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Piperine was chemically characterized by reaction with acids. This was done by dropping concentrated $\ce{HNO3},$ $\ce{HCl},$ and $\ce{H2SO4}$ to a sample of piperine. Both $\ce{HNO3}$ and $\ce{HCl}$ produced yellow solutions while sulfuric acid produced a blood-red solution.

What happens to piperine as it reacted with $\ce{H2SO4}$ and why it turned blood-red? Also, what is the difference of that reaction with the reaction of piperine with $\ce{HNO3}$ and $\ce{HCl}?$

Edit: Late addition for viewer's convenience:

The piperine structure is shown below:

Piperine

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  • $\begingroup$ Would help if you gave the structure of Piperine $\endgroup$ – Waylander Sep 24 '19 at 10:49
  • $\begingroup$ Interesting question... The reactions with the acids have likely changed the conjugated system, thus resulting in different wavelengths of light being absorbed by the resultant compounds. Nitration may occur with conc. nitric acid but as to why both nitric acid and hydrochloric acid both give a yellow solution, it is indeed puzzling. $\endgroup$ – Tan Yong Boon Sep 24 '19 at 14:44
  • $\begingroup$ The methylenedioxyphenyl system is very electron rich so protonation seems a possibility $\endgroup$ – Waylander Sep 24 '19 at 16:38
  • $\begingroup$ @Waylander: I have added the structure for your convenience. $\endgroup$ – Mathew Mahindaratne Sep 24 '19 at 16:39
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I think OP is confused with the colorimetric method of detection of piperine, which uses the concentrated sulfuric acid as one of reactant with chromotropic acid, 1,8-dihydroxynaphthalene-3,6-disulfonic acid (Chromotropic Acid Test; Ref.1 & 2). Accordingly, this colorimetric method, developed in 1965, relied on hydrolyzing the methylenedioxy group of piperine by chromotropic acid in the presence of concentrated sulfuric acid to formaldehyde, upon which a definite color was developed with chromotropic acid (Ref.3). Color intensity is proportional to the amount of piperine present. Water inhibits color development, but a small amount of methyl alcohol is necessary.

Note: It is good to know that chromotropic acid test is very sensitive to presence of formaldehyde and detect them in very low concentrations (Ref.4). It is also fact that chromotropic acid is specific to formaldehyde (Ref.2).

References:

  1. Leila Gorgani, Maedeh Mohammadi, Ghasem D. Najafpour, Maryam Nikzad, “Piperine—The Bioactive Compound of Black Pepper: From Isolation to Medicinal Formulations,” Comprehensive Reviews in Food Science & Food Safety 2017, 16(1), 124-140 (https://doi.org/10.1111/1541-4337.12246).
  2. Clark E. Bricker, Hilding R. Johnson, "Spectrophotometric Method for Determining Formaldehyde," Ind. Eng. Chem. Anal. Ed. 1945, 17(6), 400–402 (https://doi.org/10.1021/i560142a021).
  3. Horace D. Graham, “Quantitative Determination of Piperine. I. The Komarowsky Reaction,” J. Food Sci. 1965, 30(4), 644–650 (https://doi.org/10.1111/j.1365-2621.1965.tb01818.x).
  4. Jingping Zhang, David Thickett, Lorna Green, "Two Tests for the Detection of Volatile Organic Acids and Formaldehyde," Journal of the American Institute for Conservation 1994, 33(1), 47–53 (doi:10.2307/3179669).
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