The Sn2 substitution, nucleophilic, bimolecular reaction

The abbreviation Sn2 conveys the information sub-stitution-nucleophilic-bimolecular . The reaction is essentially the displacement of one group, a leaving group, by another group, a nucleophile. It is a bimolecular reaction, since kinetic data indicate that two species are involved in the rate-determining step  

Differences in electronegativities (see Section 2.7) between carbon and the leaving group atom lead to bond polarity. This confers a partial positive charge on the carbon and facilitates attack of the nucleophile. As the nucleophile electrons are used to make a new bond to the carbon, electrons must be transferred away to a suitable acceptor in order to maintain carbon s octet. The suitable acceptor is the electronegative leaving group. 

Essentials of Organic Chemistry Paul M De wick 2006 John Wiley Sons, Ltd 

The rate of an Sn2 reaction depends upon several variables. These are  

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