Under the influence of , the reaction follows an mechanism. Ethanol is protonated by HX to convert -OH into a good leaving group, which departs to form the ethyl carbocation. The halide ion then attacks the carbocation to yield the alkyl halide. The rate-determining step is carbocation formation after protonation, so the overall rate depends on the acid strength of HX. Stronger acids generate higher concentrations of the protonated intermediate, accelerating the substitution.
Acid strength order (by ):