How to Identify Primary Secondary and Tertiary Amines Using Chemical Tests
Amines are classified into different categories namely, primary, secondary, and tertiary depending on the number of carbons which are bonded directly to the nitrogen atom. Primary amines have just one carbon that is bonded to the nitrogen. Secondary amines, on the other hand, have two carbons which are bonded to the nitrogen. And lastly, the tertiary amines have three carbons that are bonded to the nitrogen. The system of classification of all these amines is superficially similar in the way we have classified alcohols.
However, the important difference is that in alcohols we count bonds to the carbon that carry the OH group. In the case of amines, we count the carbons which are bonded to the nitrogen. All of these primary, secondary and tertiary amine show varying chemical properties and have observable physical changes. They have their uses typically in the commercial and industrial applications. Today, we will learn about the identification of primary amine, secondary amine, tertiary amine, the primary amine, secondary aromatic amine and tertiary amine definition, and the primary, secondary, tertiary amine formula.
Structure of Primary Amine, Secondary Amine, Tertiary Amine
Let us look at how primary amine and secondary amine and tertiary aromatic amine look like. Take a look at the figure given below.
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Properties of Primary and Secondary Amines
Amines, irrespective of them being primary, secondary, or tertiary amine groups, usually have specific distinct properties about them, for example, their characteristic odours. These odours usually resemble that of rotten eggs or fishes. Aliphatic amines are known to be less dense than water and are stronger bonds of ammonia compared to the aromatic amines.
The major industrial applications of the primary, secondary amine are to make dyes, rubber, synthetic fibres and resins, and pharmaceuticals. However, specific tests are done for the identification of primary amines, secondary amines, and tertiary amines. Amongst the most popular tests is the Hinsberg test. The reaction which is produced from this test is known as the Hinsberg reaction. Let us learn about the Hinsberg test and Hinsberg reaction in detail.
Hinsberg Test
The Hinsberg test is a test to identify the primary, secondary and tertiary amines. In this test, the given amine is to be shaken well along with Hinsberg reagent in the presence of an aqueous alkali, which can be either NaOH or KOH. Then, a reagent constituting of benzenesulfonyl chloride and an aqueous sodium hydroxide solution is to be added to a substrate. A primary amine will tend to form a soluble sulfonamide salt. Acidification of this salt precipitates then the sulfonamide of this primary amine. A secondary amine is to undergo the same reaction which will directly tend to form an insoluble sulfonamide. A tertiary amine, however, will not react with the sulfonamide but it is insoluble. On adding dilute acid, this particular insoluble amine gets converted to a soluble ammonium salt. In this manner, the reaction can distinguish between all the three types of amines.
You must know that the tertiary amines can react with the benzenesulfonyl chloride under various conditions since the Hinsberg test described above is not absolute. However, the Hinsberg test for identification of different types of amines is only valid when the reaction speed, temperature, concentration, and solubility are taken into consideration.
The Hinsberg reaction is shown in the figure below.
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Hence, the Hinsberg test is highly effective for the identification of primary amines, secondary amines, and tertiary amines. Amines are generally used for many different purposes, for example, a great variety of different medicines, photographs, etc. not only this but also the amine used for the synthesis of insecticides and rocket propellants. Hence, amines have an entire host of several different uses in the industry apart from their traditional chemical uses. Amines also have their use in the heavy-duty military functions, for example, the production of synthetic fibres, and the production of Kevlar. This is known to be the main component in making bulletproof vests and helmets to protect the soldiers in warfare.
FAQs on Amines Identification and Qualitative Analysis in Organic Chemistry
1. What are amines in chemistry?
Amines are organic derivatives of ammonia (NH3) in which one or more hydrogen atoms are replaced by alkyl or aryl groups. They contain a nitrogen atom with a lone pair of electrons, which makes them basic in nature.
- Primary amine (1°): R–NH2
- Secondary amine (2°): R2NH
- Tertiary amine (3°): R3N
2. How do you identify primary, secondary, and tertiary amines?
Primary, secondary, and tertiary amines are identified using the Hinsberg test and reactions with nitrous acid (HNO2).
- Primary amine: Forms a soluble sulfonamide in alkali and gives brisk effervescence of N2 with HNO2.
- Secondary amine: Forms an insoluble sulfonamide and produces a yellow oily nitrosamine with HNO2.
- Tertiary amine: Does not react with Hinsberg reagent and forms a soluble ammonium salt with HNO2.
3. What is the carbylamine test for amines?
The carbylamine test is a chemical test used to identify primary amines by the formation of foul-smelling isocyanides. Only primary amines react with chloroform and alcoholic KOH on heating:
R–NH2 + CHCl3 + 3KOH → R–NC + 3KCl + 3H2O.
- Positive test: Offensive isocyanide smell
- Applicable to: Aliphatic and aromatic primary amines
- Secondary and tertiary amines: No reaction
4. How does the Hinsberg test identify amines?
The Hinsberg test distinguishes amines based on their reaction with benzenesulfonyl chloride (C6H5SO2Cl).
- Primary amine: Forms a sulfonamide soluble in alkali due to acidic hydrogen.
- Secondary amine: Forms a sulfonamide insoluble in alkali.
- Tertiary amine: Does not form sulfonamide; remains insoluble but dissolves in dilute acid.
5. How do amines react with nitrous acid?
Amines react differently with nitrous acid (HNO2) depending on their class.
- Primary aliphatic amine: Produces alcohol with evolution of nitrogen gas:
R–NH2 + HNO2 → R–OH + N2↑ + H2O - Primary aromatic amine: Forms stable diazonium salt at 0–5°C.
- Secondary amine: Forms yellow nitrosamine.
- Tertiary amine: Forms soluble ammonium salt.
6. What is the difference between aliphatic and aromatic amines in identification tests?
The key difference is that aromatic primary amines form stable diazonium salts with nitrous acid, while aliphatic primary amines do not.
- Aliphatic primary amine: Releases N2 gas immediately.
- Aromatic primary amine: Forms diazonium salt (Ar–N2+Cl−) at 0–5°C.
- Aromatic amines are generally less basic due to resonance stabilization.
7. Why are amines basic in nature?
Amines are basic because the nitrogen atom contains a lone pair of electrons that can accept a proton (H+). When an amine reacts with an acid, it forms an ammonium salt:
R–NH2 + HCl → R–NH3+Cl−.
- Aliphatic amines: More basic due to +I effect of alkyl groups.
- Aromatic amines: Less basic due to resonance delocalization.
8. How can you distinguish between primary, secondary, and tertiary amines using the carbylamine test?
Only primary amines give a positive carbylamine test, while secondary and tertiary amines do not react.
- Primary amine: Produces foul-smelling isocyanide (positive test).
- Secondary amine: No reaction.
- Tertiary amine: No reaction.
9. What is the chemical formula of a simple amine example?
A simple example of an amine is methylamine (CH3NH2), which is a primary amine derived from ammonia.
- General formula of primary amine: R–NH2
- Example: Ethylamine (C2H5NH2)
- Tertiary example: Trimethylamine ((CH3)3N)
10. What are common laboratory tests for identification of amines?
Common laboratory tests for amine identification include the carbylamine test, Hinsberg test, and reaction with nitrous acid.
- Carbylamine test: Detects primary amines by isocyanide formation.
- Hinsberg test: Differentiates 1°, 2°, and 3° amines.
- Nitrous acid test: Produces N2 gas, nitrosamine, or diazonium salt depending on type.
- Litmus test: Shows basic nature (turns red litmus blue).
