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In Solomons' Organic Chemistry, in a section discussing leaving groups, it is stated that$-$

Among the halogens, an iodide ion is the best leaving group and a fluoride ion is the poorest: $$I− > Br− > Cl− >> F−$$ The order is the opposite of the basicity in an aprotic solvent: $$F− >> Cl− > Br− > I−$$

But the order of basicity of halides is $F->Br->Cl->I-$ so the first relation makes sense, as leaving groups must depart as weak bases.

My question is why does the order change in an aprotic solvent? What am I missing?

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    $\begingroup$ I believe you have misrepresented the text. It is not claiming that a separate ranking in terms of better leaving groups is established for an aprotic solvent, but rather that such ranking (regardless of solvent) is inverse with respect to the ranking of halogens in terms of basicity. $\endgroup$
    – Sam202
    Commented Apr 29 at 7:40
  • $\begingroup$ @Sam202 I thought so as well, but was confused by the explicit mention of aprotic solvent, as basicity of halides remain same regardless of nature of solvent . $\endgroup$
    – Eisenstein
    Commented Apr 29 at 7:55
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    $\begingroup$ Consider formatting guides for texts and formulas/equations/expressions including mhchem \ce{} specifics. $\endgroup$
    – Poutnik
    Commented Apr 29 at 9:28

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