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A benzene derivative $ \ce{C_8H_x} $ did not produce white precipitate with the ammonical silver nitrate solution but decolorised the cold dilute alkaline $\ce{KMnO4}$ solution. The compound is having _____ number of hydrogen atoms.

  1. The absence of white precipitate indicates it is not an alkyne.
  2. The benzene ring must have an alkene group attached with it such that it decolourises $\ce{KMnO4}$

Now for the question, the answer key tells it must be styrene, however I argue that the answer should be one homologous higher than styrene, because styrene's resonance doesn't allow it to react with $\ce{KMnO4}$.

Is my reasoning right?

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    $\begingroup$ No, it would react just fine. Also, the next one in the homologous series would be C9, not C8. $\endgroup$ Mar 21 at 8:24
  • $\begingroup$ Oh my, I think I overlooked that part about $C_8$. Due apologies. But isn't the double bond in conjugation with the benzene? Shouldn't the electron delocalization prevent further attack? $\endgroup$
    – Akilan SS
    Mar 21 at 9:41
  • $\begingroup$ It is in conjugation, but that's not enough to prevent it from most attacks. $\endgroup$ Mar 21 at 9:45
  • $\begingroup$ Hmm makes sense. Thanks for helping out! $\endgroup$
    – Akilan SS
    Mar 21 at 9:47

1 Answer 1

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Styrene has been reported to react with potassium permanganate to yield 1-phenyl-1,2-ethanediol (at room temperature, albeit promoted by either ultrasound or hydrogen peroxide), phenylglyoxylic acid (albeit at 70 °C), and benzaldehyde (at room temperature, albeit in the presence of alumina). None of the above-mentioned methods strictly adhere to the "cold dilute alkaline KMnO4 solution" mentioned. Yet, I argue that styrene is more than happy to react with KMnO4 if given a little nudge. Meanwhile, phenylacetylene also reacts with KMnO4, yet again requiring a little heat (refluxing DMC). Nonetheless, as @jimchmst pointed out, phenylacetylene couldn't be the right answer because it reacts with silver nitrate.This leads us to conclude that the correct answer is styrene.

References:

  1. https://doi.org/10.1016/S0040-4039(98)01671-2
  2. https://doi.org/10.1039/C5RA01646K
  3. https://doi.org/10.1021/ja01265a511
  4. https://doi.org/10.1055/s-2001-16760
  5. https://doi.org/10.1055/s-1978-24783
  6. https://orgsyn.org/demo.aspx?prep=CV5P0517
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