Proposed mechanism B: formation of an intermediate dimeric adduct

A possible mechanism that fits all the experimental evidence so far involves nucleophilic attack of the usual tetrahedral intermediate on another aldehyde to give an intermediate adduct. This adduct could then form the products directly by hydride transfer. You may not like the look of this last step, but the mechanism was proposed and evidence is needed to disprove it.

Which step would be rate determining for this mechanism? It could not be step 1 since, if this were the case, the rate law would be fi rst order with respect to the aldehyde rather than the observed second-order relationship. Also, if the reaction is carried out in water labelled with oxygen-18, the oxygen in the benzaldehyde exchanges with the 18O from the solvent much faster than the Cannizzaro reaction takes place. This can only be because of a rapid equilibrium in step 1 and so step 1 cannot be rate determining.

So, for mechanism B, either step 2 or step 3 could be rate determining—either case would fi t the observed rate law. Step 2 is similar to step 1: in both cases an oxyanion nucleophile attacks the aldehyde. Since the equilibrium in step 1 is very rapid, it is reasonable to suggest that the equilibrium in step 2 should also be rapid and thus that the hydride transfer in step 3 must be rate determining. So mechanism B can fi t the rate equation. How can mechanism B be ruled out? One way is to change the attacking nucleophile. The Cannizzaro reaction works equally well if methoxide is used in a mixture of methanol and water. If mechanism B were correct, the reaction with methoxide would be as follows.

One of the products would be different by this mechanism: benzyl methyl ether would be formed instead of benzyl alcohol. None is observed experimentally. Moreover, under the conditions of the experiment, benzyl methyl ether does not react to form benzyl alcohol, so it cannot be the case that the ether is formed but then reacts to form the products. Mechanism B can therefore be ruled out.

اخر الاخبار

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المزيد

مواضيع ذات صلة

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Cross metathesis

Ene-yne metathesis

Metathesis catalysts

Alkene metathesis

Attack of carbenes on lone pairs

Nitrenes are the nitrogen analogues of carbenes

Rearrangement reactions

Insertion into C–H bondsInsertion into C–H bonds

Making cyclopropanes