Nucleophilic substitution reaction is that in which attacking nucleophile replaces a leaving group.
Attacking nucleophile: specie with an electron free pair or negative charge and ability to replace already present nucleophile i.e OH-
Leaving group: A specie with electron deficiency or positive charge, negative charge i.e Cl- What is SN1 Reaction:-
SN1 indicates the unimolecular nucleophilic substitution reactions. Their rate determining step of the mechanism depends on the decomposition of a single molecular species (substrate). So the rate of an SN1 reaction can be expressed by: rate = K [R-LG]
Furthermore, SN1 is a multi-step reaction, which forms an intermediate and several transition states …show more content…
In this mechanism, separation of leaving group and formation of new bond happen synchronously. Therefore, two molecular species involve with the rate determining step, and this leads to the term bimolecular nucleophilic substitution reaction or SN2. The rate of the SN2 reaction can be expressed by rate = K [R-LG] [Nu–]. In inorganic chemistry, this reaction also called “associative substitution” or “interchange mechanism.” The following figure illustrates the mechanism of SN2 reaction.
Here, nucleophile attacks the opposite direction of the leaving group. Thus, SN2 reaction always leads to an inversion of stereochemistry. This reaction works best with methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile. In addition, the stability of the leaving group as an anion and the strength of its bond to the carbon atom both affect the rate of reaction.
The following figures illustrate the energy profile diagram of SN1 and SN2 reactions. Difference Between SN1 and SN2 Reactions
SN1 Reaction: SN1 Reaction is unimolecular and a first order reaction. So substrate affects the reaction rate.
SN2 Reaction: SN2 Reaction is bimolecular or a second order reaction. So, both substrate and nucleophile affect the reaction rate.
SN1 Reaction: This is expressed as rate = K [R-LG]
SN2 Reaction: This is expressed as rate = K’ [R-LG] [Nu–]