# What is the reaction between piperine and concentrated sulfuric acid?

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:

• Would help if you gave the structure of Piperine Sep 24 '19 at 10:49
• 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. Sep 24 '19 at 14:44
• The methylenedioxyphenyl system is very electron rich so protonation seems a possibility Sep 24 '19 at 16:38
• @Waylander: I have added the structure for your convenience. Sep 24 '19 at 16:39

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. The abstract of Ref.3 tells them all:

A highly stable reagent consisting of para-dimethylaminobenzaldehyde (I) or para-hydroxybenzaldehyde (II) or other cylic aldehydes, plus thioureau in concentrated sulfuric acid, has been developed for use in determining piperine. The slightly yellow reagent, if stored under refrigeration in a brown bottle, maintains a constant chromogenic capacity for 4–6 weeks. Piperine when heated with this reagent for 15 min at 100°C develops a red color (I max. 490 mμ) or purple color (II max. 570 mμ) whose intensity is proportional to the amount of piperine present. Water inhibits color development, but a small amount of methyl alcohol is necessary. Ethyl alcohol cannot be used, since it develops color with the reagent. Critical factors for reproducible, quantitative assay are the concentration of acid and thiourea in the reagent and the time and temperature of heating the reaction mixture. For I, the most sensitive of the aldehydes used, good precision, with a standard deviation of it ± 3470, was attained. Between the levels of 0.01 and 0.06μM of piperine per ml of reagent, there is a linear relation which may be described by the least-squares equation.

The theory behind the test is concentrated $$\ce{H2SO4}$$ would knocked off formaldehyde from its cyclic acetal form in piperine and formaldehyde then react with formaldehyde specific reagent to form the red color. What happen with the other two acids, $$\ce{HCl}$$ and $$\ce{HNO3}$$, is they can also remove the cyclic acetal protective group from piperine forming free catechol moiety. In the absence of formaldehyde specific reagent in the solution, the highly conjugated catechol moiety predominate to give the supposed color.

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).