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Topic: When pKa’ does not correlate with electrophilicity  (Read 465 times)

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Offline owk9688

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When pKa’ does not correlate with electrophilicity
« on: January 13, 2019, 03:04:53 PM »
Hey!

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?

Offline phth

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Re: When pKa’ does not correlate with electrophilicity
« Reply #1 on: January 14, 2019, 03:59:02 AM »
The rate depends on the pKa and the nature of the electrophile. The dimethyl sulfate can react in two positions making it faster even though the pka's are different, and solvent effects play a role in the reactions.

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