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Here's a couple of examples of what I mean. Firstly, in the reaction with thionyl chloride to make a carboxylic acid:

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Secondly, in this specific example of a hydroboration reaction:

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I don't really understand why you cant attack from the hydroxide group and then proton transfer. Any suggestions?

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    $\begingroup$ Can you draw a resonance structure which localizes -ve charge on the hydroxide ? $\endgroup$ – J. LS May 28 '15 at 15:24
  • $\begingroup$ You could cleave the bond to the acidic proton though $\endgroup$ – RobChem May 28 '15 at 15:43
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    $\begingroup$ Yes, but are either of these reactions taking place under basic conditions ? The first one has HCl as a byproduct... $\endgroup$ – J. LS May 28 '15 at 16:40
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For both the examples that you mentioned, the carbonyl oxygen has its lone pair localized while the other oxygen's lone pair is delocalized with the double bond. The better nucleophile is the one which has greater electron density.

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  • $\begingroup$ But what is attacking is not the lone pair on the carbonyl oxygen but the pi bond electrons. Your reasoning is therefore not correct. $\endgroup$ – Tan Yong Boon Feb 18 '18 at 5:26
  • $\begingroup$ @TanYongBoon That's not correct; seems mechanism was wasn't completely precise in question, if you thought so. $\endgroup$ – Mithoron Feb 18 '18 at 22:38

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