Identification of Primary Amines
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
1. What are primary, secondary, and tertiary amines, and how are they classified?
Amines are classified based on the number of carbon-containing groups (alkyl or aryl) attached directly to the nitrogen atom.
- Primary (1°) amines have one carbon group and two hydrogen atoms attached to the nitrogen (e.g., Ethylamine, CH₃CH₂NH₂).
- Secondary (2°) amines have two carbon groups and one hydrogen atom attached to the nitrogen (e.g., Diethylamine, (CH₃CH₂)₂NH).
- Tertiary (3°) amines have three carbon groups and no hydrogen atoms attached to the nitrogen (e.g., Triethylamine, (CH₃CH₂)₃N).
2. What is the Hinsberg test and how does it distinguish between the three types of amines?
The Hinsberg test is a classic chemical method used to differentiate between primary, secondary, and tertiary amines. It involves reacting the amine with Hinsberg's reagent (benzenesulphonyl chloride, C₆H₅SO₂Cl) in the presence of an aqueous alkali like NaOH. The different types of amines produce visibly different results:
- Primary (1°) amine: Reacts to form a sulphonamide, which is soluble in the alkali.
- Secondary (2°) amine: Reacts to form a sulphonamide, which is insoluble in the alkali and precipitates out.
- Tertiary (3°) amine: Does not react with the reagent under these conditions.
3. Why is the product from a primary amine in the Hinsberg test soluble in alkali, while the product from a secondary amine is not?
The difference in solubility is due to the presence of an acidic hydrogen. The sulphonamide formed from a primary amine still has one hydrogen atom attached to the nitrogen. The strongly electron-withdrawing benzenesulphonyl group (–SO₂) makes this hydrogen acidic enough to be removed by the alkali (NaOH), forming a soluble salt. In contrast, the sulphonamide from a secondary amine has no hydrogen atom on the nitrogen, so it cannot act as an acid and thus remains insoluble in the alkali.
4. Why do tertiary amines not react with the Hinsberg reagent?
Tertiary amines fail to react with the Hinsberg reagent (benzenesulphonyl chloride) because they lack a crucial component for the reaction: a hydrogen atom directly attached to the nitrogen atom. The reaction mechanism involves the removal of this hydrogen along with the chlorine from the sulphonyl chloride to form HCl. Without this hydrogen, the initial step of the reaction cannot proceed.
5. What is the Carbylamine test and which type of amine does it identify?
The Carbylamine test, also known as the isocyanide test, is a specific identification test for primary amines only (both aliphatic and aromatic). In this test, the amine is warmed with chloroform (CHCl₃) and an alcoholic solution of potassium hydroxide (KOH). The formation of an isocyanide (or carbylamine), which has a highly unpleasant and foul odour, confirms the presence of a primary amine. Secondary and tertiary amines do not give a positive result for this test.
6. How does the Azo dye test work for identifying primary aromatic amines?
The Azo dye test is a confirmatory test specifically for primary aromatic amines, such as aniline. The test occurs in two main steps:
- Diazotisation: The primary aromatic amine is treated with nitrous acid (generated in-situ from NaNO₂ and cold HCl) at a low temperature (0-5 °C) to form a benzene diazonium salt.
- Coupling Reaction: This diazonium salt is then coupled with a phenol or another aromatic amine (like β-naphthol), resulting in the formation of a brightly coloured azo dye (e.g., orange, red, or yellow). The appearance of this colour is a positive test.
7. Are there any physical properties that can help in the preliminary identification of amines?
Yes, certain physical properties can provide initial clues. Odour is a key characteristic. Lower aliphatic amines typically have a strong, fishy smell. Aromatic amines, like aniline, have a distinct, unpleasant odour. Also, many amines are liquids that may darken over time upon exposure to air and light due to oxidation. While not definitive, these observations can help in the preliminary identification process in a laboratory setting.
8. What are some important limitations of using the Hinsberg test for amine identification?
While widely taught, the Hinsberg test has limitations. The results are not always absolute and can be influenced by reaction conditions. For example, some tertiary amines might react with the reagent under forcing conditions, and the solubility of the sulphonamide from a primary amine can decrease if the alkyl chain is very long. Therefore, while it is a useful qualitative test for simple amines as per the CBSE syllabus, modern definitive identification relies on spectroscopic methods like NMR (Nuclear Magnetic Resonance) and IR (Infrared) spectroscopy.
