Reactive intermediates in study of mechanism


• Detection of intermediate:


• It may not be possible for many reactions to isolate the postulated intermediate even though present.

• Their presence may be proved with the help of certain spectroscopic methods such as IR, UV, NMR, EPR etc.

• Raman spectroscopy was used to know the presence of nitronium ion NO2+ in the nitration of benzene.

• EPR can be used to detect free radicals in the reaction as they are paramagnetic when not paired.

• CIDNP (Chemically Induced Dynamic Nuclear Polarization) also can help in showing the presence of free radicals. It is an NMR technique.

Presence of radicals may be proved by carryong out certain reactions 

also.

• If a doubly bonded compound with the known stereochemistry is added in the reaction mixture of a reaction believed to be involving a free radical mechanism and one stereoisomer is found to be converting to the other, it shows the presence of free radicals in the reaction.


Trapping of an intermediate:

• It is acceptable evidence in proving the existence of an intermediate in areaction to carry out another well known reaction the expected intermediatemay undergo. It is termed as trapping of the intermediate.

• Benzyne intermediates are highly unstable and if a reaction is believed toinvolve a benzyne inter mediate, based on the out come of the reaction, it ispossible to confirm its presence by adding a diene and isolating the Diel-Alderproduct formed due to its reaction with benzyne.