Q24.70 P - Chemistry: Molecular Nature Of Matter And Change

Question

The oxidation of methanol to formaldehyde can be accomplished by reaction with chromic acid: 

 ${\mathbf{\text{6H}}}^{\mathbf{\text{+}}}{\mathbf{\text{(aq)+3CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{OH(aq) +2H}}}_{\mathbf{\text{2}}}{\mathbf{\text{CrO}}}_{\mathbf{\text{4}}}\mathbf{\text{(aq)}}\mathbf{\to }{\mathbf{\text{3CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{O(aq)+2Cr}}}^{\mathbf{\text{3+}}}{\mathbf{\text{(aq)+8H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l)}}$

The reaction can be studied with the stable isotope tracer  _{${}^{\mathbf{\text{18}}}\mathbf{\text{O}}$} and mass spectrometry. When a small amount of CH318OH is present in the alcohol reactant, ${{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}}^{\mathbf{\text{18}}}\mathbf{\text{O}}$  forms. When a small amount of  ${\mathbf{\text{H}}}_{\mathbf{\text{2}}}{\mathbf{\text{Cr}}}^{\mathbf{\text{18}}}{\mathbf{\text{O}}}_{\mathbf{\text{4}}}$ is present,  ${{\mathbf{\text{H}}}_{\mathbf{\text{2}}}}^{\mathbf{\text{18}}}\mathbf{\text{O}}$ forms. Does chromic acid or methanol supply the O atom to the aldehyde? Explain.

Step-by-Step Solution

Answer

Methanol provides the aldehyde with the oxygen atom.

Step 1: The given data

From the given data, the balanced chemical equation of the reaction is as shown below:

 ${\mathbf{\text{6H}}}^{\mathbf{\text{+}}}{\mathbf{\text{(aq)+3CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{OH(aq) +2H}}}_{\mathbf{\text{2}}}{\mathbf{\text{CrO}}}_{\mathbf{\text{4}}}\mathbf{\text{(aq)}}\mathbf{\to }{\mathbf{\text{3CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{O(aq)+2Cr}}}^{\mathbf{\text{3+}}}{\mathbf{\text{(aq)+8H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l)}}$

Methanol oxidises into formaldehyde. So, this methanolic -OH oxygen remains the same.

Step 2: Explanation

Methanol supplies the O atom to the aldehyde since the atom of oxygen in the methanol reactant appears in the product as formaldehyde. 

However, the oxygen atom in the chromic acid is the oxygen that forms water in the product side.

Hence, methanol supplies the O-atom to the aldehyde.