Question

The compound $l - $(N-ethyl-N-methyl)-propanamine forms non-superimposable mirror images.
But this compound does not show optical activity because of the:
A.Absence of a chiral N atom
B.Presence of a chiral N atom
C.Presence of lone pair on N atom
D.Rapid flipping of one form into the other

Answer 1

Hint: Non-Superimposable images: Those compounds in which the valency of the central atom is filled by different compounds or atoms, are known as non-superimposable images of the compounds. They are also known as achiral compounds.

Complete step by step answer:
First of all we will discuss chiral and achiral atoms.
Chiral atom: The atom in which all the valencies of the centre atom are filled by different atoms or groups of atoms, are known as chiral centres. If any compound has chiral centre then the compound may be chiral or may not be. For example: Carbon shows four valencies. So in the compound if the four valencies of carbon are filled by different atoms or groups of atoms, then that carbon is said to be chiral carbon.
A chiral atom: The atom in which the valency of the centre atom is either filled by all the same atom or group of atoms or two same and different i.e. at least two groups attached to the centre atom must be same. For example: Methane. In methane the four valencies of the carbon are filled by the same atoms i.e. hydrogen. So it is an achiral carbon.
Superimposable and non-superimposable images:
If a compound is the same in its image, then it is known as a superimposable image. And if a compound is not the same as its image, then it is known as a non-superimposable image. Generally the chiral centre is non-superimposable because they have different atoms attached to the centre. And the achiral centre may be superimposable or non-superimposable depending on the position of the group in the compound.
For example: if we place our right hand on the left hand then we see that they are not superimposable on each other. So this is the example of non-superimposable images.
Now in the question we are given with the compound $l - $ (N-ethyl-N-methyl)-propanamine. This compound forms non-superimposable images. But they do not show optical activity because the rapid flipping of one form to another form i.e. \[l\] to $d$ transformations.

So, the correct answer is Option D .

Note:
Enantiomers: They are the mirror images of chiral compounds. They are non-superimposable on each other. L and D configurations are the enantiomers of the molecules. In L configuration hydroxyl group is present at left side while in D configuration the hydroxyl group is present at right side.