7

Nature of Transition State:The Hammond Postulate

If two states, for example, a transition state and an unstable intermediate, occur
consecutively during a reaction process and have nearly the same energy content,
their interconversion will involve only a small reorganisation of molecular structure.

Or in other words...a reactive intermediate resembles the transition state the more the
closer the intermediate is in energy to the transition state in question.

Evidence for intermediacy of

σ-complex

2.2.

Electrophilic Nitration

General reaction scheme

The electrophile in this case is:

A linear molecule, isoelectronic with carbon dioxide.NO2+ can be isolated as salts
which are extremely powerful nitrating agents (commercially available):

General reaction scheme:

Evidence for a two step process:

H

H

H

H

SbF5

-120oC

HSO3F

+

HF/BF3

BF-
4

+

characterised by 1H-NMR

can be isolated as a
bright yellow solid

R

R

NO2

+

H2O

H2SO4

HNO3

+

NO2+

+

HSO4-

+

NO2+

R

O2N

H

slow

R

O2N

H

+H2O

fast

r.d.s.