Question

Total number of possible structure of amines of molecular formula \(\mathrm{C}_{4} \mathrm{H}_{11} \mathrm{~N}\), which on reaction with benzene sulphonyl chloride to form a white precipitate that is insoluble in alkali is (A) 1 (B) 2 (C) 3 (D) 4

Short answer

The total number of possible structures of amines with the molecular formula \(C_4H_{11}N\) that form a white precipitate upon reaction with benzene sulphonyl chloride, which is insoluble in alkali, is 3. These structures are primary amines: Butylamine, Iso-butylamine, and Tert-butylamine. The correct answer is (C) 3.

Step-by-step solution

Understanding the types of amines

Amines can be classified into three types based on the number of hydrogen atoms they have bonded to the nitrogen atom: primary (1°), secondary (2°), and tertiary (3°). Primary amines have one hydrogen atom and two alkyl groups attached to the nitrogen atom, secondary amines have two hydrogen atoms and one alkyl group attached, and tertiary amines have three alkyl groups attached with no hydrogen atom.

Identify the reaction with benzene sulphonyl chloride

Benzene sulphonyl chloride reacts with amines to form a compound called an N-alkylbenzenesulphonamide. This reaction only occurs with primary (1°) and secondary (2°) amines, as tertiary amines do not have any hydrogen atoms on the nitrogen atom to form the required compound.

Find all possible primary and secondary amine structures with the given molecular formula C4H11N

For primary amines, we can have the following structures: 1. \(CH_3(CH_2)_3NH_2\) (Butylamine) 2. \(CH_3CH_2CH(CH_3)NH_2\) (Iso-butylamine) 3. \(CH_3C(CH_3)_2NH_2\) (Tert-butylamine) And for secondary amines, we can have the following structures: 1. \(CH_3(CH_2)_2NHCH_3\) (Di-ethylamine) 2. \(CH_3CH_2N(CH_3)_2\) (Di-methyl ethylamine) 3. \(CH_3CH(CH_3)NHCH_3\) (Isopropyl methylamine) Note there are two additional structures (iso-propyl ethylamine and t-butylamine) that exist in the \(C_4H_{11}N\) configuration, but we do not count them under primary or secondary amines in this case.

Determine which structures form insoluble white precipitates in alkali

Primary amines (1°) will form N-alkylbenzenesulphonamides that are insoluble in alkali when they react with benzene sulphonyl chloride. However, secondary amines (2°) will form soluble N,N-dialkylbenzenesulphonamides. So, we only need to consider primary amines to find the possible structures. Among the primary amines we listed earlier, there are 3 structures: 1. Butylamine 2. Iso-butylamine 3. Tert-butylamine Thus, the total number of possible structures is 3. So, the correct answer is (C) 3.

Key concepts

Amines Classification

Amines are organic compounds that feature nitrogen atoms with a lone pair of electrons, making them basic and very reactive. Understanding the classification of amines is crucial for predicting their chemical behavior and reactions. The classification is based on the number of carbon-containing groups, known as alkyl or aryl groups, attached to the nitrogen atom.

There are three main types of amines:

• Primary Amines (1°): These have one alkyl or aryl group attached to the nitrogen atom, along with two hydrogen atoms. They can be represented as RNH₂, where R is an alkyl or aryl group.
• Secondary Amines (2°): These have two alkyl or aryl groups attached and one hydrogen atom, with the general formula R₂NH.
• Tertiary Amines (3°): These have three alkyl or aryl groups attached to the nitrogen atom and no hydrogen atoms, represented by R₃N.

The exercise given presented a situation where we needed to identify amines that would react with benzene sulphonyl chloride to form a precipitate insoluble in alkali. Only primary and secondary amines can undergo this reaction. Tertiary amines, lacking a hydrogen atom on the nitrogen, are not reactive in this context.

Benzene Sulphonyl Chloride Reaction

The reaction of amines with benzene sulphonyl chloride (also known as the Hinsberg reagent) is a classic method to distinguish between different types of amines. Benzene sulphonyl chloride, represented as C₆H₅SO₂Cl, is used to identify primary and secondary amines, which possess different solubility characteristics once they have reacted with this reagent.

When primary (1°) amines react with benzene sulphonyl chloride, they form N-alkylbenzenesulphonamides. These products are insoluble in alkali, leading to the formation of a white precipitate, which is the basis of the textbook exercise. Secondary (2°) amines, on the other hand, react to form N,N-dialkylbenzenesulphonamides, which are soluble in alkali and hence, do not form a precipitate. Tertiary (3°) amines cannot react with benzene sulphonyl chloride to form N-alkylbenzenesulphonamides due to the absence of a hydrogen atom on the nitrogen.

N-Alkylbenzenesulphonamide Formation

The formation of N-alkylbenzenesulphonamide is an essential concept to grasp to understand the chemical behavior of primary amines in the reaction with benzene sulphonyl chloride. To form this compound, a primary amine undergoes a substitution reaction where the sulfonyl group from the benzene sulphonyl chloride replaces one of the hydrogen atoms on the nitrogen.

The general reaction can be depicted as follows:
RNH₂ + C₆H₅SO₂Cl → RNH-SO₂-C₆H₅ + HCl

The product, an N-alkylbenzenesulphonamide, has a nitrogen atom bonded to an alkyl group and a sulphonyl benzene group. The key feature for testing solubility is the basic nitrogen which remains after the reaction. In alkali, only the primary amine's reaction product is insoluble, making it possible to distinguish it from secondary and tertiary amines. The reasoning behind this exercise was to apply these principles to deduce which amines, given their molecular structure, would lead to the formation of insoluble N-alkylbenzenesulphonamides when reacted with benzene sulphonyl chloride.