanolic ammonia in a sealed tube under pressure:

CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br

Why EXCESS Ammonia is Required

The primary amine product (ethylamine) also has a lone pair on nitrogen and is an even stronger nucleophile than ammonia. If equimolar quantities are used, ethylamine reacts with remaining bromoethane to form diethylamine, triethylamine, and finally a quaternary ammonium salt. Using a large excess of ammonia ensures that haloalkane molecules collide almost exclusively with ammonia, maximising the yield of the primary amine.

Method 2: Reduction of Nitriles (Preparation of Pure Primary Amines)

Nitriles can be reduced cleanly to primary amines using lithium tetrahydridoaluminate (LiAlH4) in dry ether followed by dilute acid, or by catalytic hydrogenation using hydrogen gas over a nickel catalyst (H2 / Ni):

R-C≡N + 4[H] -> R-CH2-NH2

Key Synthetic Advantage: Unlike haloalkane substitution, nitrile reduction yields exclusively primary amine with zero contamination from secondary or tertiary amines.

Method 3: Reduction of Nitrobenzene to Phenylamine

Nitrobenzene is converted to phenylamine industrially and in the laboratory in two stages:

Reflux nitrobenzene with tin (Sn) and concentrated hydrochloric acid (HCl):
C6H5NO2 + 6[H] -> C6H5NH3+Cl- + 2H2O (forms the phenylammonium salt)
Add aqueous sodium hydroxide (NaOH) to liberate the free amine base:
C6H5NH3+Cl- + OH- -> C6H5NH2 + H2O + Cl-

5. Quaternary Ammonium Salts as Cationic Surfactants

When a tertiary amine reacts with a haloalkane, a quaternary ammonium salt is produced:

(CH3)3N + CH3Cl -> (CH3)4N+ Cl- (tetramethylammonium chloride)

Cationic Surfactant Structure & Applications

Quaternary ammonium salts with one or two very long non-polar alkyl hydrocarbon chains (e.g. C16 to C18) act as cationic surfactants:

Hydrophobic tail: The long non-polar hydrocarbon chain dissolves readily in non-polar grease and oils.
Hydrophilic head: The positively charged nitrogen cation (N+) interacts electrostatically with polar water molecules and negatively charged surfaces.
Uses: Fabric softeners and hair conditioners. Fabric fibres and wet hair naturally carry negative surface charges. The positive cationic heads bind electrostatically to the fabric or hair surface, leaving the smooth, non-polar hydrocarbon chains facing outward to reduce friction, eliminate static charge, and impart softness.

6. Worked Synthesis & Mechanism

Worked Example 1: Synthesis of Propylamine without Secondary Amine Impurities

A chemist wishes to synthesise propylamine (CH3CH2CH2NH2) from bromoethane. Outline the reaction steps, reagents, and conditions, and explain why this route gives a pure product.

Step 1: Nucleophilic substitution to form a nitrile:

Reflux bromoethane (CH3CH2Br) with aqueous-ethanolic potassium cyanide (KCN):
CH3CH2Br + CN- -> CH3CH2CN + Br-
Product: Propanenitrile (extends chain to 3 carbons).

Step 2: Reduction of the nitrile:

Pass propanenitrile and hydrogen gas over a finely divided nickel catalyst at elevated temperature:
CH3CH2CN + 2H2 -> CH3CH2CH2NH2
Product: Propylamine.

Advantage: Only the nitrile functional group is reduced, guaranteeing a 100% pure primary amine free of secondary or tertiary amine side products.

Worked Example 2: Industrial Synthesis of Phenylamine

Write equations showing how benzene is converted to phenylamine via nitrobenzene.

Stage 1: Nitration of benzene:

C6H6 + HNO3 -> C6H5NO2 + H2O
Reagents: Concentrated HNO3 and concentrated H2SO4 at 50 - 55 degrees C.

Stage 2: Reduction to phenylamine:

C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O
Reagents: Tin (Sn) and concentrated HCl under reflux, followed by addition of NaOH(aq).

Active Recall

Unit Flashcards

Master definitions, mechanisms, and key equations with spaced repetition flashcards.

Practice Flashcards →

Mock Assessment

Exam Practice & AI Grader

Complete Section A MCQs and Section B written questions with instant AI marking and mark schemes.

Start Exam Practice →