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A nucleophile should get better at its job if it can donate a pair of electrons more easily. We say that a more electronegative atom should be less nucleophilic , generally. I understand this is because the more EN an atom gets, the more strongly it pulls electrons towards itself. But why does the fact that such an atom would ,generally, have more electron density not override this?

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    $\begingroup$ As a donor atom gets more and more electronegative, the outer orbitals are more closer to the nucleus and electrons in those orbitals become lower in energy, i.e the energy of the HOMO decreases, so it becomes progressively difficult for them to donate as now they need to give away the electrons which are more stable in a sense. $\endgroup$ – Soumik Das Feb 7 at 6:54
  • $\begingroup$ Yes. But does the partial negative charge make them less stable? $\endgroup$ – Sal_99 Feb 8 at 2:28
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Electronegativity and nucleophilicity are different concepts. Electronegativity refers to how strongly an atom pulls bonding electrons towards itself. Nucleophilicity refers to ability of atom to donate a lone pair of electrons. Different factors affect nucleophilicity, such as orbital size, charge of ion, size of molecule, hybridisation etc etc.

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  • $\begingroup$ Yes. But does the fact that an atom has a partial negative charge help its nucleophilicity? $\endgroup$ – Sal_99 Feb 8 at 2:27
  • $\begingroup$ Yes. Read that I wrote "charge of ion" as a factor affecting nucleophilicity. $\endgroup$ – Mr Organic Feb 8 at 2:31
  • $\begingroup$ And that is where I am confused. More electronegatvity - - - > more partial negative (ceteris paribus) - - - - > more nucleophilicity. But more electro negative - - - > less able to give up electrons because of stability - - - > less nucleophilicity. Which one dominates? $\endgroup$ – Sal_99 Feb 8 at 2:55
  • $\begingroup$ @Sal_99 stop thinking about electronegativity affecting nucleophilicity so much. EN is just a minor factor affecting nucleophilicity. Bulkiness of the base and ionic charge are larger factors at play. Anyways, I'm treading on thin waters, a generalisation is that the more electronegative the atom, the worse nucleophile it is. $\endgroup$ – Mr Organic Feb 8 at 9:00

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