Preperation Of Amines

Question 1

Reduction of nitro compounds

Answer 1

Nitro compounds when reduced by passing hydrogen gas in the presence of finely divided nickel,
palladium or platinum or by reduction with metals in acidic medium, we get amines.
R-NO2 + H2. ( Pd/ethanol——>
R-NH2
Nitrobenzene when reduced using H2 in presence of Ni, Pd or Pt Or, by using iron or tin and HCl,
we get aniline.
Reduction with iron and hydrochloric acid is preferred because FeCl2 formed gets hydrolysed to release
hydrochloric acid during the reaction. Thus, only a small amount of hydrochloric acid is required to initiate
the reaction.

Reaction refer note

Question 2

Ammmonolysis of alky halides

Answer 2

Alkyl halides when treated with alcoholic solution of ammonia at 373 K, undergo nucleophilic
substitution reaction to give a mixture of primary, secondary and tertiary amines and quaternary
ammonium salt (R4N+X –
). This process of cleavage of C-X bond by ammonia is called ammonolysis
(Hofmann’s Ammonolysis).
R-X + NH3 (alc.) ——->
R-NH2 + R2NH + R3N + R4N+X –
If large excess of ammonia is used, primary amine is formed as the major product. The order of reactivity
of alkyl halides with amines is RI > RBr >RCl.

Question 3

Reduction of nitriles

Answer 3

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

Reaction refer note

Question 4

Reduction of amides

Answer 4

Acid amides on reduction with lithium aluminium hydride give amines.

Reaction refer note

Question 5

Gabriel phthalimide synthesis

Answer 5

This method is used for the preparation of aliphatic primary amines only. Phthalimide when treated
with ethanolic solution of potassium hydroxide to form potassium salt of phthalimide which on
heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine

Reaction refer note

Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo
nucleophilic substitution with the anion formed by phthalimide.

Question 6

Hoffmann bromomide reaction

Answer 6

When an amide is treated with bromine and ethanolic solution of NaOH, we get a 10
amine. This
reaction is called Hoffmann bromamide degradation reaction. It is used for the conversion of an
amide to a primary amine with one carbon atom less than that present in the amide.

Reaction refer note

Question 7

Physical properties

Answer 7

1) lower amines have fishy smell
2) solubility
3) boiling point

Question 8

Boiling point

Answer 8

Boiling point: Primary and secondary amines are associated through intermolecular hydrogen
bonding. This intermolecular association is more in primary amines than in secondary amines. Tertiary
amines do not have intermolecular association due to the absence of hydrogen atom. Therefore, the order of
boiling points of isomeric amines is as follows: Primary > Secondary > Tertiary

Question 9

Solubility

Answer 9

Solubility: Lower aliphatic amines are soluble in water because they can form hydrogen bonds with
water molecules. However, solubility decreases with increase in molar mass of amines due to increase in size
of the hydrophobic alkyl part. Higher amines are insoluble in water. Amines are soluble in organic solvents
like alcohol, ether and benzene.

Question 10

Basic character of amines

Answer 10

Amines react with acids to form salts. This reaction shows that they are basic
in nature.
R-NH2 + HX ——>
R-NH3+X–

C6H5-NH2 + HCl ——–>
C6H5NH3+Cl –
Aniline
Anilinium chloride
In amine, there is an unshared pair of electrons on nitrogen atom and hence it can donate this
electron pair. So it acts as a Lewis base. Basic character of amines can be expressed in terms of Kb and pKb.
(pKb = - log Kb)
Greater the value of Kb, smaller will be the pKb value and stronger will be the base.
Aliphatic amines are stronger bases than ammonia. This is due to the +I effect of alkyl groups
leading to greater electron density on the nitrogen atom. But aromatic amines are weaker bases than
ammonia. This is due to the electron withdrawing nature of the aryl group

Question 11

Basicity of amines is related to ?

Answer 11

Basicity of amines is related to their structure. Basic character of an amine depends upon the ease of
formation of the cation by accepting a proton from the acid. As the stability of the cation increases, the
basicity also increases.