Test for Amines - Practice Questions & MCQ

Action of Nitrous Acid:
Nitrous acid can distinguish between primary, secondary and tertiary amines. Primary amines on reaction with nitrous acid form Alcohols with the evolution of dinitrogen gas. Secondary amines form N- Nitrosoamines which have a characteristic yellow oily texture. Tertiary amines do not react with Nitrous acid and there is no visible change in the reaction mixture.

The reaction occurs as follows: 

Action of Nitrous Acid

Hoffmann's Test
Separation of primary, secondary and tertiary amines by Hoffmann's method. The mixture of three amines is treated with diethyl oxalate. The primary amine forms a solid oxamide, a secondary amine gives a liquid oxamic ester while tertiary amine does not react.

Hinsberg's Test
The Hinsberg test which is used to distinguish between primary, secondary, and tertiary amines, is based upon sulfonamide formation. In this test, an amine is reacted with benzene sulfonyl chloride. If a product forms, the amine is either a primary or secondary amine, because tertiary amines do not form stable sulfonamides. If this sulfonamide that formed is dissolved in aqueous sodium hydroxide solution, it is a primary amine. But if it is insoluble in aqueous sodium hydroxide, then it is a secondary amine. The sulfonamide of a primary amine is soluble in an aqueous base because it still possesses an acidic hydrogen on the nitrogen, which can be lost to form a sodium salt.

Hoffmann's Mustard Oil Reaction
When ethylamine is heated with carbon disulphide and mercuric chloride, ethyl isothiocyanate is formed which has smell like mustard oil.

$\mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2+\mathrm{CS}_2+\mathrm{HgCl}_2 \rightarrow \mathrm{C}_2 \mathrm{H}_5 \mathrm{NCS}+\mathrm{HgS}+2 \mathrm{HCl}$

When any primary amine(aliphatic or aromatic) is heated with chloroform and alcoholic potassium hydroxide solution, isocyanide(carbylamine) is formed which has a very unpleasant smell. This test is called carbylamine test or isocyanide test. The reactions occur as follows:

$\begin{aligned} & \mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2+\mathrm{CHCl}_3+3 \mathrm{KOH} \rightarrow \mathrm{C}_2 \mathrm{H}_5 \mathrm{NC}+3 \mathrm{KCl}+3 \mathrm{H}_2 \mathrm{O} \\ & \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+\mathrm{CHCl}_3 \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NC}+3 \mathrm{KCl}+3 \mathrm{H}_2 \mathrm{O}\end{aligned}$

It is to be noted that Aromatic primary amines also respond to this test as shown in the reaction above.

Alkylation
Amines undergo alkylation with RX and undergo complete alkylation and this is called exhaustive alkylation, but with Me₂SO₄ amines undergo monomethylation. 1^o and 2^o amines are also methylated by heating HCHO and excess of HCOOH at 100^oC. This reaction is known as Eschweiler-Clarke methylation. The reaction occurs as follows:

Acylation
1^o and 2^o aliphatic and aromatic amines react with acid chlorides (RCOCl), anhydrides and esters by SN² reaction is called acylation reaction. The reaction is carried out in the presence of a base stronger than amine, such as pyridine, which removes HCl so formed and shifts the equilibrium to the product side. The reaction occurs as follows:

Nitrous acid(HNO₂) is obtained in situ by the reaction of sodium nitrite (NaNO₂) with dil. HCl. The reaction occurs as follows:

$\mathrm{NaNO}_2+\mathrm{HCl} \rightarrow \mathrm{HNO}_2+\mathrm{NaCl}$

1^o aliphatic amines react with HNO₂ to form aliphatic diazonium salts, which, being unstable, liberate N₂ gas quantitatively and form alcohols. Quantitative evolution of N₂ is used in the estimation of amino acids and proteins.

The reaction occurs as follows:

Mechanism

Nitrous acid can distinguish between primary, secondary and tertiary amines. Primary amines on reaction with nitrous acid form Alcohols with the evolution of dinitrogen gas as seen above. 

Secondary amines form N- Nitrosoamines which have a characteristic yellow oily texture. Tertiary amines do not react with Nitrous acid and there is no visible change in the reaction mixture.

The reaction occurs as follows: 

Action of Nitrous Acid