Amines

This chapter introduces amines, which are organic compounds derived from ammonia. You will learn to classify them as primary, secondary, or tertiary, and name them using IUPAC and common systems. The core of the chapter focuses on their structure, particularly the lone pair of electrons on the nitrogen atom which is responsible for their characteristic basic nature. A significant portion is dedicated to comparing the basic strength of different amines, considering factors like the inductive effect, resonance, and solvation.

The main practical skills involve methods of preparation and chemical reactions. Key preparation methods include the ammonolysis of alkyl halides, reduction of nitro compounds, Gabriel phthalimide synthesis, and the Hofmann bromamide degradation reaction. You will study reactions such as acylation, alkylation, and the important reactions with nitrous acid, which serve as a test to distinguish between the different classes of amines. For aromatic amines like aniline, you will also cover electrophilic substitution reactions.

Students often find the relative comparison of basicity challenging, as it requires balancing multiple electronic and steric effects. The mechanisms of name reactions also require careful attention. This chapter builds directly on concepts from General Organic Chemistry, especially electronic effects, and uses reactions from the 'Haloalkanes and Haloarenes' chapter.

Formulas for Amines

Hofmann Ammonolysis: $R-X + NH_3 \rightarrow RNH_2 \xrightarrow{R-X} R_2NH \xrightarrow{R-X} R_3N \xrightarrow{R-X} [R_4N]^+X^-$ Produces a mixture of 1°, 2°, 3° amines and a quaternary ammonium salt.
Reduction of Nitro Compounds: $Ar-NO_2 + 6[H] \xrightarrow{Sn/HCl} Ar-NH_2 + 2H_2O$ A common method for preparing aromatic amines like aniline.
Gabriel Phthalimide Synthesis: $C_6H_4(CO)_2NK + R-X \rightarrow C_6H_4(CO)_2NR \xrightarrow{NaOH(aq)} R-NH_2$ Used for the synthesis of pure primary aliphatic amines.
Hofmann Bromamide Degradation: $R-CONH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O$ An amide is converted to a primary amine with one less carbon atom.
Basicity Constant of an Amine: $K_b = \frac{[RNH_3^+][OH^-]}{[RNH_2]}$ Measures the strength of an amine as a base in aqueous solution.
pKb value: $pK_b = -log_{10}(K_b)$ A smaller pKb value indicates a stronger base.
Carbylamine Reaction (Isocyanide Test): $R-NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} R-NC + 3KCl + 3H_2O$ A test for primary amines, which produce a foul-smelling isocyanide.
Diazotisation of Aromatic Amines: $Ar-NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278K} Ar-N_2^+Cl^- + NaCl + 2H_2O$ Forms a stable benzenediazonium salt at low temperatures.
Hinsberg's Test: $C_6H_5SO_2Cl + RNH_2 \rightarrow C_6H_5SO_2NHR$ The product from a 1° amine is soluble in alkali. Used to distinguish 1°, 2°, and 3° amines.