Strong nucleophile but weak bases

[FeralisExtremum]

Hey everyone,

I've been reviewing Orgo and I'm having some trouble understanding how a strong nucleophile can be a weak base. Chad provides a list of these strong nucleophiles but weak bases: CN, N3, Cl, Br I, SH, SR (all negatively charged ions).

I don't understand conceptually how something could be a good nucleophile but a weak base (I understand the difference between nucleophiles and bases and why something can be a good base but a bad nucleophile, but the opposite condition does not make any sense to me).

Related to this, I also was curious why the halide anions are weak bases to begin with: for example, they are weaker bases than H2O, despite having a negative charge. According to Chad's CARDIO method of ranking bases, a negative charge should be a sign of a strong base, but that doesn't seem to be the case here. I understand that HI, HBr, HCl, etc are all strong acids so the conjugate bases are weak, but again conceptually I do not understand why they are weak bases given their negative overall charge. Are they subject to some weird super stability because of their filled octet?

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[Dotoday]

Simpky speaking. Strong nucleophiles want to donate electrons. Strong bases want to accept protons. For example F- is very electronegative and therefore would not want to donate electrons, but would accept a proton. I- or CN- on the other hand are not as electronegative and woukd like to donate electrons. Hope this helps. Good luck.

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[DHAVEN]

These weak bases that are strong nucleophiles include Azides, Halides and Thiols. They will easily donate their electrons, and take part in E1 or Sn1 mechanisms with carbocation intermediates. Their conjugate acids Hydrazoic acid, Hydrohalides, and Sulfides are going to usually be strong bases, as I understand. That's another way to distinguish them from strong nucleophilic good bases. Strong bases will have weak conjugate acids.