An SN2 reaction is characterised by

  • A. a two step mechanism through a carbocation
  • B. first order kinetics involving only the alkyl halide
  • C. a single step in which the nucleophile attacks as the leaving group departs, with second order kinetics
  • D. the formation of a free radical intermediate

Explanation

The nucleophile approaches from the side opposite the leaving group and bond forming and bond breaking occur together through a five coordinate transition state, so the rate depends on the concentrations of both the halide and the nucleophile. Because attack is from the back, the configuration at that carbon is inverted, an effect known as Walden inversion. Primary halides follow this route because they are least hindered.

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