Amines: Nomenclature, Classification and Its Basic Character

Amines

Eg:

Preparations

Reduction of Nitro Compounds

Nitro compounds are reduced to amines either by catalytic hydrogenation or chemically by using a metal and acid.

Ammonolysis

If ammonia is used in excess primary amine is the major product.

If alkyl halide is in excess, secondary amine, tertiary amine and finally quarternary ammonium salt can be formed.

The above method is known as Hofmann's ammonolysis method. The reactivity of the various halides decreases in the following order.

RI > R - Br > R - Cl

Disadvantage - mixture of amines.

Gabriel Pthalimide Synthesis

Pure primary amine is only obtained.

Reduction of Nitriles

This reaction is used for ascent of series.

Reduction of Amides

Secondary and Tertiary amides give secondary or tertiary amines.

Hoffman Bromamide Degradation

Acid amides on reaction with bromine in the presence of alkalis at about 343 K give primary amines.

The amine formed by this method has one carbon atom less than the parent compound.

This reaction involves migration of alkyl group from carbonyl with the elimination of CO₂.

Reductive Amination of Carbonyl Compounds

Reduction of products obtained on nucleophilic substitution of ammonia and its derivatives.

If we use 1^o amine in place of ammonia, it will yield 2^o amine.

Physical Properties

Lower members are gases while higher members are liquids. Among, aryl amines, lower members are liquids but higher members are solids.

Methyl amine and ethyl amine have ammoniacal smell but higher amines have fishy smell.

Boiling points

Amines are polar and have higher boiling points than equivalent non-polar compounds.

Only primary and secondary amines have higher boiling points than tertiary amines due to hydrogen bonding between nitrogen of one and hydrogen of another molecule.

Orde of b.p.-Primary > Secondary > Tertiary

However amines have lower boiling points than those of corresponding alcohols or carboxylic acids due to lower hydrogen bonding.

Solubility

Smaller amines are soluble in water but it decreases with inceasing size.

the solubility of amines in water is less than that of alcohols.

Basicity of Amines

In amines, there is a lone pair of electrons on nitrogen. The tendency of nitrogen to share these electrons with acids is responsible for the basic character of amines.

Basic character of aliphatic amines should increase with increase of alkyl substitution due to +I effect of alkyl group. Hence expected attitude is:

tertiary amine > secondary amine > primary amine > ammonia.

However, the order of basicity in aqueous solution does not follow the expected trend:

This is due to

steric effect: crowding of alkyl groups around N atom increases from 1^o to 3^o amines and stability decreases.

hydration effect: Greater the extent of H - bonding in protonated amine more will be its stabilization which increases from 3^o to 1^o.

The resultant of all factors cause 2^o amine to be still more basic than 1^o amine.

Solublity of aniline

Low stability of anilium ion results in low stability.

The electron releasing groups (like -OCH₃, -CH₃, -NH₂ etc) enhance the availability of unshared electrons on nitrogen and increases the basic strength.

Electrons withdrawing groups like (-NO₂, -CN, -X etc) affect the stability of an aromatic ammonium cation and decreases the basic strength of parent aromatic amine.

Chemical Properties:

Salt Formation

Amines being basic react with acids to form salts.

These are soluble in water but insoluble in non-polar solvents treatment with aqueous hydroxide regenerate the amines.

Acylation

Aliphatic and aromatic primary and secondary amines undergo acylation through nucleophilic substitution of acid derivatives such as acid halides or anhydrides to form amides.

Acid chlorides are stronger acylating agents when compared to anhydrides and esters.

Tertiary amines do not react with acid derivatives

Formation of Sulphonamides

Primary amines combine with benzene sulphonyl chloride (C₆H₅SO₂Cl) to form sulphonamides.

which is solube in alkali(due to acidic hydrogen).

The product formed by the action of secondary amine on benzene sulphonyl chloride does not dissolve in an alkali.

Tertiary amines do not react with C₆H₅SO₂ Cl.

Hence Hinsberg's reagent is used for separation of primary, secondary and tertiary amines.

With Carbonyl Compounds

Primary amines react with aldehydes and ketones to yield imines also called Schiff bases.

Carbylamine Reaction

In presence chloroform and alcoholic potash, an alkyl isocyanide (carbylamine) is formed which gives an offensive smell. Only given by primary amines, hence used as a test.

With Nitrous Acid

Primary aromatic amine reacts with nitrous acid to form a diazonium salt which is stable but low temperatures.

On reaction with phenol, an aromatic diazonium salt undergoes a coupling reaction to form a dye.

Primary aliphatic amines form diazonium salts which are unstable and liberate nitrogen to form a carbonium ion which forms an alcohol or on proton elimination gives an alkene.

Secondary amines, both aliphaptic and aromatic give yellow colored oily product insoluble in mineral acids.

Aliphatic tertiary amines form water soluble salts with nitrous acid.

Aromatic tertiary amines on reaction with nitrous acid, undergo electrophilic substitution with nitro sonium ion at the p-position of the phenyl ring.

So this can be used to distinguish amines.

Electrophilic Substitution

All amino groups -NH₂, -NHR, -NR₂ are most powerful activating groups in aromatic electrophilic substitution reactions are ortho and para directing.

The high reactivity is due to the fact that in addition to other resonating structures that stabilize intermediate carbonium ion in aromatic electrophilic substitution,

This results in instantaneous bromination of aniline at all of the three O, p – positions.

For controlling bromination also, the amide derivatives are used which restores aniline on hydrolysis.