I am reading an old method for making triphenylchloromethane (Org. Synth. 1943, 23, 100):

$\hspace{30 mm}$Reaction

When I draw out the structures of the molecules, I see this:

enter image description here

But the $\mathrm{S_N1}$ mechanism doesn't make sense to me since $\ce{OH-}$ is a bad leaving group, and there is nothing to protonate it.

  • $\begingroup$ SN1 reactions are based on the stability of carbocation (R+), whereas SN2 reactions are based on the steric hinderance in R group. $\endgroup$ – Sujith Sizon Dec 7 '15 at 2:42

I find it unlikely that the substitution proceeds via the hydroxyl group directly leaving. If there is any evidence that this reaction proceeds via direct $\mathrm{S_N1}$ of $\ce{OH-}$ by $\ce{Cl-}$, I would be very interested to see it. When an alcohol and an acyl chloride are mixed, I think it is much more probable that they will initially form the ester via nucleophilic acyl substitution:

$\hspace{27 mm}$Acyl substitution

Presumably, this ester is rather unstable because of steric repulsions between the three phenyl groups and the ester group. So now, the $\ce{-OAc}$ group ($\ce{Ac} = \ce{COCH3}$) can leave as a relatively stable carboxylate ion to give the resonance-stabilised carbocation:

$\hspace{35 mm}$SN1

The carbocation is then intercepted by chloride ions that were formed earlier to give the product you want. So, the second part of the reaction is a $\mathrm{S_N1}$ substitution of $\ce{OAc-}$ by $\ce{Cl-}$.

$\ce{OAc-}$ ($\mathrm{p}K_\mathrm{aH} = 4.76$) is a much better leaving group than $\ce{OH-}$ ($\mathrm{p}K_\mathrm{aH} = 15.7$).

There is another pathway possible, since $\ce{HCl}$ is formed from the initial reaction of the alcohol with the acyl chloride. If $\ce{HCl}$ is present in appreciable quantities, it can protonate the original alcohol and you could perhaps have a direct $\mathrm{S_N1}$ of $\ce{H2O}$ by $\ce{Cl-}$.

$\hspace{10 mm}$SN1

  • $\begingroup$ It all came back to me! Thank you very much for the detailed answer! $\endgroup$ – greg Dec 7 '15 at 3:08

Yes, the hydroxide is a bad leaving group but look the structure if the hydroxide ion leave. It's a triphenyl cation! It's very stable carbocation (very high mesomeric effect). Then the reactive hydroxide react with acyl chloride and liberate Cl- that react with phenyl cation.


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