Preparation Of Amines And Their Physical Properties Note For Class 12 Che

Preparation Of Amines And Their Physical Properties

Now as you know about Amines from our previous Article. In this article, you will get to know about the Preparation of Amines, Physical Properties of Amines and Basicity of Amines, which are important topics of NCERT Chemistry Class 12. Enjoy Reading!!

Methods of Preparation of Amines

1. By reduction of nitro (RNO₂) compounds –

Nitro compounds can be reduced either by catalytic hydrogenation using molecular hydrogen or by chemical reduction, usually by metal and acid. This method is not used when the molecule also contains some other easily hydrogenated group, such as a carbon double bond. The Chemical reduction method is often carried out by adding hydrochloric acid to a mixture of the nitro compound and metal (granulated tin or iron).

2. By reduction of nitriles (RCN)

Alkyl and aryl cyanides or nitriles can be reduced to their corresponding primary amines using lithium aluminum hydride (LiAlH4) or catalytic hydrogenation.

3. By reduction of amides (RCONH₂)

Amides can directly be converted into their corresponding amines, by treating the amide with a mixture of base and bromine (KOH + Br2). This reaction is named a Hofmann Bromide reaction.

The reaction is written as:

RCONH₂ + Br₂ + 4KOH → RNH₂ + K₂CO₃ + 2KBr + 2H₂O

In the above reaction, the amine formed has one carbon less than that of the corresponding amide. Due to the loss of carbon atom, this reaction is also called as Hofmann degradation of amides.

4. By ammonolysis of alkyl halide

Aliphatic halogen compounds are converted into amines by treatment with aqueous (or) alcoholic solution of ammonia. This reaction is carried out either by allowing the reactants to stand together at room temperature or by heating them under pressure. Displacement of halogen by NH₃ produces the amine salt, from which free amine can be liberated with hydroxide ion.

Primary alkyl halides undergo substitution very easily as compared to tertiary alkyl halides, which undergo elimination very easily. The primary amine formed acts as a base and keep on reacting with the alkyl halide until each of the hydrogen atoms of amine gets replaced with alkyl groups. Thus, reaction with an alkyl halide yields a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt. Here, primary amine is obtained as a major product by taking a large excess of ammonia.

5. Gabriel phthalimide synthesis

This method is used for the preparation of primary amines. Phthalimide is treated with ethanolic potassium hydroxide and forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine. Aromatic primary amines are not prepared by this method as aryl halides do not undergo the reaction with the anion formed by phthalimide.

PHYSICAL PROPERTIES OF AMINES

i. The lower aliphatic amines are gaseous in nature having a fishy smell.

ii. Primary amines with three or more carbon atoms are liquid and higher members are all solids.

iii. Lower amines are soluble in water because they can form hydrogen bonds with water, but the solubility decreases with an increase in the hydrophobic alkyl group.

iv. Amines have boiling points greater than the hydrocarbons of comparable molecular mass. This is due to the fact that they have the ability to associate through intermolecular hydrogen bonding.

v. Boiling points of various types of amines are in the order given below:

Basicity of amines

Amines act as Lewis bases (a compound or ionic species that has an ability to donate an electron pair to an acceptor compound). The nitrogen atom bears a lone pair of electrons; thus, it acts a Lewis base. The basic character of amines is expressed in terms of their K_b and pK_b values. K_b is the base dissociation constant, while pK_b is the -log of the constant.

All aliphatic amines are strong bases than NH₃ because alkyl groups donate electrons to the nitrogen atom, which is more electronegative than the alkyl group. The inductive effect makes the electron density on the alkyl amine’s nitrogen greater than the nitrogen of ammonium; hence, lone pair of nitrogen in aliphatic amines is freely available for donation. Similarly, primary, secondary, and tertiary alkyl amines are more basic than ammonia. The aromatic amines are weaker bases than NH₃ owing to the electron-withdrawing nature of the aryl group. In aromatic amines, the lone pair on the nitrogen atom is involved in resonance with the benzene ring (the electrons are with both the aromatic ring and the nitrogen atom). So, they are less available for donation.

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