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Why are thioethers ($\ce{RSR}$) not better leaving groups than ethers ($\ce{ROR}$)? I learnt that better bases are poorer leaving groups. Here, clearly the poorer base is thioether, as the lone pair is on a larger atom.

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    $\begingroup$ Then you should unlearn it. Being a leaving group is only tangentially related to basicity, as you can see from this table. $\endgroup$ – Mithoron May 13 '17 at 21:19
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    $\begingroup$ What this chart doesn't cover (and would help you with your understanding) is what is meant by "leaving group ability?" Is it kinetics? Is it thermodynamics? You can't rationalize if you don't know what you're trying to rationalize. $\endgroup$ – Zhe May 13 '17 at 21:21
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    $\begingroup$ @Zhe It's from Wikipedia.... $\endgroup$ – user38977 May 14 '17 at 4:08
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    $\begingroup$ @Mithoron It would be better if you explain why.... $\endgroup$ – user38977 May 14 '17 at 4:14
  • $\begingroup$ @blue Let's assume we are talking about thermodynamics. Then look at it by considering the reverse reaction. Is it easier to S-alkylate RSR or to O-alkylate ROR? Kinetics would be a little trickier to rationalise, as steric hindrance and other effects come into play. $\endgroup$ – user6376297 May 14 '17 at 7:16

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