Internal nucleophilic substitution, SNi

The reaction between an alcohol and thionyl chloride proceeds via an internal nucleophilic substitution, SNi, in which retention is also observed at the substituted carbon. 

Clearly, the NEt2 group is involved. The nitrogen is a nucleophile and can do an internal nucleophilic substitution (SNi), a very fast reaction for entropy reasons because two different molecules do not have to come together. 

This conversion of 101 to 102 is a substitution, but the intermediacy of 104 leads to an intramolecular transfer of chlorine to displace sulfur dioxide. Reactions such as this are classified as internal nucleophilic substitution, which is abbreviated by the symbol SnI. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

In a few reactions, nucleophilic substitution proceeds with retention of configuration, even where there is no possibility of a neighboring-group effect. In the SNi mechanism (substitution nucleophilic internal) part of the leaving group must be able to attack the substrate, detaching... 

SN2Ar SNAr SNi See SNAr. Nucleophilic aromatic substitution, also called SN2Ar. Substitution nucleophilic internal. 

The Sni (substitution nucleophilic internal) reaction, equation 8.10," is denoted a (Dm + D -h An) reaction, where D without a subscript refers to a tmimolecular dissociation. This system for labeling reaction mechanisms did not meet with wide acceptance, however, and most of the chemical literature still utilizes the Ingold system to describe substitution reactioirs. ... 

The Sni mechanism (substitution-nucleophilic-internal), as originally proposed, postulated a front-side displacement occurring via a four-center transition state. It is now clearly established that such a process has never been observed, and that all the reactions proposed to proceed by SnI mechanisms involve ion pairs. The Sni mechanism was initially suggested to explain the observed stereochemistry of chlorosulfite decomposition. Chlorosulfite esters are formed by reaction of alcohols with thionyl chloride, and are the key reactive intermediates in the conversion of alcohols to chlorides with thionyl chloride. Nucleophilic attack by the chloride of the chlorosulfite ester was considered to be concerted with cleavage of the C-O bond ... 

---