Question

Question. Which of the amines listed next is resolved into enantiomers? In each case, explain why interconversion of the enantiomers does or does not take place.

1. Cis-2-methylcyclohexanamine
2. N-ethyl-N-methylcyclohexanamine
3. N-methylaziridine
4. Ethylmethylanilinium iodide
5. Methylethylpropylisopropylammonium iodide

Short answer

1. Resolvable as there are two asymmetric carbons and carbon does not invert.
2. Not resolvable, as the nitrogen atom is free to invert.
3. Not resolvable, as the structure is symmetric.
4. Not resolvable, even though the nitrogen is quarternary, one of the groups is a proton which can exchange rapidly allowing for conversion.

5. Resolvable, as the nitrogen is quarternary and cannot invert when bonded to carbons.

Step-by-step solution

Explanation of part (a)

Compounds which are optically active can be resolved. If compound has bulkier substituents attached to it, then rotation gets restricted and it becomes optically active. Tertiary amines are chiral but they exist as racemic mixtures because of nitrogen inversion.

Cis-2-methylcyclohexanamine is resolvable as there are two asymmetric carbons and carbon does not invert.

Resolvable compound

Explanation of part (b)

Compounds which are optically active can be resolved. If compound has bulkier substituents attached to it, then rotation gets restricted and it becomes optically active. Tertiary amines are chiral but they exist as racemic mixtures because of nitrogen inversion.

N-methyl-N-ethylcyclohexanamine is not resolvable, as the nitrogen atom is free to invert.

Explanation of part (c)

Compounds which are optically active can be resolved. If compound has bulkier substituents attached to it, then rotation gets restricted and it becomes optically active. Tertiary amines are chiral but they exist as racemic mixtures because of nitrogen inversion.

N-methylaziridine is not resolvable as the structure is symmetric, it means plane of symmetry exists and molecule becomes optically inactive.

Explanation of part (d)

Compounds which are optically active can be resolved. If compound has bulkier substituents attached to it, then rotation gets restricted and it becomes optically active. Tertiary amines are chiral but they exist as racemic mixtures because of nitrogen inversion.

Ethylmethylanilinium iodide is not resolvable even though the nitrogen is quaternary and one of the groups is a proton which can exchange rapidly allowing for inversion.

Explanation of part (e)

Compounds which are optically active can be resolved. If compound has bulkier substituents attached to it, then rotation gets restricted and it becomes optically active. Tertiary amines are chiral but they exist as racemic mixtures because of nitrogen inversion.

Methylethylpropylisopropylanilinium iodide is resolvable as nitrogen atom is quaternary and cannot invert when bonded to carbons.