What is SNi Reaction. ?

 SNi Reaction

What is SNi reaction ?

The term SNi stands for substitution nucleophilic internal. In SN1 and SN2 reaction mechanism, the racemization and inversion of the configuration of the product take place.

But Hughes, Ingold has shown that optically active 1-phenyl ethanol reacts with thionyl chloride to give 1-phenyl ethyl chloride with complete retention of configuration.

Normally the product should be inversion of configuration. so former can be explained by internal mechanism which is called SNi mechanism.

Example of SNi reaction:

Mechanism of SNi reaction: 

The solvents used in SNi reaction is diethyl ether or tetra hydro furan. The mechanism of SNi reaction shown below. 

Stereo chemistry of SNi reaction:

During the SNi reaction there occurs a internal transformation through a cyclic transition state to which attack of chloride from front side leads to retention in configuration. 

That is, both the reactant and products are having same configuration. 

 Why pyridine does not used as a solvent in SNi reaction ? 

Reaction of 1-phenyl thanol with thionyl chloride is an example of SNi reaction. In this reaction,ether is used as a solvent. Pyridine does not used as a solvent instead of ether. 

Because, pyridine react with thionyl chloride to form pyridinium chloride. The chloride ion is free for attact on crabon atom of substrate. 

Hence, chloride ion attact the carbon atom from back side. As a result, inversion of configuration of the product takes place. 

So the mechanism is not a  SNi, the mechanism is actually SN2.  

Summary

• What is stereo chemistry of SNi reaction ? 
• Why does pyrridine not used as solvent in SNi reaction?  
• What is SNi reaction ? give examples. 

Read more : Short description of HCl chemistry