My question is, how do we predict the products of a cross-Cannizzaro disproportionation in presence of a concentrated base? i.e, How to judge which of the aldehydes (both don't have any alpha hydrogen) is reduced and which one is oxidized?

If we are asked

$$\ce{CH3-O-C6H4-CHO + HCHO ->[OH^- (conc)] ?}$$

Case: if we see acc to RDS, which is hydride transfer, we must see which compound is better hydride donor. Then it will be $\ce{CH3-O-C6H4-CHO}$. This will get oxidized, and $\ce{HCHO}$ reduced.

Case: if we see which is a better nucleophilic substrate, $\ce{HCHO}$ will get oxidised, and $\ce{CH3-O-C6H4-CHO}$, reduced.

Now my problem is, which of the above cases is correct? i.e, do we see which one of the substrates is better $\ce{H-}$ donor, or which one of them is a better nucleophilic substrate?

Please advise which case is correct, and how to approach such problems.

  • $\begingroup$ Please have a look at this tutorial shows you how math and chemical formulae can be nicely formatted on this site. In their current form the chemical equations in your question are rather hard to read. $\endgroup$ – Philipp Nov 21 '14 at 15:35
  • $\begingroup$ Philipp, I know a bit of latex, typed this in a hurry, sorry about the horrible equations... $\endgroup$ – Saurabh Raje Nov 21 '14 at 16:19
  • $\begingroup$ No problem, thanks for taking the time and improving the formatting. $\endgroup$ – Philipp Nov 21 '14 at 16:48
  • $\begingroup$ Oh, its the least I could do... $\endgroup$ – Saurabh Raje Nov 21 '14 at 16:51

Canizzaro Reaction

$$\ce{A +B->[\text{conc. OH}^-]A' +B'}$$

If A has more partial positive charge, equivalently better hydride donor or more reactive, it will reduce and B will oxidise.Exception is formaldehyde where it will always oxidise to formate as it's hydrate is more stable in aqeous medium.

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  • $\begingroup$ So you mean to say that the one which is a better nucleophillic substrate will be reduced, and not oxidized? $\endgroup$ – Saurabh Raje Nov 22 '14 at 3:25
  • $\begingroup$ Moreover, if A has more positive charge, how is it a better $$\ce{H^-}$$ donor? Shouldn't we look for a higher electron density for a better donor? $\endgroup$ – Saurabh Raje Nov 22 '14 at 3:26

To answer this, first let us look at the mechanism of the Cannizzaro reaction in general. It is depicted in scheme 1 as a cyclic transition state. The cycle itself isn’t as important, the realisation that one aldehyde is present as a (deprotonated) hydrate after the action of $\ce{OH-}$ is.

mechanism of a Cannizzaro reaction
Scheme 1: General mechanism of a Cannizzaro reaction.

In your reaction, you are reacting formaldehyde with a (substituted) benzaldehyde. The question boils down to ‘can we predict, which aldehyde will form a hydrate mor easily?’ And the answer is: we can!

$$\begin{array}{cc}\hline \text{carbonyl} & \text{hydrate ratio} \\ \hline \ce{H2CO} & 99~\% \\ \ce{MeCHO} & 60~\% \\ \ce{Me2CO} & < 1~\% \\ \hline\end{array}$$

We have a clear tendency for formaldehyde to almost exclusively form hydrates under standard conditions. Thus, we can conclude that formaldehyde will alwys be the hydrate partner, end up oxidised and reduce its reaction partner as shown in scheme 2.

Cannizzaro reaction with formaldehyde
Scheme 2: Cannizzaro reduction of an aldehyde with formaldehyde as sacrificial aldehyde.

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The Cannizzaro reaction, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position. The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol.

I believe that your fist compound is an isomer of methoxybenzaldehyde, but the other compound, "hcho" is... Formaldehyde?

Methoxybenzaldehyde would disproportionate in their corresponding carboxylic acid and alcohol if a base were present, which is not, so it will not undergo Canizzaro disproportion.

I'm not sure if formaldehyde can undergo Canizzaro reaction to yield methanol plus formic acid, but a base would be surely needed.

If you talk about a crossed Cannizzaro reaction, in which an aldehyde is used in combination with a more valuable chemical. In this variation, the reductant is formaldehyde, which is oxidized to sodium formate, and the other aldehyde is transformed into the correspondent alcohol. But you also need base.

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  • $\begingroup$ I am sorry, I should have mentioned that I needed the answer for cross cannizaro only. $\endgroup$ – Saurabh Raje Nov 21 '14 at 16:18
  • $\begingroup$ And yes, that is formaldehyde $\endgroup$ – Saurabh Raje Nov 21 '14 at 16:18
  • $\begingroup$ So there you have it :) Formaldehyde is always acting as reductant, and it's oxidized to sodium (or whichever counterion your base would have) formate. $\endgroup$ – Altered State Nov 21 '14 at 16:40
  • $\begingroup$ Please see the question once again, maybe you will be able to understand my basic doubt, which is, How to predict the products of any cross cannizaro dispropotionation? Thank you for answering the particular example, but please let me know which of the cases(reasoning) is correct? $\endgroup$ – Saurabh Raje Nov 21 '14 at 16:48
  • $\begingroup$ useless; he's not asking what canizzaro reaction is? $\endgroup$ – RE60K Nov 21 '14 at 17:51

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