Amines

Definition and Classification

Amines are organic compounds derived from ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups. They are classified as primary (1°), secondary (2°), or tertiary (3°) amines based on the number of carbon-containing groups attached to the nitrogen atom.

Structure and Hybridisation

The nitrogen atom in amines is sp³ hybridised, resulting in a pyramidal molecular geometry. The lone pair of electrons on nitrogen influences the shape and reactivity of amines. Because of this lone pair, the nitrogen atom is trigonal pyramidal rather than planar, and amines act as Lewis bases by donating the lone pair to acids or electrophiles. In solution, tertiary amines do not form intermolecular hydrogen bonds as effectively as primary and secondary amines, which also affects their properties.

Nomenclature

Amines are named using common or IUPAC systems. The IUPAC names are systematic and based on the longest carbon chain attached to the amino group. In IUPAC nomenclature, the suffix -amine is used for the principal amino group, and alkyl groups attached to nitrogen are indicated by N- prefixes. For example, CH₃NH₂ is methanamine, C₂H₅NH₂ is ethanamine, and (CH₃)₂NH is N-methylmethanamine. When the amino group is not the principal functional group, it may be named as an amino substituent.

Preparation Methods

Amines can be prepared by various methods including reduction of nitro compounds, ammonolysis of alkyl halides, reduction of nitriles, Gabriel phthalimide synthesis, reduction of amides, and Hoffmann bromamide degradation. In the reduction of nitro compounds, nitroalkanes or nitroarenes give the corresponding amines on reduction. Ammonolysis of alkyl halides involves heating an alkyl halide with alcoholic ammonia, though a mixture of primary, secondary and tertiary amines may form. Reduction of nitriles gives primary amines with one carbon atom more than the starting nitrile. The Gabriel phthalimide synthesis is a useful method for preparing pure primary aliphatic amines. Reduction of amides gives amines, and Hoffmann bromamide degradation converts an amide into a primary amine with one carbon atom less than the parent amide.

Physical Properties

Amines exhibit varying physical states depending on molecular weight, have characteristic boiling points influenced by hydrogen bonding, and show solubility trends in water and organic solvents. Lower members are gases or liquids with fishy odours, while higher members are usually liquids or solids. Primary and secondary amines form intermolecular hydrogen bonds, so their boiling points are generally higher than those of corresponding hydrocarbons and lower than those of alcohols. Tertiary amines cannot form intermolecular hydrogen bonds among themselves, so their boiling points are usually lower than those of primary and secondary amines of comparable molar mass. Lower amines are fairly soluble in water because they can hydrogen-bond with water, but solubility decreases as the hydrocarbon part increases.

Chemical Properties

Amines are basic due to the lone pair on nitrogen and undergo reactions such as alkylation, acylation, carbylamine reaction, bromination, nitration, sulphonation, and reactions with nitrous acid. Their basicity is due to the availability of the lone pair for protonation, and aliphatic amines are generally more basic than ammonia. They form alkylammonium salts with acids. On alkylation, amines form higher amines, while acylation with acyl chlorides or acid anhydrides gives amides. Primary amines with chloroform and alcoholic KOH give isocyanides in the carbylamine reaction. Aniline undergoes electrophilic substitution such as bromination, nitration and sulphonation readily because the amino group activates the benzene ring. With nitrous acid, primary aliphatic amines evolve nitrogen gas, primary aromatic amines form diazonium salts, and secondary amines give N-nitrosamines.

Solved Examples

Example 1: Write the structure and IUPAC name of the amide that forms propanamine by Hoffmann bromamide degradation.

Solution: Propanamine has three carbon atoms. Hoffmann bromamide degradation removes one carbon from the amide. Therefore, the amide must have four carbons: butanamide (CH₃CH₂CH₂CONH₂).

Example 2: Identify the amine formed by Hoffmann degradation of benzamide.

Solution: Benzamide (C₆H₅CONH₂) undergoes Hoffmann degradation to form aniline (C₆H₅NH₂), a primary aromatic amine.

Practice Set

• Level 1 (Easy): Define primary, secondary, and tertiary amines with examples.
• Level 2 (Moderate): Explain the Gabriel phthalimide synthesis and its limitation.
• Level 3 (Challenging): Compare the basicity order of methyl-substituted amines in aqueous solution and explain the reason.

Answer Key

• Level 1: Primary amines have one alkyl/aryl group attached to nitrogen (RNH₂), secondary have two (R₂NH), tertiary have three (R₃N). Examples: methylamine (1°), dimethylamine (2°), trimethylamine (3°).
• Level 2: Gabriel phthalimide synthesis involves formation of potassium phthalimide, reaction with alkyl halide to form N-alkyl phthalimide, and hydrolysis to yield primary amine. It cannot prepare aromatic primary amines.
• Level 3: Basicity order for methyl-substituted amines in aqueous solution is (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃. This is due to inductive effect, solvation, and steric hindrance affecting proton acceptance.

Quick Reference Table

• Classification: Primary (RNH₂), Secondary (R₂NH), Tertiary (R₃N)
• Structure and hybridisation: Nitrogen in amines is sp³ hybridised and usually has a pyramidal shape because of one lone pair
• Preparation: Reduction of nitro compounds, ammonolysis of alkyl halides, reduction of nitriles, Gabriel synthesis, Hoffmann degradation
• Physical properties: Boiling points are influenced by hydrogen bonding; lower amines are soluble in water, while solubility decreases with increase in carbon chain length
• Basicity: Tertiary > Secondary > Primary > Ammonia (gas phase); in aqueous solution, basicity is influenced by inductive effect, solvation and steric hindrance
• Reactions: Alkylation, acylation, carbylamine test, bromination, nitration, sulphonation, reaction with nitrous acid
• Diazonium salts: Formed by diazotisation of primary aromatic amines at low temperature; used in synthesis of dyes and aromatic compounds

Common Mistakes and Misconceptions

• Confusing primary, secondary, and tertiary amines by counting the alkyl or aryl groups attached to nitrogen incorrectly.
• Assuming all amines have the same basic strength; their basicity changes with structure and the surrounding medium.
• Thinking Gabriel phthalimide synthesis can prepare aromatic amines; it is used only for primary aliphatic amines.
• Ignoring the lone pair of electrons on nitrogen when explaining the basic nature and reactivity of amines.
• Mixing up the basicity order of amines in aqueous solution with the gas-phase order.
• Forgetting that steric hindrance reduces protonation of bulky tertiary amines in water.
• Assuming nitrous acid gives the same product with all amines; primary, secondary, and tertiary amines react differently.
• Confusing the carbylamine reaction, which is given only by primary amines, with reactions of secondary or tertiary amines.
• Thinking diazonium salts are stable at room temperature; aromatic diazonium salts must be kept cold.

Glossary

• Amines: Organic compounds derived from ammonia by replacement of hydrogen atoms with alkyl or aryl groups.
• Diazonium Salts: Compounds containing the diazonium group (N₂⁺) attached to an aromatic ring, used as intermediates in organic synthesis.
• Gabriel Phthalimide Synthesis: A method to prepare primary amines using phthalimide and alkyl halides.
• Hoffmann Bromamide Degradation: Reaction converting amides to primary amines with one less carbon atom.
• Basicity: The ability of a compound to accept protons, influenced by electronic and steric factors.
• sp³ Hybridisation: Mixing of one s and three p orbitals to form four equivalent orbitals in a tetrahedral arrangement.