CBSE Class 12 Chemistry Chapter 13: Amines NCERT Solutions

To classify amines, we examine the number of alkyl or aryl groups directly attached to the nitrogen atom.

1. This structure represents an amino group (-NH₂) attached to a carbon chain (implied). Since the nitrogen atom is attached to only one carbon atom (in the implied chain), it is a primary amine.
2. This structure shows a nitrogen atom bonded to two methyl groups (<math>CH_3</math>) and one implied carbon atom (which would complete the valency of nitrogen if it were part of a larger molecule, or if it's ammonia derivative like N(CH3)3). Assuming it's a derivative where N is bonded to 3 carbons, and two are methyls, the third is implied. If the structure is meant to be N(CH3)3, then it is a tertiary amine. However, if it's part of a larger molecule where N is bonded to two methyl groups and one other carbon, it's tertiary. Given the context of other examples, this is likely intended as a representation of a tertiary amine where nitrogen is bonded to three carbon atoms, two of which are methyl groups. Thus, it is a tertiary amine.
3. The structure is <math>(C_2H_5)_2CHNH_2</math>. Here, the amino group (-NH₂) is attached to a carbon atom which is bonded to two ethyl groups (<math>C_2H_5</math>) and one hydrogen atom. Since the nitrogen atom is attached to only one carbon atom (the one in the CH group), this is a primary amine.
4. The structure is <math>(C_2H_5)_2NH</math>. Here, the nitrogen atom is bonded to two ethyl groups (<math>C_2H_5</math>) and one hydrogen atom. Since the nitrogen atom is bonded to two carbon atoms (from the two ethyl groups), this is a secondary amine.

Summary:

Primary amines: (i) and (iii)

Secondary amine: (iv)

Tertiary amine: (ii)

The molecular formula <math>C_4H_{11}N</math> indicates that the amines are saturated and contain one nitrogen atom. We need to find all possible structural arrangements (isomers) of carbon and nitrogen atoms that satisfy this formula and then determine their IUPAC names and the type of isomerism they exhibit.

Part 1 & 2: Structures and IUPAC Names

We can systematically draw the isomers by considering the arrangement of the four carbon atoms and the position of the amino group (-NH₂) for primary amines, or the arrangement of alkyl groups around the nitrogen for secondary and tertiary amines.

A. Primary Amines (R-NH₂):

1. Straight chain of 4 carbons:

   Structure: $CH\_3-CH\_2-CH\_2-CH\_2-NH\_2$

   IUPAC Name: Butanamine (or Butan-1-amine)

2. Branched chain (3 carbons main chain, methyl group at C2):

   Structure: $CH\_3-CH\_2-CH(CH\_3)-NH\_2$

   IUPAC Name: 2-Methylpropanamine (or 2-Methylpropan-1-amine)

3. Branched chain (3 carbons main chain, methyl group at C2, NH2 at C2):

   Structure: $CH\_3-CH(NH\_2)-CH\_2-CH\_3$

   IUPAC Name: Butan-2-amine

4. Branched chain (3 carbons main chain, two methyl groups at C2):

   Structure: $CH\_3-C(CH\_3)(NH\_2)-CH\_3$

   IUPAC Name: 2-Methylpropan-2-amine

B. Secondary Amines (R-NH-R'):

1. Ethyl and Methyl groups:

   Structure: $CH\_3-CH\_2-NH-CH\_3$

   IUPAC Name: N-Methylethanamine

2. Two identical Ethyl groups:

   Structure: $CH\_3-CH\_2-NH-CH\_2-CH\_3$

   IUPAC Name: N-Ethylethanamine (Commonly Diethylamine)

3. Propyl and Methyl groups (branched propyl):

   Structure: $CH\_3-CH(CH\_3)-NH-CH\_3$

   IUPAC Name: N-Methylpropan-2-amine

C. Tertiary Amines (R-N(R')-R''):

1. Two Methyl groups and one Ethyl group:

   Structure: $CH\_3-CH\_2-N(CH\_3)\_2$

   IUPAC Name: N,N-Dimethylethanamine

2. One Methyl group and two identical Methyl groups (This is not possible with C4H11N, as it would require 3 methyls and 1 carbon, total 4 carbons, but N would be bonded to 4 carbons, which is not an amine). Let's re-evaluate. The possible tertiary amines are formed by distributing 4 carbons around N.

   Possible combinations for R, R', R'' summing to 4 carbons:

   • 1, 1, 2: e.g., Methyl, Methyl, Ethyl -> N,N-Dimethylethanamine (already listed)
   • 1, 2, 1: Same as above.
   • 2, 1, 1: Same as above.

   Let's consider the carbon skeleton first: Butane skeleton or Isobutane skeleton.

   If the carbon skeleton is butane (4 carbons in a line):

   • Tertiary amine: CH3-CH2-N(CH3)-CH3 (N,N-Dimethylethanamine)

   If the carbon skeleton is isobutane (3 carbons in a line, 1 methyl branch):

   • Tertiary amine: CH3-C(CH3)(CH3)-NH2 (2-Methylpropan-2-amine - primary, already listed)
   • Tertiary amine: CH3-CH(CH3)-N(CH3)-CH3 (N,2-Dimethylpropanamine)

   So, the tertiary amines are:

   1. N,N-Dimethylethanamine ($CH\_3-CH\_2-N(CH\_3)\_2$)
   2. N,2-Dimethylpropanamine ($CH\_3-CH(CH\_3)-N(CH\_3)-CH\_3$)

Part 3: Types of Isomerism Exhibited

Different pairs of amines exhibit various types of isomerism:

• Chain Isomerism: Isomers have the same molecular formula and functional group but differ in the arrangement of the carbon chain. For example, Butanamine (straight chain) and 2-Methylpropanamine (branched chain) are chain isomers. Similarly, N-Ethylethanamine and N,2-Dimethylpropanamine can be considered related through chain isomerism of the alkyl groups.
• Position Isomerism: Isomers have the same molecular formula and carbon skeleton but differ in the position of the functional group (the amino group). For example, Butan-1-amine and Butan-2-amine are position isomers. Also, N-Methylethanamine and N-Ethylmethanamine (if it existed with C4H11N) would be position isomers.
• Functional Isomerism: Isomers have the same molecular formula but belong to different functional groups. Primary, secondary, and tertiary amines are functional isomers of each other. For example, Butanamine (1°), N-Methylethanamine (2°), and N,N-Dimethylethanamine (3°) are functional isomers.
• Metamerism: This type of isomerism occurs in secondary and tertiary amines due to the difference in the nature of alkyl groups attached to the nitrogen atom, while the total number of carbon atoms remains the same. For example, N-Methylethanamine ($CH\_3-CH\_2-NH-CH\_3$) and N-Ethylmethanamine (if it existed with C4H11N, e.g., $CH\_3-NH-CH\_2-CH\_2-CH\_3$ - N-Propylmethanamine) are metameric. Another example from the list is N-Ethylethanamine ($CH\_3-CH\_2-NH-CH\_2-CH\_3$) and N-Methylpropan-2-amine ($CH\_3-CH(CH\_3)-NH-CH\_3$).

Specific examples from the source text:

• Pairs (a) and (b) exhibit position isomerism (Butanamine and Butan-2-amine).
• Pairs (a) and (c) exhibit chain isomerism (Butanamine and 2-Methylpropanamine).
• Pairs (a) and (d) exhibit chain isomerism (Butanamine and 2-Methylpropan-2-amine).
• Pairs (b) and (c) exhibit chain isomerism (Butan-2-amine and 2-Methylpropanamine).
• Pairs (b) and (d) exhibit chain isomerism (Butan-2-amine and 2-Methylpropan-2-amine).
• Pairs (e) and (f) exhibit metamerism (N-Methylethanamine and N-Ethylmethanamine - assuming the source meant N-Propylmethanamine for (f) to be a metamer). The source lists (e) N-Methylethanamine and (f) N-Ethylethanamine. These are not metameric, but functional isomers. Let's assume (f) was intended to be N-Propylmethanamine. If we strictly follow the source's listed pairs: (e) $CH\_3-CH\_2-NH-CH\_3$ (N-Methylethanamine) and (f) $CH\_3-CH\_2-NH-CH\_2-CH\_3$ (N-Ethylethanamine). These are not metameric. However, the source states (e) and (f) exhibit metamerism. This implies a misunderstanding in the source's pairing or naming. Let's consider the common interpretation: N-Methylethanamine ($C\_2H\_5NHCH\_3$) and N-Propylmethanamine ($CH\_3NHCH\_2CH\_2CH\_3$) are metameric. The source's pair (e) and (f) might be intended to represent different alkyl groups around N.
• The source states pairs (e) and (f) exhibit metamerism. Let's assume (e) is N-Methylethanamine ($CH\_3CH\_2NHCH\_3$) and (f) is N-Ethylethanamine ($CH\_3CH\_2NHCH\_2CH\_3$). These are not metameric. A correct metameric pair would be N-Methylethanamine and N-Propylmethanamine.
• The source states pair (g) exhibits metamerism. Pair (g) is Methylpropan-2-amine ($CH\_3CH(CH\_3)NHCH\_3$). This is a secondary amine. If we consider N-Ethylethanamine ($CH\_3CH\_2NHCH\_2CH\_3$) as another secondary amine, they are metameric.
• All primary amines exhibit functional isomerism with secondary and tertiary amines and vice-versa.