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What is the Inversion of Amines ?

Inversion of amines takes place at room temperature and required 64 k j per mole energy.
An amine such as ethyl methyl amine has a tetrahedral (sp3) nitrogen atom, and the nitrogen lone pair in an sp3 orbital. However this tetrahedral amine structure is not static. It is found that the nitrogen atom inverts. The lone pair disappears from one face, moves through the nucleus, and reappears on the opposite face. The lone pair repels the ethyl group, the hydrogen atom, and the methyl group, so as the lone pair reappears, these groups move away. As the lone pair is tunneling through the nucleus, the nitrogen has just three attachments so this nitrogen atom is trigonal planar with sp2 hybridization. Overall flipping of amines is called as inversion of Amine. This magical act is also known as quantum mechanical tunneling.

The tetrahedral amine structures are in equilibrium. Furthermore, these structures are enantiomers. All physical properties of enantiomers are identical except for the direction in which they rotate plane polarized light). From this we can conclude that the two enantiomeric amines are present in equal amounts and have equal stability .

There are two conditions in which a inversion of nitrogen atom does not take place.
1: No Lone Pair:
2: Ring Strain:
When the amine nitrogen is not part of a ring, this bond angle change is easily and inversion takes place. However, if the nitrogen atom is part of a three-membered or a four-membered ring then inversion is significantly retarded by strain.

Effect of Inversion on basicity of amines:
The rate of nitrogen inversion also correlates with, among other things, hybridzation. But in cases where these "other things", such as ring constraints, limit nitrogen inversion, then the rate of nitrogen inversion may not correlate with hybridization. So the rate of nitrogen inversion does not always correlate with hybridization and is therefore not a good indicator of lone pair "availability".
 It is clear that basicity of amines reduced by inversion of amines.

ILLUSTRATIVE EXAMPLE (1): 1-Azabicyclo[2,2,1]heptane is more basic than triethylamine why?.
 SOLUTION:  1-Azabicyclo[2,2,1]heptane is more basic than triethylamine . Because in case of triethylamine the lone pair of electrons is less available in the latter due to rapid nitrogen inversion. Nitrogen inversion is not possible in the bicyclic amine.

ILLUSTRATIVE EXAMPLE (2): Which of the following compound is more basic?
SOLUTION: Option (A) is least basic because Option (B) and (C) both have restriction of inversion hence more basic while option (C) is least basic than option (B) due to –I effect of N-atom to another reduce availability of lone pair of nitrogen atom.

ILLUSTRATIVE EXAMPLE (3):The correct order of decreasing basicity of the compounds is :
SOLUTION:  III > II > IV > I
In Case of Option (III) their is restriction of inversion take place at bridge head nitrogen atom hence it is most basic than (II), while (II) is more basic then (VI) because its lp does not delocalized  while lp of (IV) is delocalized with ring hence less basic  but (I) is least basic to all due to presence of three strong  -I group.

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