Question

Why are aldehydes easily oxidized?

Answer 1

An `"aldehyde"` is necessarily situated at a primary carbon. Why so? It is thus sterically predisposed to reaction.

Explanation:

When we oxidize a primary alcohol, it is often very difficult to stop at the aldehyde stage, i.e.

`RCH_2OH rarr RC(=O)H rarr RCO_2H`

The oxidation numbers of the ipso carbon from left to right, are `-I`, `+I`, and `+III`. Because the ipso carbon is sterically unhindered, reaction can occur at this centre relatively easily. Because oxidation is thermodynamically downhill, oxidation to the carboxylic acid is facile.

I once had a colleague with brilliant hands at these sorts of oxidations. He was good, he was quick, and he used to do all these oxidations with chromic acid. He assessed the stage of the reaction by thin layer chromatography, and he could invariably stop at the aldehyde. His professor (very unfairly!) complained that his results were not reproducible, but that's an old cop-out. He could reproduce his results; his successors had to work hard to emulate him.