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  1. Why does $\ce{O-}$ show +I effect? On the other hand why does $\ce{OH-}$ show -I effect? both oxygens are electron rich and are negatively charged. my teacher says that charge on the species is a headache for the species ,so according to me both the oxygens should show +I effect, that is donating electrons tendency. Plus they have lone pairs too.

  2. How can we extend the similar explanation for the halides? they have 3 lone pairs so they should also not take up more electrons.

  3. If carbon is double bonded to oxygen then is there any inductive effect taking place over there?

Organic is confusing me. Help would be appreciated.

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closed as too broad by Jan, Wildcat, bon, M.A.R., Todd Minehardt Oct 9 '15 at 21:41

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Three questions in one! $\endgroup$ – Jan Oct 9 '15 at 15:51
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    $\begingroup$ Organic is confusing me, too. Voting to close... $\endgroup$ – Todd Minehardt Oct 9 '15 at 21:40
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Lone pairs can sometimes be delocalized. An acceptable rule of thumb is that if an atom possesses lone pairs and is connected to an sp2 atom, then the two atoms participate in resonance. So consider phenol. Oxygen has lone pairs. It's connected to an sp2 carbon. And so yes, the oxygen can participate in resonance with the sp2 carbon it's connected to, and also the other sp2 carbons in the aromatic ring. We can ascertain the fact that every atom is phenol is sp2 by noting that phenol is planar.

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In general, a negative formal charge makes delocalization a (more) favorable move as this decreases the electron (charge) density on one atom. Spreading out electron density between or among multiple atoms generally lessens reactivity.

Now, your question is why would oxygen exhibit a +I or electron-donating effect despite its high electronegativity. Well, it can participate in resonance donation as explained above; see phenol and its conjugate base, the phenoxide ion, for examples. Examining both in terms of electrophilic aromatic substitution also reveals that the oxygen atom in both is strongly electron donating. True, oxygen can withdraw electron density via sigma bonds (inductive effect) but doing so is unfavorable especially in the phenoxide ion in which the oxygen already bears a negative formal charge. As your teacher explained, bearing charge isn't something that is very favorable.

And in general, you're going to find that resonance effects outweigh inductive effects. Hence the reason why an sp2 oxygen connected to another sp2 atom generally exhibits a +I effect. Consider dimethyl malonate and methyl acetoacetate . Which is more acidic? From inductive effects alone we'd say dimethyl malonate because the oxygen is highly electronegative as opposed to an sp3 carbon and should withdraw more electron density via the inductive effect. From a resonance-standpoint we'd say the opposite; the oxygen in the ester donates electron density via resonance more than a methyl group can inductively donate electron density. And you'd have to realize that the resonance-standpoint is more important in evaluating the acidities of the two weak acids.

A common exception to resonance>inductive effect in organic chemistry is an sp2 atom connected to a halogen. Yes, the halogen has lone pairs and these can be delocalized into the adjacent p-orbital. But the halogens (except possibly fluorine) are much bigger than many other atoms, and therefore the magnitude of resonance donation is limited. So the great electronegativity of the halogens trumps their magnitude resonance donation, and we can see this in the context of electrophilic aromatic substitution again; halogens are ortho-para directors (delocalize the lone pairs in the Lewis structures to see why) but are also mild deactivators.

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