Nucleophilicity: Polarizability vs Charge Distribution

[77deuce]

Hey! The TPR book I'm reading says that nucleophilicity increases as you move down within a group on the periodic table because it increases polarizability due to the negative charge being more prone to distortion around larger atoms. This make sense, but...

I remember learning that having a negative charge on a larger atom will delocalize the charge , thus making it more stable and decreasing nucleophilicity. (TPR doesn't mention this)

Which of these two phenomena has more powerful effects/ trumps the other?

(According to TPR, a negatively charged sulfur is more nucleophilic than hydroxide bc sulfur is more polarizable)

The pKa of Hydrosulfuric acid is lower than that of water, so isn't that more evidence that hydroxide would be be more nucleophilic that a negative sulfur?

Let me know if I am just completely mistaken. Thanks!

tl;dr Does nucleophilicity increase or decrease as you go down a group? Why?

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[aldol16]

TPR is wrong. Nucleophilicity and going down a column in the periodic table must be contextualized in terms of solvent. If you're in a polar protic solvent, nucleophilicity will increase going down a column because H-bonding ability decreases going down a column. Therefore, the solvent will H-bond with the nucleophile and therefore diminish nucleophilicity via solvation. That's why fluoride is a poor nucleophile in polar protic solvents.

However, in polar aprotic solvents, nucleophilicity decreases going down a column. This is because H-bonding doesn't exist anymore and thus nucleophilic reactions categorically are accelerated in polar aprotic solvents. Here, nucleophilicity correlates well with basicity. That is, the fluoride ion is a strong base and will react very quickly with nucleophiles because it's unstable on its own.