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With respect to bimolecular substitution and bimolecular elimination reactions, both electron delocalization and polarizability have an analogous effect on a species, making them good leaving groups.

For example I- is a good leaving group since its negative charge is spread out over a large area.

Also, the tosylate ion is a good leaving group because of electron delocalization where its electrons are spread out over a large area due to resonance stabilization.

However, the analogy seems to break down when considering their nucleophilicity in aprotic media, I- is a good nucleophile and tosylate is not.

Is there a fundamental problem of the analogy between polarizability and electron delocalization?

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  • $\begingroup$ Acetate is not a 'good' leaving group. I think you have your nucleophilicity and leaving group ability backwards. $\endgroup$
    – Lighthart
    Jun 24, 2016 at 16:09
  • $\begingroup$ I replaced acetate with tosylate, but I think you're missing the point. Why does spreading charge out over a large space make resonance structures and polarizable species more stable -- but, increasing resonance decreases nucleophilicty, whereas increasing size and polarizability increases nucleophilicity? They're both spreading charge out over a large area. $\endgroup$
    – Ryan David Ward
    Jun 24, 2016 at 16:24

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TL;DR: Nucleophilicity is primarily a kinetic phenomena. Leaving group ability is a thermodynamic phenomena.

The stabilization of an anion by delocalization of charge means that the resulting products will have lower energies than alternate situations. In accordance with the Hammond postulate (a few logical steps skipped), transition state barriers with low energy products will be smaller than those with higher energy products.

However, for a bimolecular reaction to occur, the molecule must have 'proper collision speed and orientation'. Diffuse charges (over multiple atoms) generally make this more difficult. This extra facet of the of the geometry of the transition state is why the equivalent stability argument does not apply to both leaving group ability and nucleophilicity.

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  • $\begingroup$ Whoa, I was not expecting such a succinct answer. Thank you. Can you comment on bases, which are also said to be thermodynamic? $\endgroup$
    – Ryan David Ward
    Jun 24, 2016 at 16:41
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    $\begingroup$ Explicitly not true, but a useful shortcut: Basicity is the thermodynamic aspect of nucelophilicity. Strong bases are higher in energy and thus, according to the Hammond postulate, transition state structures in bimolecular reactions will be close to the structure of the strong base, and hence activation energy barriers will be smaller. I would encourage you to read the Wiki page on Hammond Postulate: en.wikipedia.org/wiki/Hammond%27s_postulate $\endgroup$
    – Lighthart
    Jun 24, 2016 at 16:47
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Nucleophilicity of a chemical species depends on the 'availability' of charge on the nucleophile. As you pointed out, acetates are bad nucleophiles. This is because the negative charge is delocalised over the 3 centres (O-C-O) and not readily available to any electrophile.

I- is a good nucleophile since it is spherical and polarisable, the charge being available.

The concepts of delocalisation and polarisability are independent from each other and any resemblance in a particular situation should be treated as coincidental and not read into too much.

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  • $\begingroup$ While I appreciate your answer, it just seems like you're restating the premise. I understand that the two concepts are different, but they seem pretty analogous. Don't they both distribute negative charge over a large space, which contributes to stability? Is there a third concept that the "polarizability" and "electron delocalization" have in common, like "charge delocalization"? $\endgroup$
    – Ryan David Ward
    Jun 24, 2016 at 15:16

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