Amines

When the hydrogen atom of ammonia is replaced by an alkyl group, it is called an amine.

\underset{Ammonia}{NH_{3}}\ \xrightarrow[-H]{+R}\ \underset{Amine}{R-NH_{2}}

Classification:

On the basis of the number of alkyl groups present in the nitrogen atom, amines are classified into three classes.

1. Primary (1°) amine: When one hydrogen atom of ammonia is replaced by an alkyl group, the amine is called a primary amine. Its functional group is -NH₂.

2. Secondary (2°) amine: When two hydrogen atoms of ammonia are replaced by an alkyl group, the amine is called a secondary amine. Its functional group is -NH-.

3. Tertiary(3°) amine: When three hydrogen atoms of ammonia are replaced by an alkyl group, the amine is called a tertiary amine. Its functional group is:

Nomenclature

Primary amine (1°)

Secondary (2°) amine

Tertiary (3°) amine

Isomerism

1. Chain isomerism

2. Positional isomerism

3. Functional isomerism (1°,2° and 3° amine)

4. Metamerism

Q. Write the structure and IUPAC names of all possible isomers formed by C₃H₉N. Also, indicate 1°, 2° and 3° amine.

Separation of Primary, Secondary, and Tertiary amines by Hofmann method:

1. Treatment with diethyl oxalate: The mixture of amines is treated with diethyl oxalate. Following changes takes place.

i. Primary amine forms diethyl oxamide which is solid.

ii. Secondary amine forms dialkyl oxamic ester which is liquid.

iii. Tertiary amine doesn’t react as it doesn’t contain a replaceable hydrogen atom on nitrogen.

2. Separation of unreacted tertiary amine: Tertiary amine is more volatile than oxamide and oxamic ester, so separated by distillation.

3. Separation of oxamide and oxamic ester: Solid oxamide and liquid oxamic ester are separated by filtration.

4. Regeneration of primary and secondary amine: Solid oxamide and liquid oxamic ester are separately boiled with KOH to regenerate the respective amine.

Preparation of primary amines:

1. From haloalkane: Alkyl halide heated with excess ammonia gives primary amine.

\begin{align*} R-X+NH_{3}&\rightarrow R-NH_{2}+HX\\ \underset{Chloroethane}{CH_{3}CH_{2}Cl}+NH_{3}&\rightarrow \underset{Ethanamine}{CH_{3}CH_{2}NH_{2}}+HCl\end{align*}

2. From nitrile: Alkyl cyanide (alkanenitrile) reduced with H₂Ni, LiAlH_4, or Na/C₂H₅OH gives primary amine.

\begin{align*} R-CN+4[H]&\xrightarrow{LiAlH_{4}}R-CH_{2}NH_{2}\\ \underset{Ethanenitrile}{CH_{3}CN}+4[H]&\xrightarrow{LiAlH_{4}} \underset{Ethanamine}{CH_{3}CH_{2}NH_{2}} \end{align*}

3. From nitroalkane: Reduction of nitroalkane gives primary amine.

\begin{align*} R-NO_{2}+6[H]&\xrightarrow{LiAlH_{4}}R-NH_{2}+2H_{2}O\\ \underset{Nitromethane}{CH_{3}NO_{2}}+6[H]&\xrightarrow{LiAlH_{4}} \underset{Methanamine}{CH_{3}NH_{2}} + H_{2}O \end{align*}

4. From amides:

i. By reduction of amides:

ii. Hofmann’s bromamide reaction (decarbonylation): Amides on reaction with Br₂ in presence of aq. KOH gives primary amines. This reaction is also called Hofmann Hypobromite or degradation reaction.

Physical Properties:

1. Lower members are gases while higher members are liquid. Methylamine and ethylamine have ammoniacal smells but higher amines have fishy smells.
2. Lower members of amine are soluble in water due to the formation of an intermolecular hydrogen bond with water.

Higher members are insoluble in water due to increases in hydrophobic hydrocarbon parts.

3. All amines except tertiary amines have high boiling points because of the formation of the intermolecular hydrogen bonds.

Primary amine > Secondary amine > Tertiary amine
Decreasing order of boiling point

Chemical Properties of amine:

1. Basicity of amine: Amines are basic in nature due to the presence of lone pair of electrons on the nitrogen atom. They donate lone pair of electrons to an electron-deficient molecule and behave as a lewis base. They release OH^– ion in an aqueous medium.

R-NH_{2} + H_{2}O \rightleftharpoons RNH_{3}^{+} + OH^{-}

The presence of an alkyl group increases the electron density on the nitrogen atom which makes the amines more basic than ammonia.

\underset{\substack{+I\ effect\\ (more\ basic)}}{R\rightarrow NH_{2}}\ \ \underset{\substack{No\ +I\ effect\\ (less\ basic)}}{NH_{3}}

Comparison of basic strength of primary, secondary, and tertiary amine:

The basic strength of primary, secondary, and tertiary amines can be explained on the basis of the following three factors.

i. Inductive (+I) effect

The presence of alkyl group increases the electron releasing capacity of nitrogen and the basic strength of amines increases from primary to tertiary.

ii. Steric effect

The size of an alkyl group is larger than the hydrogen atom. It hinders the attack of acid on amines and thus decreases the basic nature. Greater the number of the alkyl group, greater will be the steric hindrance and lesser will be the basic strength of amine.

iii. Hydration effect

Amines get protonated by accepting H⁺ ions in an aqueous medium. The water molecules form hydrogen bonding with the protonated amine. Greater the extent of hydrogen bond in protonated amines, more will be stabilization, and high will be the basic strength of amines.

All these three factors favour secondary amines. So secondary amine becomes most basic in the aqueous medium.

Secondary > Primary > Tertiary
Decreasing order of basic strength

In the gaseous or non-aqueous state, the hydration effect is absent. So basicity depends on the inductive effect.
Tertiary > Secondary > Primary

• Action with conc. HCl: Amines reacts with conc. HCl to form ammonium chloride (salt).

2. Action with alkyl halide (alkylation): Primary amines react with an alkyl halide to form secondary and tertiary amines with a quaternary ammonium salt.

3. Action with acid chloride and anhydride (Acylation): Primary amines react with acid chloride to form N-substituted amide.

4. Carbylamine reaction / Isocyanide test: Primary amines react with chloroform in presence of alcoholic KOH to form isocyanide or carbylamine which is a bad-smelling compound.

Test of 1° , 2° and 3° amine (Nitrous acid test):

1. Primary amine reacts with nitrous acid to give alcohol with brisk efforvation of nitrogen gas.

2. Secondary amine reacts with nitrous acid to form nitrosoamine which is a yellow oily layer.

3. Tertiary amine reacts with nitrous acid to form nitrite salt.