The oxidation of methanol to formaldehyde can be accomplished by reaction with chromic acid: $$ \begin{array}{r} 6 \mathrm{H}^{+}(a q)+3 \mathrm{CH}_{3} \mathrm{OH}(a q)+2 \mathrm{H}_{2} \mathrm{CrO}_{4}(a q) \longrightarrow \\ 3 \mathrm{CH}_{2} \mathrm{O}(a q)+2 \mathrm{Cr}^{3+}(a q)+8 \mathrm{H}_{2} \mathrm{O}(I) \end{array} $$ The reaction can be studied with the stable isotope tracer \({ }^{18} \mathrm{O}\) and mass spectrometry. When a small amount of \(\mathrm{CH}_{3}{ }^{18} \mathrm{OH}\) is present in the alcohol reactant, \(\mathrm{CH}_{2}{ }^{18} \mathrm{O}\) forms. When a small amount of \(\mathrm{H}_{2} \mathrm{Cr}^{18} \mathrm{O}_{4}\) is present, \(\mathrm{H}_{2}{ }^{18} \mathrm{O}\) forms. Does chromic acid or methanol supply the \(\mathrm{O}\) atom to the aldehyde? Explain.

Stable isotope tracers are atoms with a different number of neutrons but the same number of protons. This difference allows scientists to track the movement of specific atoms within chemical reactions. In our oxidation of methanol example, we use the stable isotope \({}^{18}\text{O}\). This isotope helps to identify the source of the oxygen atom in the formaldehyde production. By substituting the normal oxygen with \({}^{18}\text{O}\) into methanol (\(\text{CH}_{3}{}^{18}\text{OH}\)) and chromic acid (\(\text{H}_{2}\text{Cr}^{18}\text{O}_{4}\)), we can observe where the \({}^{18}\text{O}\) ends up in the reaction products using mass spectrometry.

The chromic acid reaction is a chemical process where chromic acid (\(\text{H}_{2}\text{CrO}_{4}\)) oxidizes alcohols to aldehydes or ketones. In this reaction:

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

Here, methanol (\(\text{CH}_{3}\text{OH}\)) is oxidized to formaldehyde (\(\text{CH}_{2}\text{O}\)). During oxidation, chromic acid acts as the oxidizing agent, reducing in the process to \(\text{Cr}^{3+}\). This reaction produces water (\(\text{H}_{2}\text{O}\)) as a byproduct. Through isotope tracing, we determined that the oxygen in the formaldehyde originates from methanol, whilst the oxygen in the water comes from chromic acid.

Formaldehyde (\(\text{CH}_{2}\text{O}\)) is formed through the oxidation of methanol. During this reaction,\(3\text{CH}_{3}\text{OH}\) (methanol molecules) interact with \(\text{6 H}^{+}\) and \(\text{2 H}_{2}\text{CrO}_{4}\) (chromic acid) producing \(3\text{CH}_{2}\text{O}\) (formaldehyde), \(\text{2 Cr}^{3+}\), and \(\text{8 H}_{2}\text{O}\) as products. From the isotope tracer experiment, we confirmed that the oxygen atom within the formaldehyde originates from the oxygen in methanol, indicating a direct transfer during the oxidation reaction. Hence, methanol supplies the oxygen atom to formaldehyde (\(\text{CH}_{3}{}^{18}\text{OH}\) forms \(\text{CH}_{2}{}^{18}\text{O}\)). This is an essential concept in understanding the oxidation process and the role of reactant molecules in forming specific products.