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Both have 2p-2p overlap and oxygen and fluorine have similar electronegativities but OH is strongly activating while fluorine is deactivating

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    $\begingroup$ What does the oxygen have attached that the fluorine doesn't, start from there. $\endgroup$ – AS_1000 Aug 18 '17 at 15:10
  • $\begingroup$ How could only the presence of a hydrogen bond, which slightly decreases OH's inductive effect completely reverse the trend from deactivating to strongly activating? $\endgroup$ – xasthor Aug 18 '17 at 15:14
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    $\begingroup$ Think of another interaction that the hydrogen might be involved with, look at the resonance structures as well. $\endgroup$ – AS_1000 Aug 18 '17 at 15:16
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F is the most electronegative element of the periodic table.Due to its -I effect, it shifts the electron density of any alkyl group towards itself, resulting in less density of electron cloud over the alkyl or phenyl group.This deactivates the ring for electrophilic substitution reactions.However , oxygen also being an electronegative element , has capacity to donate its lone pair electrons forming a back bond in case their is a deficiency in alkyl group.In case of cyclic rings that is in aromatic structures , the lone of oxygen is in resonance with the ring during attack of the reagent , that is during formation of sigma complex.However this is not shown by fluorine due to its high electronegativity.Do not get confused with other halogens.Cl ,Br and I are ring activating groups as they also can form the same back bond as oxygen.

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