SN2 type transition state

The ethylation of toluene by diethylhalonium ions gives ethyltoluenes with ortho para isomer ratios between 0.60 and 0.96. The ortho para isomer ratios obtained for the alkylation of toluene in conventional Friedel-Crafts ethylations range from 1.17 to 1.84 (average 1.60). Such differences are considered to be due to the steric ortho effect caused by diethylhalonium ions, and are in accordance with the most probable displacement reaction on the bulky diethylhalonium ions by the aromatic substrate. This can be envisioned to proceed through an SN2-type transition state involving no free alkyl cations [Eq. (5.109)]. 

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