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Water dissociates into the hydroxide and hydronium ions but, why does this happen if the hydroxide ion is such a poor leaving group. Alcohols don't undergo $\ce{S_{N}1}$ reactions because the hydroxide ion doesn't dissociate so why does it happen in water?

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Only 1 in 10,000,000 water molecules dissociate.

HI, HBr and HCl fully dissociate, and I-, Br-, and Cl- and are much better leaving groups.

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  • $\begingroup$ what is it exactly that makes the hydroxide ion such a bad leaving group? $\endgroup$ – RobChem Dec 15 '14 at 13:28
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    $\begingroup$ Weak bases (conjugate bases of strong acids) are good leaving groups. Strong bases are bad leaving groups. H-, CH3-, etc. would be even worse than OH-. $\endgroup$ – DavePhD Dec 15 '14 at 13:35
  • $\begingroup$ Is SH- a good leaving group? $\endgroup$ – RobChem Dec 15 '14 at 13:38
  • $\begingroup$ I would expect SH- to be a poor leaving group, but not quite as bad as OH-, based upon the pKa of H2S being 6.9. I don't have any personal experience with it though. $\endgroup$ – DavePhD Dec 15 '14 at 13:51
  • $\begingroup$ @DavePhD You are avoiding the issue there I think, because the next logical question would be: "Then why is hydroxide such a strong base?". This question is more or less equivalent to what OP asked. $\endgroup$ – Jori Dec 17 '14 at 9:04
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Water dissociating into H+ and OH- isn't the same concept as free OH- leaving. A more accurate comparison would be forming NaOH by dropping Na metal in water, because in that case, OH- leaves water to become part of a new molecule as opposed to just ionizing in solution for a second before it becomes full water again.

Alcohols are very capable of SN1. Water is the leaving group as it is obviously protonated first. Benzyl alcohol to benzyl chloride, t-butanol to t-butyl bromide, etc.

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