H2O is best leaving group or H2S?
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Answer:
Your reasoning is correct but there are more factors that determine e- pair availability and it turns out that H20 is more basic. Clearly their is a balance between the larger negative charge and the availability of the lone pairs. In the case of H20 versus H2S, the larger negative charge favors H20 being more basic.
Consider a protonated alcohol and thioalcohol(the one which has S in place of O). Now we have two scenarios , since it is protonated O and S carry a positive charge --- now the question is which is the most unstable one??? Similar to asking which is most electronegative?? Exactly ---> "O" is the most electronegative and it's kind of hates the lack of an electron...hence it quickly pulls its bonded electrons(to carbon) to leave as H2O. S also does it, but not as quickly as O does...hence H2o is a better leaving group.