Class 12 Chemistry · Chapter 9 NotesAmines

Amines are considered derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl groups. The nitrogen atom is trivalent and carries an unshared pair of electrons. Its orbitals are sp3 hybridised, giving amines a pyramidal geometry. Because of the lone pair, the C–N–E bond angle (where E is C or H) is slightly less than 109.5°; for example, it is about 108° in trimethylamine. Amines are classified as primary (1°), secondary (2°) or tertiary (3°) depending on how many hydrogen atoms of ammonia have been replaced. If one hydrogen is replaced, the amine is RNH2 or ArNH2, a primary amine. Replacement of two hydrogens gives R–NHR′, a secondary amine. Replacement of all three gives R3N or RNR′R″, a tertiary amine. Amines are called simple when all alkyl or aryl groups are the same, and mixed when the groups are different.

• Primary amineRNH2 or ArNH2
• Secondary amineR2NH or R–NHR′
• Tertiary amineR3N, RNR′R″ or R2NR′
• Nitrogen in amines is sp3 hybridised with a pyramidal shape

In the common system, aliphatic amines are named by prefixing the alkyl group to 'amine' as one word, such as methylamine. When two or three identical groups are present, the prefixes di or tri are used, as in dimethylamine or trimethylamine. In the IUPAC system, primary amines are named as alkanamines by replacing the 'e' of the parent alkane with 'amine'. For example, CH3NH2 is methanamine. If more than one amino group is present, numbers indicate their positions and the final 'e' of the hydrocarbon suffix is retained, as in ethane-1,2-diamine. For secondary and tertiary amines, the locant N is used to show substituents attached to nitrogen. For example, CH3NHCH2CH3 is N-methylethanamine and (CH3CH2)3N is N,N-diethylethanamine. Aromatic amines have –NH2 directly attached to a benzene ring. C6H5NH2 is commonly called aniline, which is also an accepted IUPAC name; in the IUPAC system it can also be called benzenamine.

• Common namealkylamine, dialkylamine or trialkylamine
• IUPAC name of primary aminealkanamine
• Use N-locants for groups attached to nitrogen in secondary and tertiary amines
• Aniline or benzenamineC6H5NH2

Amines can be prepared by several methods. Nitro compounds are reduced to amines using hydrogen gas with finely divided nickel, palladium or platinum, or by metals in acidic medium. Reduction with iron scrap and hydrochloric acid is preferred because the FeCl2 formed hydrolyses to release hydrochloric acid, so only a small amount is needed to start the reaction. Alkyl or benzyl halides undergo ammonolysis with ethanolic ammonia in a sealed tube at 373 K, replacing the halogen with an amino group. This method gives a mixture of primary, secondary and tertiary amines and a quaternary ammonium salt, but a large excess of ammonia favours the primary amine. Nitriles on reduction with lithium aluminium hydride (LiAlH4) or by catalytic hydrogenation give primary amines and increase the carbon chain by one atom. Amides on reduction with LiAlH4 also yield amines. Gabriel phthalimide synthesis gives primary amines: phthalimide reacts with ethanolic potassium hydroxide, then an alkyl halide, and finally alkaline hydrolysis gives the primary amine. Aromatic primary amines cannot be prepared this way because aryl halides do not undergo nucleophilic substitution with the phthalimide anion. Hoffmann bromamide degradation treats an amide with bromine in aqueous or ethanolic sodium hydroxide; the alkyl or aryl group migrates from carbonyl carbon to nitrogen, giving a primary amine with one carbon less than the amide.

• Reduction of nitro compoundsH2 with Ni, Pd or Pt, or Fe/HCl
• Ammonolysis of alkyl halidesR–X + NH3 → RNH2 (major product with excess NH3)
• Reduction of nitriles or amides with LiAlH4
• Gabriel phthalimide synthesisfor primary amines only, not aryl amines
• Hoffmann bromamide degradationamide + Br2 + NaOH → primary amine with one carbon less

Lower aliphatic amines are gases with a fishy odour. Primary amines with three or more carbon atoms are liquids, and still higher ones are solids. Aniline and other arylamines are usually colourless but become coloured on storage due to atmospheric oxidation. Lower aliphatic amines are soluble in water because they form hydrogen bonds with water molecules. Solubility decreases as molar mass increases because the hydrophobic alkyl part becomes larger; higher amines are essentially insoluble in water. Amines are soluble in organic solvents like alcohol, ether and benzene. Primary and secondary amines form intermolecular hydrogen bonds through nitrogen and hydrogen, so they associate. Primary amines have two hydrogen atoms available for hydrogen bonding, so association is greater than in secondary amines. Tertiary amines have no hydrogen attached to nitrogen, so they cannot form such intermolecular hydrogen bonds. Therefore, the boiling point order for isomeric amines is primary > secondary > tertiary. Amines have lower boiling points than alcohols of similar molar mass because nitrogen is less electronegative than oxygen and forms weaker hydrogen bonds.

• Lower aliphatic aminesgases with fishy odour
• Solubility in water decreases with increasing molar mass
• Boiling point order for isomeric aminesprimary > secondary > tertiary
• Amines have lower boiling points than alcohols of similar molar mass

Amines are reactive because of the difference in electronegativity between nitrogen and hydrogen and the unshared pair of electrons on nitrogen. They behave as nucleophiles and as Lewis bases. Amines react with acids to form salts, and these salts regenerate the parent amine when treated with a base like NaOH. Amine salts are soluble in water but insoluble in organic solvents like ether, which helps separate amines from non-basic organic compounds. The basic strength of amines is expressed using Kb or pKb; a larger Kb or smaller pKb means a stronger base. Aliphatic amines are stronger bases than ammonia due to the +I effect of alkyl groups, which increases electron density on nitrogen. Aromatic amines are weaker bases than ammonia because the aryl group withdraws electrons and the lone pair on nitrogen is in conjugation with the benzene ring. In aniline, the lone pair is less available for protonation, and aniline is more stable than the anilinium ion because it has more resonating structures. In aqueous solution, the basicity order is affected by inductive effect, solvation and steric hindrance. For methyl-substituted amines, the order is (CH3)2NH > CH3NH2 > (CH3)3N > NH3. For ethyl-substituted amines, it is (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3.

• Amines are Lewis bases due to the lone pair on nitrogen
• Aliphatic amines are stronger bases than ammonia; aromatic amines are weaker
• pKbsmaller value means stronger base
• Basicity in aqueous solution depends on inductive effect, solvation and steric hindrance

Amines undergo alkylation with alkyl halides. Primary and secondary amines react with acid chlorides, anhydrides or esters in acylation, replacing hydrogen of –NH2 or >N–H with an acyl group to form amides; the reaction is carried out with a base stronger than the amine, such as pyridine, to remove HCl. Reaction with benzoyl chloride is called benzoylation. Primary amines, both aliphatic and aromatic, react with chloroform and ethanolic potassium hydroxide on heating to form foul-smelling isocyanides or carbylamines; this carbylamine reaction is a test for primary amines, and secondary and tertiary amines do not give it. With nitrous acid, prepared in situ from a mineral acid and sodium nitrite, primary aliphatic amines form unstable aliphatic diazonium salts that liberate nitrogen gas and give alcohols. Aromatic amines react with nitrous acid at low temperature (273–278 K) to form diazonium salts. Secondary and tertiary amines react differently. Benzenesulphonyl chloride, known as Hinsberg's reagent, reacts with primary amines to form sulphonamides that are soluble in alkali because the N–H hydrogen is acidic. Secondary amines form N,N-disubstituted sulphonamides that are not acidic and are insoluble in alkali. Tertiary amines do not react. This helps distinguish and separate primary, secondary and tertiary amines. Aromatic amines undergo electrophilic substitution. The –NH2 group is ortho- and para-directing and strongly activating. Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline. To prepare monosubstituted derivatives, the –NH2 group is protected by acetylation with acetic anhydride, then substitution is carried out, followed by hydrolysis. Direct nitration of aniline gives tarry oxidation products and, in strongly acidic medium, the anilinium ion is meta-directing, so some meta derivative forms. Acetylation controls nitration and gives the p-nitro derivative as the major product. Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate, which on heating at 453–473 K gives p-aminobenzenesulphonic acid, called sulphanilic acid. Aniline does not undergo Friedel-Crafts reactions because it forms a salt with aluminium chloride, the Lewis acid catalyst, giving nitrogen a positive charge that strongly deactivates the ring.

• Acylationamines + acid chloride/anhydride → amides
• Carbylamine reactionprimary amine + CHCl3 + alc. KOH → isocyanide (foul smell)
• Hinsberg's reagent distinguishes primary, secondary and tertiary amines
• Aniline + Br2 water → 2,4,6-tribromoaniline (white precipitate)
• Acetylation protects –NH2 and controls electrophilic substitution

Diazonium salts have the general formula RN2+X−, where R is an aryl group and X− may be Cl−, Br−, HSO4− or BF4−. They are named by adding 'diazonium' to the parent hydrocarbon name followed by the anion name. For example, C6H5N2+Cl− is benzenediazonium chloride. Primary aliphatic amines form highly unstable alkyldiazonium salts, while primary aromatic amines form arenediazonium salts that are stable for a short time in solution at low temperature (273–278 K). Benzenediazonium chloride is prepared by diazotisation: aniline reacts with nitrous acid, produced from sodium nitrite and hydrochloric acid, at 273–278 K. It is a colourless crystalline solid, readily soluble in water, stable in cold but reacts with water when warmed, and decomposes easily in the dry state. Benzenediazonium fluoroborate is water insoluble and stable at room temperature. Reactions of diazonium salts involve either displacement of nitrogen or retention of the diazo group. Displacement reactions introduce Cl−, Br−, I−, CN−, F−, H, OH− or NO2− into the aromatic ring. Sandmeyer reaction uses Cu(I) ion and the corresponding halide or cyanide; Gatterman reaction uses halogen acid with copper powder. Iodobenzene forms with potassium iodide. Fluoroboric acid followed by heating gives aryl fluoride. Hypophosphorous acid or ethanol reduces the diazonium salt to arene. Warming to 283 K hydrolyses it to phenol. Diazonium fluoroborate with aqueous sodium nitrite and copper gives a nitro compound. In coupling reactions, the diazo group is retained: benzenediazonium chloride couples with phenol at the para position to form p-hydroxyazobenzene, and with aniline to form p-aminoazobenzene. These azo compounds are coloured and used as dyes. Diazonium salts are valuable intermediates because they allow introduction of groups such as –F, –Cl, –Br, –I, –CN, –OH and –NO2 into aromatic rings, including compounds not easily made by direct substitution.

• General formula of diazonium saltRN2+X−
• Diazotisationaniline + NaNO2 + HCl at 273–278 K → benzenediazonium chloride
• Sandmeyer reactiondiazonium salt + Cu(I) halide/cyanide → aryl halide or cyanide
• Coupling reactiondiazonium salt + phenol/aniline → azo dye
• Diazonium salts help introduce –F, –Cl, –Br, –I, –CN, –OH and –NO2 into aromatic rings