CHAPTER 8: Question 14
Simple Answers
In an aziridine, the nitrogen atom is sp3 hybridized, and so
has preferred bond angles of 109o. In aziridines (and azetidines),
this bond angle is distorted to 60o (90o for an azetidine),
resulting in a strained structure. However, in the transition state for
inversion of the conformations, the nitrogen atom is sp2 hybridized
since the R group is coplanar with the carbon atoms of the three membered
ring as shown below. Thus the preferred bond angles around the nitrogen
atom in the transition state are 120o, and hence the transition
state is more strained than the starting material. This raises the energy
of the transition state and hence slows down the rate at which the two
conformations interconvert. Five and six membered ring, nitrogen containing
heterocycles are able to accommodate the sp2 hybridized nitrogen
atom without introducing strain into the ring system, so the rate at which
these compounds undergo 'umbrella' inversion is similar to that of acyclic
amines.
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