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At school, I did an experiment of synthesizing resorcinol-formaldehyde resin. When I added $\ce{NaOH}$ and gave some heat, the color of resorcinol-formaldehyde resin inside the test tube became reddish-brown. Can anyone tell me why?

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    $\begingroup$ What do you think happen if you do the same with the green tea infusion or generally phenolic compounds? And why? $\endgroup$
    – Poutnik
    Commented May 23, 2021 at 14:33
  • $\begingroup$ Because you have made a polymer. $\endgroup$ Commented May 24, 2021 at 3:32
  • $\begingroup$ @Poutnik What has the green tea to do in this problem ? $\endgroup$
    – Maurice
    Commented May 24, 2021 at 12:28
  • $\begingroup$ @Maurice Phenolic OH groups and tendency of their conjugate bases in alkalic solutions to 1/ get intensified red/brown colour due electron delocalization 2/ to get oxidized, condensed and further deepening of colour. $\endgroup$
    – Poutnik
    Commented May 24, 2021 at 12:36
  • $\begingroup$ @Poutnik. So the red color is due to the oxidation of resorcin, and it has nothing to do with the polymerization with methanal. Does this happen only when there is an excess of resorcin in the mixture ? $\endgroup$
    – Maurice
    Commented May 24, 2021 at 14:35

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The polymerization of resorcinol creates multiple (spaghetti-like) chains containing the aromatic rings of resorcinol. This is a simultaneous growth in multiple directions. Eventually, the chains trap each other in the solid state. In the analogy used, it is like stirring a pot of spaghetti and then freezing the content.

It may happen that an arene ring is just in proximity of an arene ring of an other chain. Given a growing polymer chain may be flexible, it may happen that there are intramolecular "close couples", too. If these arenes are close enough to each other, their $\pi$-electron systems may interact with each other as if they were coupled as one. This extension of the $\pi$-electron system may move the observable absorption band toward longer wavelengths. Then,

  • If there are just enough of these "close encounters" per unit of volume with the same arene-arene distance, the perception of color may be intensified (e.g., pale yellow -> bright yellow).

  • With variation of the arene-arene distance, the intermolecular interaction of these $\pi$-electron systems equally varies. This may yield the absorption of light of different wavelengths / the subtraction of different colors from the white light by these arenes. Thus it may happen that the color of the polymer is not uniform. If the range of light absorption is wide enough, the polymer may appear just black.

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