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I've been given this mechanism by my lecturer for the formation of a tosylate. It claims that NEt3 is needed to remove the hydrogen from the O+ in the second step. However surely the Cl- from TsCl would remove the H and no NEt3 is needed?

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    $\begingroup$ No, $\ce{Cl-}$ is a much worse base than $\ce{NEt3}$. $\endgroup$ – ringo Apr 16 '16 at 22:58
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No, $\ce{Cl-}$ will not remove the $\ce{H+}$ because it is not basic enough. The $\mathrm{p}K_\mathrm{a}$ of $\ce{Cl-}$'s conjugate acid ($\ce{HCl}$) is $-7$. The $\mathrm{p}K_\mathrm{a}$ of $\ce{NEt3}$'s conjugate acid ($\ce{HNEt3+}$) is $10.75$ (source). This means that $\ce{HCl}$ almost completely dissociates in solution, and is very unlikely to form $\ce{HCl}$ molecules. $\ce{NEt3}$, on the other hand, is much more basic and will easily pick up the $\ce{H+}$.

Also the mechanism is incorrect. A lot of the arrows are wrong. See here (pyridine serves the same function as $\ce{NEt3}$):

Tosyl ester formation

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    $\begingroup$ pKa of Et3NH+: 10.75 in water I'm fairly sure that website got it mixed up. All the pKb values cited (including the anilines, pyridine etc.) are actually pKaH values $\endgroup$ – orthocresol♦ Apr 17 '16 at 1:10
  • $\begingroup$ This sulfur-oxygen bond movement is... eh. $\endgroup$ – SendersReagent Apr 17 '16 at 8:25

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