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OH is a strong base which makes it a good nucleophile. Why then is it a good leaving group?

Also, why are bad nucleophiles good leaving groups? What are the criteria a good leaving group should fulfill?

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closed as unclear what you're asking by Jan, Todd Minehardt, jerepierre, Nilay Ghosh, Jon Custer Feb 24 '17 at 2:46

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    $\begingroup$ I think $OH^-$ is not a good leaving group as far as I know. That is the reason why we protonate it and then water is released as a good leaving group.Strong bases such as OH−, OR− tend be poor leaving groups, due their inability to stabilize a negative charge.For a good leaving group the stabilization of charge is one of the factors. $\endgroup$ – Piyush Raut Feb 23 '17 at 15:12
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    $\begingroup$ Iodide is a great nucleophile and an excellent leaving group. $\endgroup$ – Zhe Feb 23 '17 at 15:24
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    $\begingroup$ What in the world made you think it is? $\endgroup$ – Mithoron Feb 23 '17 at 15:39
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I can’t quite follow you because hydroxide is one of the lousiest leaving groups known to chemistry that are still considered potential leaving groups (mostly by students, that is).

Note that nucleophilicity and basicity are unrelated as are nucleophilicity and leaving group ability. Here are two examples:

  • hydroxide is a pretty good nucleophile, a very strong base and a lousy leaving group.

  • iodide is a pretty good nucleophile, a very weak base and a very good leaving group.

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Good leaving groups are groups that are stabilized in the reaction medium. In general, the conjugate base for a strong acid is a good leaving group, and indeed, you can apply the same rationale for why something is the conjugate base of a strong acid to why it's a good leaving group.

This, of course, refutes your claim that hydroxide is a good leaving group since it is not the conjugate base of a strong acid.

Nucleophicility is a much harder concept to quantify in terms of what is important, and I think it's best if I not attempt a lengthy answer here.

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