So I know that pKa is a thermodynamic property while electrophilicity would be a kinetic one, but I’m encountering a real life example right now that I can’t seem to wrap my head around:
in my lab I was charged with alkylating a bunch of phenols/phenoxides for others to use in their experiments. The reactions were clean but I noticed that dimethylsulfate finished the fastest (by TLC), faster even than methyl iodide. Also, butyl chloride took more than a full day to fully react (dimethylsulfate was done in 3 hours).
I guess I’m just really surprised bc in terms of pKa, sulfate should be far inferior of a leaving group compared to iodide and chloride, but with iodide the rates are essentially the same and even though chloride seems to be a far better leaving group, the butyl chloride was absolutely sluggish compared to the dimethylsulfate sulfate. Is the chloride example purely a steric issue? I know it’s the difference between methyl and primary but I really would not have thought that it would be so dramatic. And if it is, why then do methyl iodide and dimethylsulfate react at similar rates even with a pKa’ difference of nearly 6 orders of magnitude?