Cannizzaros Interpretation

Compounds contained whole numbers of atoms as Dalton postulated. 

Avogadro s hypothesis was correct—equal volumes of gases under the same conditions contain the same number of molecules. 

TABLE 2.1 Relative Mass Data for Several Cases Containing Carbon 

Compound Relative Molecular Mass Percent Carbon (by mass) Relative Mass of Carbon Present 

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According to Cannizzaros interpretation of Avogadro s hypothesis, the same volumes of any two gases have the same number of particles, provided they are at the same pressure and temperature. The number of atoms within 1 liter of oxygen, therefore, is the same as the number of atoms within 1 liter of hydrogen. To find the relative mass, you need only divide the two masses. Chlorine atoms, therefore, are 3.165 g/0.0900 g = 35.2 times more massive than hydrogen atoms. 

Cannizzaro s work was so convincing because he collected data on so many compounds. Although he couldn t absolutely prove that his atomic mass values were correct (because he had no way to verify absolutely the formulas of the compounds), the consistency of the large quantity of data he had collected eventually convinced virtually everyone that his interpretation made sense and that the relative values of atomic mass that he had determined were correct. The confusion was finally over. Chemistry had the universal (relative) mass standards that it needed. 

An additional complication in mechanistic interpretation is the fact that methoxide (reactions are often run in methanol) serves also as an effective hydride donor towards aldehydes and the derived formaldehyde may participate in a crossed Cannizzaro reaction. The conditions of the Cannizzaro reaction are fairly extreme (strong base, reasonably high temperatures) and complex mechanistic behavior is therefore not surprising. 

Alkah-catalysed keto-enolic equilibrium c and internal Cannizzaro-type reaction f, represented in Scheme 13.5, should be taken into account when interpreting the experimental data. 

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