Chapter 9 - Amines

Amines are organic derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups. The chapter studies their structures, preparation, basic character and reactions, then introduces diazonium salts as versatile intermediates in aromatic chemistry.

Amines occur in amino acids, alkaloids, neurotransmitters, medicines, dyes and many industrial products. For Class 12 students, the key is to connect the nitrogen lone pair with basicity, nucleophilicity and reaction behaviour.

Classification, nomenclature and structure

Amines are primary, secondary or tertiary depending on whether one, two or three carbon groups are bonded to nitrogen. This classification counts groups attached directly to nitrogen; it is not based on the nature of a neighbouring carbon as it is for alcohols and haloalkanes.

In simple amines, nitrogen is approximately sp3 hybridised and has a trigonal-pyramidal shape with a lone pair. In aromatic amines such as aniline, the lone pair interacts with the benzene ring, affecting bond character and basicity.

Preparation of amines

Amines can be prepared by reduction of nitro compounds, nitriles and amides, ammonolysis of haloalkanes and other specific methods. Reduction route selection depends on the functional group and desired product.

Gabriel phthalimide synthesis is useful for preparing primary alkyl amines without a mixture of higher amines, but it is not suitable for ordinary aryl amines. Hofmann bromamide degradation converts a primary amide into a primary amine containing one fewer carbon atom. This loss of one carbon is a common conversion clue.

Physical properties

Primary and secondary amines can form intermolecular hydrogen bonds because they contain N–H bonds. Tertiary amines cannot hydrogen-bond to one another as donors, though all three classes can accept hydrogen bonds from water through the nitrogen lone pair.

Lower amines are water soluble, but solubility falls as the hydrophobic carbon portion grows. Boiling-point comparisons should consider molecular mass, hydrogen bonding and branching together.

Basic character of amines

Amines act as bases by accepting a proton through the nitrogen lone pair. Their basic strength depends on availability of that lone pair and on the stability of the protonated conjugate acid.

Alkyl groups generally donate electron density and can increase basicity, while resonance delocalisation of the lone pair in aniline makes it less available for protonation. In aqueous solution, solvation and steric effects also influence the observed order, so gas-phase and aqueous basicity orders may differ.

Substituents on aromatic amines affect basicity through inductive and resonance effects. Electron-withdrawing groups generally lower basicity, while electron-donating groups generally increase it; position can determine the strength of the effect.

Reactions of amines

Amines react with acids to form ammonium salts and can be regenerated by treatment with base. Alkylation can produce higher amines and ultimately quaternary ammonium salts. Acylation of primary or secondary amines forms amides, while tertiary amines lack an N–H bond and do not undergo ordinary acylation in the same way.

The carbylamine test is given by primary amines, both aliphatic and aromatic, and produces a foul-smelling isocyanide. The Hinsberg test uses differences in sulphonamide formation and solubility to distinguish primary, secondary and tertiary amines.

Aniline undergoes electrophilic substitution readily because the amino group strongly activates the ring and directs substitution to ortho and para positions. Under strongly acidic conditions, however, protonation changes the electronic effect of the group. Protection by acylation is often used to control unwanted multiple substitution or reaction conditions.

Diazonium salts

Primary aromatic amines react with nitrous acid at low temperature to form arenediazonium salts. Temperature control is important because these salts are stable only under limited conditions in aqueous solution.

The diazonium group can be replaced by chlorine, bromine, iodine, fluorine, cyano, hydroxyl or hydrogen through different reactions. Sandmeyer, Gattermann, Balz-Schiemann and related transformations make diazonium salts valuable for preparing aromatic compounds that may be difficult to obtain by direct substitution.

Diazonium salts also undergo coupling with activated aromatic rings to form azo compounds, many of which are intensely coloured dyes.

What can students study on Teachoo?

• classification and IUPAC nomenclature;

• structure and hydrogen bonding;

• reduction, ammonolysis and other preparation methods;

• Gabriel synthesis and Hofmann bromamide degradation;

• physical properties and solubility;

• basicity of aliphatic and aromatic amines;

• effects of resonance, induction, solvation and steric hindrance;

• alkylation, acylation, carbylamine and Hinsberg reactions;

• electrophilic substitution of aniline;

• preparation and reactions of diazonium salts;

• azo coupling and dye formation; and

• NCERT exercises and organic conversions.

Common mistakes to avoid

• Classify an amine by the number of carbon groups on nitrogen, not by the carbon skeleton.

• Do not use one memorised basicity order for every medium.

• Remember that aniline is less basic than ammonia because its lone pair is delocalised into the ring.

• Gabriel synthesis does not provide ordinary primary aryl amines.

• Hofmann bromamide degradation produces an amine with one fewer carbon atom.

• The carbylamine test is specific to primary amines.

• Maintain the required low temperature during diazotisation.

Best way to study Amines

First master classification and preparation routes. Then compare basicity by asking two questions: how available is the lone pair, and how stable is the protonated form in the stated medium? Make a small table for the distinguishing tests. Finally, build a diazonium-salt reaction wheel showing every replacement product and reagent; it becomes a powerful tool for aromatic conversions.

Frequently asked questions

How are primary, secondary and tertiary amines classified?

They contain one, two or three alkyl or aryl groups directly attached to nitrogen respectively.

Why is aniline less basic than ammonia?

The nitrogen lone pair in aniline is delocalised into the benzene ring by resonance and is less available to accept a proton.

Why can aqueous basicity differ from gas-phase basicity?

In water, stabilisation of the protonated ion by solvation and steric access to solvent are important in addition to electron donation.

Which amines give the carbylamine test?

Primary aliphatic and primary aromatic amines give the test; secondary and tertiary amines do not.

What happens in Hofmann bromamide degradation?

A primary amide is converted into a primary amine with one carbon atom fewer than the starting amide.

Why are diazonium salts useful?

Their diazonium group can be replaced by several different groups, enabling a wide range of aromatic conversions.

Why is diazotisation performed at low temperature?

Arenediazonium salts in aqueous solution are stable only within a low-temperature range and may decompose on warming.

Learn Amines with Teachoo through basicity comparisons, distinguishing tests, diazonium reaction maps and clear conversion strategies.