Plus Two Chemistry | Amines - Full Chapter Revision | Xylem Plus Two

Understanding Amines: Structure, Properties, and Reactions

Amines are organic derivatives of ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups. They are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of carbon-containing groups directly attached to the nitrogen atom. Quaternary ammonium salts have all four hydrogen atoms replaced by organic groups, bearing a positive charge on nitrogen. Amines are fundamental to organic chemistry and biochemistry — they form the building blocks of amino acids, proteins, nucleic acids, alkaloids, and numerous pharmaceuticals. This chapter provides a comprehensive study of their classification, nomenclature, preparation methods, physical properties, and chemical reactions, which are essential for understanding both organic chemistry and biochemistry at the Class 12 level.

The basicity of amines is one of their most important properties. Aliphatic amines are stronger bases than ammonia because the alkyl groups exert an electron-donating inductive effect, increasing the electron density on the nitrogen atom and making the lone pair more available for protonation. In aqueous solution, however, the order of basicity follows: secondary (2°) > primary (1°) > tertiary (3°) > ammonia. This arises from a combination of three factors — the inductive effect increases basicity with more alkyl groups, but solvation of the protonated ammonium ion decreases with increasing steric hindrance, and steric hindrance to protonation itself increases with more bulky alkyl groups. Aromatic amines like aniline are significantly weaker bases because the lone pair on nitrogen is delocalised into the aromatic ring through resonance, making it less available for protonation. The basicity of substituted anilines is influenced by substituents — electron-donating groups increase basicity while electron-withdrawing groups decrease it.

Amines can be prepared by several important methods including the reduction of nitro compounds using Sn/HCl or catalytic hydrogenation, ammonolysis of alkyl halides (though this produces a mixture requiring separation), reduction of nitriles and amides using lithium aluminium hydride, and the Gabriel phthalimide synthesis which provides a clean method for preparing pure primary amines without contamination by secondary or tertiary products. Key chemical reactions include alkylation, acylation to form amides, the carbylamine reaction as a specific test for primary amines, the Hinsberg test using benzenesulphonyl chloride to distinguish between the three classes, and reaction with nitrous acid which produces characteristic results with each class — primary aliphatic amines give nitrogen gas, primary aromatic amines form diazonium salts, secondary amines give N-nitrosamines, and tertiary amines form nitrosamine salts. Diazonium salts derived from primary aromatic amines are extremely versatile intermediates in organic synthesis, undergoing substitution reactions (Sandmeyer reaction, Gatterman reaction, Balz-Schiemann reaction) and coupling reactions to form brightly coloured azo dyes that are widely used in the textile, food, and pharmaceutical industries.

• Amines are classified as 1°, 2°, or 3° based on the number of alkyl/aryl groups attached to the nitrogen atom.
• Aliphatic amines are stronger bases than ammonia; aromatic amines are weaker due to resonance delocalisation of the nitrogen lone pair.
• Gabriel phthalimide synthesis is a key method for preparing pure primary amines without contamination.
• The carbylamine reaction and Hinsberg test distinguish between 1°, 2°, and 3° amines.
• Diazonium salts are versatile intermediates for synthesising aromatic compounds and azo dyes.