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This question is primarily based on the Cannizzaro reaction, where a base induces a disproportionation reaction of non-enolizable aldehydes (benzaldehyde, formaldehyde, etc) to form an alcohol and a carboxylic acid.

In the video by Nile Red exploring the Cannizzaro reaction (https://www.youtube.com/watch?v=qEBC204WTKs), the product mixture contained both Benzoic acid and Benzyl alcohol and had an orange color. I am aware that the benzoic acid was in the form of sodium benzoate due to sodium hydroxide also reacting with the acid produced.

My question is, where does the colour come from?

I am guessing it is either from the benzyl alcohol or sodium benzoate, but I have learned that most organic compounds are colorless.

Moreover, if the aldehyde used was something like formaldehyde, will the orange color also form?

Thanks in advance for the clarifications.

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none of the components you mentioned (Benozic acid, Benzyl alcohol, sodium benzoate) has a yellow colour.

The colour in organic compouns is closely tied to ther delocalized sytems of pi-electrons and depends on the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). When the energy of this gap corresponds to a wavelength that is visible, the compound absorbs this wavelength and the colour of the compound is the complementary colour. A good example are condensated aromats. For benzene or naphthalene the gap between HOMO and LUMO is too big therefore, the corresponding wavelength is in the area of UV light. For anthracene, the absorbed wavelenght is violett and therefore the colour of the compound is yellow.

Back to your question. Because the reaction happends under strong basic conditions, high temperature and with many aromatic compounds, I think sidereactions will occure where multiple aromats condensate together and therefore the solution turns yellow to orange.

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  • $\begingroup$ Thanks for the answer. Could you suggest what sort of side products could produce that yellow/orange color? $\endgroup$ – Gnayus Mar 25 at 4:09
  • $\begingroup$ No, I am sorry. The reaction goes up to 90% yield so 10% of the molecules could undergo sidereactions. If I had to guess, I would not got condensated aromats but for some kind of structure similar to benzophenone were the C=O double bond can expand the conjugated pi electron system. benzophenone itself is still colourless but something similiar might become yellow or orange. $\endgroup$ – Inselino Mar 25 at 7:53
  • $\begingroup$ As it happens pure anthracene crystals are white and a solution colourless. The first absorption band is below 400nm. if its coloured there is most likely some oxidation product. $\endgroup$ – porphyrin Mar 25 at 13:17
  • $\begingroup$ Ah ok. I knew that is often the case with naphthalene, I produced white naphthalene in the lab but yellow anthracene. Thanks for the comment $\endgroup$ – Inselino Mar 26 at 7:13

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