Test for Amines

Amines

Amines are organic compounds that contain nitrogen atoms directly bonded to carbon atoms. They may be classified as primary, secondary, and tertiary amines according to the number of carbon groups the nitrogen has. It is rather important to identify the amines within the sphere of organic chemistry because each compound follows characteristic reactivity patterns. For example, one of the classical qualitative tests used to detect primary amines is the Carbylamine test. An amine reacts with chloroform and a strong base to give a foul-smelling isocyanide that functions as a test for primary amine. Alkylation and acylation are transformations that alter the physical properties and reactivity of amines. Products of reactions of amines with NaNO₂ and HCl contain diazonium salts, which become important starting materials in synthetic organic chemistry. Knowing these tests makes one not only identify amines but also helps in improving our understanding of their chemical behaviour.

Types of Tests for Amines

Carbylamine Test

The Carbylamine test is a simple and convenient test for identifying primary amines. Indeed, the reaction of such compounds with chloroform and a strong base, like potassium hydroxide, may yield a foul-smelling compound called carbylamine or isocyanide. This clearly differentiates the primary amines from the secondary and tertiary ones, with which quite different products are formed.

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 the 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.

The 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 a 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 disulfide and mercuric chloride, ethyl isothiocyanate is formed, which smells like mustard oil.

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

Alkylation and Acylation of Amines

Alkylation and acylation of amines are very important for the change in structure and properties. Alkylation is the introduction of any alkyl group into the nitrogen atom, commonly from alkyl halides. It may enhance the reactivity and solubility of the amine. Acylation is the introduction of an acyl group to the amine, usually through its reaction with acid chlorides or anhydrides, which gives an amide. These reactions are of importance in organic synthesis because they give the chemist tools for building from simple amines to more complex molecules.

Alkylation

Amines undergo alkylation with RX and undergo complete alkylation, and this is called exhaustive alkylation, but with Me₂SO_4, amines undergo monomethylation. 1^o and 2^o amines are also methylated by heating HCHO and an 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:

Reaction with NaNO₂ + HCl

One of the most important reactions of amine functionality is the reaction with sodium nitrite, along with hydrochloric acid, HCl. It is very well known that the reaction of primary amine with $\mathrm{NaNO}_2$ in an acidic medium affords diazonium salt, a high-energy species capable of further reactions like coupling with aromatic compounds and the formation of azo dyes. The reaction finds enormous application in synthetic organic chemistry, largely in dye and pharmaceutical preparations.O₂,

Nitrous acid$\left(\mathrm{HNO}_2\right)$ is obtained in situ by the reaction of sodium nitrite $\left(\mathrm{NaNO}_2\right)$ 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$\mathrm{HNO}_2$ to form aliphatic diazonium salts, which, being unstable, liberate $\mathrm{N}_2$ gas quantitatively and form alcohols. Quantitative evolution of $\mathrm{N}_2$ 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

Some Solved Examples

Question 1: Which of the following methods is neither meant for the synthesis nor the separation of amines?
1) Hinsberg method
2) Hofmann's Reaction
3) Wurtz reaction
4) Curtius reaction

Solution: The Wurtz reaction is a method used for the synthesis of alkanes from alkyl halides, not for the synthesis or separation of amines. The Hinsberg method, Hofmann's Reaction, and Curtius reaction are specifically used for the synthesis or separation of amines. Hence, the correct answer is option (3).

Question 2: What is Hinsberg's reagent?

1) $(\mathrm{C}_6\mathrm{H}_5\mathrm{COCl})$
2) $(SOCl_2)$
3) $(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{SO}_{2} \mathrm{Cl})$
4) $(COCl)_2)$

Solution: Hinsberg's reagent is $ (\mathrm{C}_6\mathrm{H}_5\mathrm{SO}_2\mathrm{Cl})$, which is used for distinguishing between primary, secondary, and tertiary amines. Option (3) is the correct answer.

Question 3: Mulliken-Barker test induces which of the following?
1) Reduction of NO₂ group to NHOH
2) Yellow precipitate
3) Fehling's test
4) White precipitate

Solution: The Mulliken Barker test involves the reduction of the NO₂ group to the NHOH group. This hydroxylamine further reacts to produce a nitroso compound. Hence, the correct answer is option (1).

Question 4: Which of the following amines gives a positive Carbylamine test?
A. $\left(\mathrm{CH}_3\right)_2 \mathrm{NH}$
B. $\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$
C. $\mathrm{CH}_3 \mathrm{NH}_2$
D. $\left(\mathrm{C}_2 \mathrm{H}_5\right)_2 \mathrm{NH}$

Solution:

Only primary amines give the Carbylamine (Isocyanide) test. Methylamine $\left(\mathrm{CH}_3 \mathrm{NH}_2\right)$ is a primary amine.

Question 5: Which of the following will not give nitrogen gas with nitrous acid at 0–5°C?

A. Ethylamine
B. Methylamine
C. Aniline
D. Dimethylamine

Solution:

Primary amines give N₂ with nitrous acid, whereas secondary amines form N-nitroso compounds and do not evolve nitrogen gas.

Hence, the correct answer is option (D)

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