6
$\begingroup$

When chloral undergoes reaction with a concentrated alkali, will it undergo Cannizarro reaction to form the corresponding alcohol and acidic salt, or should it form chloroform instead by departure of trichloro carbanion instead of the hydride ion?

I understand that the trichlorocarbanion is stabilized by the $-I$ effect of chlorine, so why not form that instead?

$\endgroup$
8
$\begingroup$

In the Cannizarro reaction an aldehyde with no alpha-hydrogens is treated with $\ce{OH^-}$. Through a series of equilibria the aldehyde goes on to disproportionate into the corresponding carboxylate and alcohol as shown in the following mechanism.

enter image description here

Again, the only requirement for the Cannizzaro reaction to occur is that the aldehyde not have any alpha-hydrogens. Formaldehyde and benzaldehyde are two very common aldehydes that undergo the reaction when treated with $\ce{OH^-}$. Chloral ($\ce{CCl3CHO}$), having no alpha-hydrogens also undergoes the Cannizzaro reaction (reference)

Edit: Response to OP's comment

A key part of the Cannizzaro mechanism is shown in the second line of the above mechanism. Here a hydrogen is transferred in a concerted step to the second molecule of aldehyde. Free $\ce{H^-}$ is not generated in the process, rather it is a smooth, concerted transfer of a hydrogen atom.

[Note: This has been proven experimentally, when the reaction is run in $\ce{D2O}$, no deuterium winds up attached to the alcohol carbon. This shows that free $\ce{H^-}$ was never involved, but rather a concerted transfer of the aldeyhdic hydrogen takes place].

Thanks to Mithoron for noting that, with chloral, the haloform reaction can apparently compete with the Cannizarro reaction. Presumably, the pathway followed depends upon the reaction conditions.

$\endgroup$
  • 1
    $\begingroup$ Your reference says on the same page that it does undergo haloform reaction... $\endgroup$ – Mithoron Jul 26 '17 at 20:36
  • 1
    $\begingroup$ @Mithoron Does the percentage ratio of haloform and Canizzaro reaction depend on the concentration of base? $\endgroup$ – Mockingbird Jul 27 '17 at 0:37
  • $\begingroup$ @Mithoron On page O-339, where abouts on the page? $\endgroup$ – ron Jul 31 '17 at 2:01
  • $\begingroup$ Penultimate chain of reactions starting with ethanol. $\endgroup$ – Mithoron Jul 31 '17 at 15:13
2
$\begingroup$

If chloral undergoes Cannizzaro reaction then the Haloform reaction with acetaldehyde or Chloral should fail.

I think Cannizzaro with Choral is not possible as CCl3- is a good leaving group rather than hydride(H-) ion.

Ref:Cannizzaro reaction mechanism

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.