<Dd> r = k (RX) (Nu) </Dd> <P> This is a key difference between the S 1 and S 2 mechanisms . In the S 1 reaction the nucleophile attacks after the rate - limiting step is over, whereas in S 2 the nucleophile forces off the leaving group in the limiting step . In other words, the rate of S 1 reactions depend only on the concentration of the substrate while the S 2 reaction rate depends on the concentration of both the substrate and nucleophile . </P> <P> It has been shown that except in uncommon (but predictable cases) primary and secondary substrates go exclusively by the S 2 mechanism while tertiary substrates go via the S 1 reaction . There are two factors which complicate determining the mechanism of nucleophilic substitution reactions at secondary carbons: </P> <P> 1) Many reactions studied are solvolysis reactions where a solvent molecule--often an alcohol, is the nucleophile . While still a second order reaction mechanistically, the reaction is kinetically first order as the concentration of the nucleophile--the solvent molecule, is effectively constant during the reaction . This type of reaction is often called a pseudo first order reaction . </P>

Where does the nucleophilic attack in an sn2 mechanism