Question

Question. Which of the following compounds are capable of being resolved into enantiomers?

(a)N-ethyl-N-propylaniline

(b) 2-ethylpiperidine

(c) 1-ethylpiperidine

(d) 1,2,2-triethylaziridine
(e)

(f)

(g)

(h)

Short answer

(a) Not resolvable due to presence of planarity in the molecule.

(b) Resolvable due to presence of asymmetric carbon.

(c) Not resolvable due to symmetric nature of a molecule.

(d) Resolvable but nitrogen inversion is slow.
(e)
Not resolvable due to symmetric nature of the molecule.
(f)
Resolvable due to presence of asymmetric nitrogen, unable to invert.

(g)
Not resolvable in conditions where the proton on nitrogen can exchange.
(h)
Resolvable due to presence of asymmetric nitrogen, unable to invert.

Step-by-step solution

Step-1. Explanation of part (a):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers.

In part (a), N-ethyl-N-propylaniline has fixed geometry and is planar due to attachment of different substituents and due to which it has plane of symmetry and is not chiral, hence it cannot undergo resolution into enantiomers. Plane of symmetry exists as the plane containing whole molecule as molecule is planar.

Not resolvable

Step-2. Explanation of part (b):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers.

In part (b), 2-ethylpiperidine has nitrogen atom which is chiral or optically active due to attachment of different substituents to it, hence it can undergo resolution into enantiomers. Chiral center has been marked with asterisk.

Resolvable

Step-3. Explanation of part (c):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers.

In part (c), 1-ethylpiperidine has plane of symmetry in its structure and thus will not be optically active, hence it cannot undergo resolution into enantiomers.

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Not resolvable

Step-4. Explanation of part (d):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers.

In part (d), 1,2,2-triethylaziridine has chiral center due to attachment of different substituents to it due to which rotation of nitrogen atom is restricted, and hence it can undergo resolution into enantiomers. Chiral center is marked with an asterisk.|

Resolvable

Step-5. Explanation of part (e):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers.

In part (e), the structure has plane of symmetry present in it passing and hence it cannot undergo resolution into enantiomers.

Not Resolvable

Step-6. Explanation of part (f):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers.

In part (f), the structure has chiral center present in it due to attachment of different groups to the nitrogen center and due to which rotation of nitrogen atom is restricted, and hence it can undergo resolution into enantiomers.

Step-7. Explanation of part (g):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers

In part (g), the structure is not resolvable in conditions where the proton on nitrogen atom can exchange. If proton exchange occurs, it means the nitrogen atom can undergo flipping due to introduction of planarity in the molecule after proton exchange. Thus the molecule is not resolvable into enantiomers.


Not resolvable if proton exchange on nitrogen occurs

Step-8. Explanation of part (h):

Compounds which are optically active can be resolved into the enantiomers and compounds which have bulkier group attached to nitrogen which restricts flipping can also undergo resolution into enantiomers.

In part (h), the structure has asymmetric nitrogen atom (marked by a star in the structure) which is unable to invert and thus will show resolution into enantiomers. It has attachment of different substituents to it which rotation of nitrogen atom is restricted, and hence it can undergo resolution into enantiomers.

Resolvable