Test for Amines
Amines are organic compounds derived from ammonia, integral in a great number of aspects in our daily lives and the wider chemical landscape. From the caffeine that wakes one up in the morning to the pharmaceuticals that keep chronic conditions at bay, amines are ubiquitous and vital. From agriculture, where they form some of the essential ingredients of fertilizers and pesticides, to the food industry, where they improve flavor and perfume, the applications of amines are vast. Thus, the techniques for identifying and analyzing amines must be learned by all chemists and biochemists, as well as by many industrial professionals.
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The tests for amines not only help in distinguishing them from other organic compounds but also throw light upon their chemical properties and reactivity. In this paper, we will consider the major tests for amines: the carbylamine test, alkylation and acylation of amines, and the reaction of amines with sodium nitrite and hydrochloric acid.
Understanding Amines and Their Identification
Amines are organic compounds that contain nitrogen atoms directly bonded with 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 applied in detecting 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 some of the transformations that change 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 behavior.
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 has smell like mustard oil.
$\mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2+\mathrm{CS}_2+\mathrm{HgCl}_2 \rightarrow \mathrm{C}_2 \mathrm{H}_5 \mathrm{NCS}+\mathrm{HgS}+2 \mathrm{HCl}$
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 Me2SO4 amines undergo monomethylation. 1o and 2o amines are also methylated by heatingHCHO and excess of HCOOH at 100oC. This reaction is known as Eschweiler-Clarke methylation. The reaction occurs as follows:
Acylation
1o and 2o 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:
$\mathrm{NaNO}_2+\mathrm{HCl} \rightarrow \mathrm{HNO}_2+\mathrm{NaCl}$
1o 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
Importance and Uses of Amines Testing
These tests carried out for amines are of immense significance both in academic research and application at industrial levels. Identification of amines is very essential in the pharmaceutical industry for drug development, as most active pharmaceutical ingredients (APIs) are composed of amine functional groups. For example, quality control may involve the use of the Carbylamine test to ensure primary amines are pure for use in formulations of drugs.
More importantly, alkylation and acylation of amines are the critical steps in the synthesis of several compounds, agrochemicals, and specialty chemicals. These processes can modulate the properties of amines towards desired applications, making them more effective in formulated compositions.
These tests allow one to have control over the behavior of amines and, therefore, to achieve core competence in the understanding of organic chemistry at an academic level. They build critical thinking and problem-solving by teaching one how to design experiments and interpret results. Case studies involving the synthesis of dyes using diazonium salts bring forth practical applications of these tests; hence, they show relevance in real-world scenarios.
These tests for amines not only provide methods for the identification and characterization of such important compounds but underline their versatility and multifunctionality in fields as varied as medicine to agriculture. Light is thrown on the feasibility of their applicability in an array of disciplines.
Recommended topic video on(Test for Amines)
Some Solved Examples
Example 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).
Example 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.
Example 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).
Summary
The identification and analysis of amines with the aid of different tests are rather very basic in organic chemistry and its applications. The Carbylamine test is the confirmatory test for primary amines, while their alkylation or acylation makes them more useful in synthesis. Their reaction with sodium nitrite and hydrochloric acid in the formation of diazonium salts is very important in organic synthesis. These tests thus find applications outside the laboratory—for example, in pharmaceuticals and agriculture—to form a base for academic research and education.
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