The following reaction takes place under second-order conditions (strong nucleophile), yet the structure of the product shows rearrangement. Also, the rate of this reaction is several thousand times faster than the rate of substitution of hydroxide ion on 2-chlorobutane under similar conditions. Propose a mechanism to explain the enhanced rate and rearrangement observed in this unusual reaction. (“Et” is the abbreviation for ethyl.

Neighbouring group participation or NGP in organic chemistryhas been defined by IUPAC as it is the interaction of a centre with a lone pairof electrons in an atom or the electrons present in asigma bond orpi bond.

And when NGP occurs it is normal for thereaction rate to be increased and there is a possibility of rearrangement. In the above reaction N has lone pair of electrons. So it attacks the carbon attached to the leaving group (chlorine). Due to this neighbouring group participation the rate increases as the leaving group leaves very easily. The hydroxyl (OH^-) attacks from the same side as the chlorine leaves. So, inversion is not observed like normal S_N2.

The nucluophile NE_t2 attacks to the second carbon and make a cyclic three membered ring intermediate. Now, the OH^- attacks the first carbon atom from the same side as the chlorine leaves consequently breaking the three membered ring. And that is why the product shows rearrangement.