The S_N2 Mechanism

As described in the previous section, a majority of the reactions thus far described appear to proceed by a common single-step mechanism. This mechanism is referred to as the S_N2 mechanism, where S stands for Substitution, N stands for Nucleophilic and 2 stands for bimolecular. Other features of the S_N2 mechanism are inversion at the alpha-carbon, increased reactivity with increasing nucleophilicity of the nucleophilic reagent and steric hindrance to rear-side bonding, especially in tertiary and neopentyl halides. Although reaction 3 exhibits second order kinetics, it is an elimination reaction and must therefore proceed by a very different mechanism, which will be described later.

The following animation shows the reaction between methylthiolate and (S)-2-bromobutane:

CH₃S^(–) + CH₃CHBrCH₂CH₃ ^___> CH₃SCH(CH₃)CH₂CH₃+ Br^(–).

The sodium cation that accompanies the anions is not shown so as to keep the diagram simple.

This animation may be manipulated at any time by click-dragging the image

References

1. http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/sn2mov.htm

2. http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx2.htm#hal5