What are Amines ?

Aliphatic And Aromatic Amines

Aliphatic amines have at least one alkyl group bonded to the nitrogen. The lone pair of electrons on the nitrogen. The lone pair of electrons on the nitrogen means that aliphatic amines behave similarly to ammonia:
• they act as nucleophiles and take part in reaction involving donation of the lone pair
• they act as Bronsted-Lowry bases (H+ acceptors).

Aromatic amines contain a benzene ring directly attached to the NH2 group.
The delocalized system of the benzene group is able to incorporate the lone pair of electrons from the nitrogen atom, meaning that aromatic amines have different properties to aliphatic amines.

Nomenclature Of Amines

• In common system, an aliphatic amine is named by prefixing alkyl group to amine, i.e., alkylamine as one word (e.g. methylamine).
• In secondary and tertiary amines, when two or more group are the same, the prefix di or tri is appended before the name of alkyl group.
• In IUPAC system amines are named as alkanamines, derived by replacement of ‘e’ of alkane by the word amine. For example, CH3NH2 is named as methanamine. In case, more than one amino group is present at different positions in the parent chair, their positions are specified by giving numbers to the carbon atoms bearing –NH2 groups and suitable prefix such as di, tri, etc. is attached to the amine. The letter ‘e’ of the suffix of the hydrocarbon part is retained.
• For example, H2N–CH2–CH2–NH2 is named as ethane -1, 2-diamine.
• In arylamines, –NH2 group is directly attached to the benzene ring. C6H5NH2 is the simplest example of arylamine. In common system, it is known as aniline. It is also an accepted IUPAC name.
• While naming arylamines according to IUPAC system, suffix ‘e’ of arene is replaced by ‘amine’. Thus in IUPAC system, C6H5–NH2 is named as benzenamine.

Structure And Classification Of Amines

Amines are classified as 1°, 2°, or 3° depending on the number of carbon groups bonded to nitrogen.

• Aliphatic amine: All carbons bonded to nitrogen are derived from alkyl groups.

CH3–NH2 (Methylamine (a 1° amine))

• Aromatic amine: One or more of the group bonded to nitrogen are aryl groups.

Physical Properties Of Amines

• They have unpleasant odors (rotting fish like ammonia).
• They are polar compounds.
Difference in electronegativity between N–H (3.0 – 2.1 = 0.9)
• 1° and 2° amines have hydrogen bonds (N–H). Weaker than alcohols (O–H)
3° amines do not form hydrogen bonds (no H atom).
• Boiling points : Hydrocarbons < Amines < Alcohols
• Almost soluble in water (hydrogen bonding).

Preparation Of Amines

Reduction of Nitro Compounds

Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in acidic medium. Nitroalkanes can also be similarly reduced to the corresponding alkanamines.

Ammonolysis of Nitro Compounds

$$\\RX\xrightarrow[]{NH_3}\underset{\mathrm{(1\degree)}}{RNH_2}\xrightarrow[]{RX}\underset{\mathrm{(2\degree)}}{R_2NH}\xrightarrow[]{RX}\underset{\mathrm{(3\degree)}}{R_3NH}\xrightarrow[]{RX}\underset{\mathrm{(4\degree)}}{R_4N}^⊖$$

Reduction of Amides

The amides on reduction with lithium aluminium hydride amines.

Nitriles on reduction with lithium aluminium hydride (LiAlH4) or catalytic hydrogenation produce primary amines. This reaction is used ascent of amine series. i.e., for preparation of amines containing one carbon atom more than the starting amine.

$$\\ \underset{\mathrm{Nitrile}}{R–C \equiv N:} \xrightarrow[]{[R]}\underset{\mathrm{Nitrile}}{R–CH_2\ddot NH_2}$$

Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. In this degradation reactinon, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom. The amine so formed contains

Gabriel Phthalimide S ynthesis

Gabriel synthesis is used for the preparation of primary amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the

Identification Of Amines

Hinsberg’s reagent (benzene sulphonylchloride) reacts with primary, secondary and tertiary amines in a
different manner. This is used for the distinction of three types of amines.

• Primary amines react with benzene sulphonyl chloride to produce N-alkyl benzene sulphonamide. Hydrogen attached to nitrogen in the product is acidic so it soluble in alkali.
• A secondary amine reacts with Hinsberg’s reagent to from N.N-dialkyl benzene sulphonamide which remains insoluble in alkali.
• A tertiary amine does not react with benzene sulphonyl chloride.

Chemical Properties Of Amines

Addition of Water Acid catalysed addition of water gives alkyl formaldehyde derivative. RNC + H2O → RN = CHOH → RNHCHO

Reduction with LAH

Reduced to N-methyl amines.

RNC → RNHCH3

On reaction with HgO or with ozone as well as with halogen and dimethylsulphoxide, oxidised to isocyanates.

Salt Formation : Since amines are weak bases, they react with strong acids to form ammonium salts. Methylamine for example reactes with strong acid yielding the corresponding methylammonium salt.

$$\\ \underset{\mathrm{Methylamine molecule}}{CH_3NH_2(g)}\space\space+\underset{\mathrm{Hydrogen chloride molecule}}{HCl(g)}\space\space\xrightarrow[]{}\space\space\underset{\mathrm{Methylamine molecule}}{CH_3N+H_3Cl–}$$

Acylation

Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides and esters by nucleophilic substitution reaction. This reaction is known as acylation. This reaction is considered as the replacement of hydrogen atom of –NH2 or > N–H group by the acyl group.

Carbylamine R eaction

Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide forms isocyanides or carbylamines which are foul smelling substances.

Uses Of Amines

• Aromatic amines are mainly used as a starting material for the production of azo dyes.
• Many drugs are designed to interfere with the action of natural amines.
• Amines are used in making artificial fibers.
• Aromatic amines are used in detergent production.