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In an SN2 reactions there is just one step, this must be the rate-determining step. The transition state for an SN2 reaction is the point about halfway through the slow step where the combined reagents reach their highest energy.
E. D. Hughes, Trans. Faraday Soc., 1941, 37, 603.
A. R. Katritzky and B. E. Brycki, J. Phys. Org. Chem., 1988, 1, 1–20.