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We used benzaldehyde for a demonstration of the Cannizzaro reaction in school, but the liquid we used appeared visibly yellow, as opposed to the colourless liquid described by Wikipedia.

A few reasons I could think of include:

  • Reaction with oxygen to form quinone-type compounds
  • Polymerisation induced by exposure to light
  • Impurities

I haven't found much discussion about this, nor do I have any candidate compounds for if my guesses above turn out to be right.

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Tl;DR: All three factors are responsible for coloring of benzaldehyde (and pretty much any organics)


For the first two factors, I would mention the paper1 linked by @Andrew. Benzaldehyde is prone to atmospheric oxidation; the reaction is accelerated in presence of light to form primarily benzoic acid mixed other aromatics and polymerized aromatics causing darkening.

For the impurity aspect, let me quote the following from a paper2 discussing about benzaldehyde purification process:

[...] It is clear that traces of impurities, namely metals, influence markedly the benzaldehyde activity. The active impurities present in pure benzaldehyde or in that of anal, grade become visible after distillation leaving the distillation residue brown to yellow coloured. The criterion for the good quality of benzaldehyde is the colorless distillation residue.

My previous discussions:

  1. Phenoquinone vs benzoquinone
  2. Air- and light-sensitivity of phenol

References

  1. THE ABSORPTION OF OXYGEN BY BENZALDEHYDE, H. J. Almquist and G. E. K. Branch Journal of the American Chemical Society 1932 54 (6), 2293-2302 DOI: 10.1021/ja01345a018
  2. Benzaldehyde oxidation test, a model reaction with radical mechanism. II. The purification of benzaldehyde and stabilization of its activity by J. GAŠPERÍK, Chem. zvesti 29 (6) 808-810 1975 (PDF)
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