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We know opposing charges attract and like charges repel. By this logic then, in the two examples on the left, the curved arrows would be show the formation of an oxygen-oxygen bond. However, these mechanisms are incorrect, and instead the mechanisms on the right take place. f

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Can anyone explain this with some more examples?

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    $\begingroup$ Because they depict different reactions? I'm slightly confused as to what you are asking about. Can you clarify what is confusing you? $\endgroup$
    – bon
    May 17, 2016 at 17:57
  • $\begingroup$ @bon I am simply asking that why 1st reaction is wrong i.e why does oxygen atom does not attack on oxygen atom . $\endgroup$
    – Koolman
    May 17, 2016 at 18:03
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    $\begingroup$ It would cause hypercoordination and oxygen isn't prone to this. $\endgroup$
    – Mithoron
    May 17, 2016 at 18:29
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    $\begingroup$ All those can be explain using with quantum mechanics. If you are able to follow it I can explain it to you in few days (my timetable is full). And there is no "attraction" it is a misunderstanding to say that. Electrons go where there are the less possible density of charge. Also I've never seen a positive oxygen which want to stay like this. And due to its electronegativity the density of electrons will be more around it than around the carbon that's with the oxygen on the alcool will react with the carbon. $\endgroup$
    – ParaH2
    May 17, 2016 at 18:41
  • $\begingroup$ @Mithoron what is hypercoordination ? $\endgroup$
    – Koolman
    May 18, 2016 at 9:37

2 Answers 2

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Arrow pushing is a tool that helps you think about the possible reactions available for a given species or set of reactants: where electrons are likely to move in order to form new bonds and break old ones. But you still have to look at the species that you get after doing the arrow pushing and consider whether they will be favorable or not. Sometimes you can get a reaction between a (formally) negatively charged atom and a positively charged atom - for example between methoxide and tert-butyl cation. Do the arrow pushing and see how that works out.

The reactions you have labeled as incorrect can be seen (via arrow pushing) to produce species that have too many bonds or electrons around a single atom (that was what was meant by the term "hypercoordination" - you could also say hypervalent - for example, having 10 valence electrons around oxygen), whereas the ones labeled as correct lead to stable or at least normally bonded species.

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In a bond, the more electronegative element will have a greater share of the electrons, and a partial negative charge to reflect this greater electron density.  The less electronegative element will have a partial positive charge to reflect the lack of electron density. 

Because in chemical reactions, electrons will flow from areas of high electron density to areas of low electron density. Knowing where the partial charges are is an important first step in determining where the molecule will react.

Formal charge is NOT the same as electron density. 

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