Iodine and fluorine leaving group

[jd12345]

A question in my test asked which of the acyl halide will hydrolyse faster in an aqueous solution of NaOH

Well the asnwer is acyl iodide becasue iodine is a better leaving group( the solution says so)
But i don't understand - fluorine will be hydrated to the greatest extent so removal of fluorine should be easy

 

[aroc91]

fluorine will be hydrated to the greatest extent so removal of fluorine should be easy

That doesn't necessarily make it more stable, though. The charge dispersion due to iodide's size makes it far more stable on its own than fluoride.

This is the same concept as halide acid strength increasing as you move down the row. Check the halide acid pKas yourself-

evans.harvard.edu/pdf/evans_pka_table.pdf