SNi

SNi or Substitution Nucleophilic internal stands for a specific but not often encountered nucleophilic aliphatic substitution reaction mechanism. A typical representative organic reaction displaying this mechanism is the chlorination of alcohols with thionyl chloride and the main feature is retention of stereochemical configuration. Thionyl chloride first reacts with the alcohol to form an alkyl chloro sulfite. In the second step the sulfite group is lost and just like in an SN1 reaction an alkyl carbocation is created. The actual nucleophile in the third reaction step is the chlorine atom attached to the sulfite group which recombines with this carbocation. The crucial difference with the standard SN1 mechanism is that since the nucleophile resides at the same side as the original leaving group i.d. the hydroxyl group, the stereochemistry is retention of configuration and not racemization. This reaction type is linked to many forms of Neighbouring group participation, for instance the reaction of the sulfur lone pair in sulfur mustard to form the cationic intermediate.

This reaction mechanism is supported by the observation that addition of pyridine to the reaction leads to inversion. The reasoning behind this finding is that pyridine reacts with the intermediate sulfite replacing chlorine. The dislodged chloride has to resort to nucleophilic attack from the rear as in a regular nucleophilic substitution.

In the complete picture for this reaction the sulfite reacts with a chlorine ion in a standard SN2 reaction with inversion of configuration. When the solvent is also a nucleophile such as dioxane two successive SN2 reactions take place and the stereochemistry is again retention. With standard SN1 reaction conditions the reaction outcome is retention via a competing SNi mechanism and not racemization and with pyridine added the result is again inversion.