How would you use Fieser-Kuhn rules to calculate the wavelength of maximum absorption for tartrazine?


Fieser-Kuhn rules states that: $\lambda_{max}=114+5M+n(48-1.7n)-16.5\ R_{endo} - 10\ R_{exo}$

  1. What would $M$ be in this example? There aren't any alkyl substituents, but what I'm not sure about ring residues. Would this refer to the $\ce{SO3Na}$ and $\ce{COONa}$?

  2. Would $n$, the number of conjugated double bonds be equal to 12?


  1. Does Pyrazole count as a ring for $R_{endo}$? So $R_{endo}=3$?

Or would Woodward-Fieser rules be more suitable to use in this scenario?

  • $\begingroup$ Have a look on mhchem for input of chemical formulae in the body of questions, answers, comments. Said page includes a test drive to familiarize yourself with this tool, too. $\endgroup$
    – Buttonwood
    Mar 4, 2023 at 11:49
  • $\begingroup$ Terminal -SO3Na groups have each two double bonds $\endgroup$
    – Maurice
    Mar 4, 2023 at 12:56

1 Answer 1


Fieser-Kuhn rules needs to be used to calculate the wavelength in this case, since there are more than 4 conjugated double bonds. Woodward-Fieser rules are only applicable for conjugated dienes and polyenes with upto 4-double bonds or less. [1]

M = 6 (SOONa, SOONa, COONa, 2 N atoms connected by triple bond,N connected to the Benzene sulfinate group)

n=12 as illustrated in the question

π‘…π‘’π‘›π‘‘π‘œ = 3


This calculation matches the experimental data as seen in [2][3][4] which list the πœ†π‘šπ‘Žπ‘₯ value of tartrazine as 425nm, 427nm, 428nm respectively.


[1] Mehta, A. (2015) Ultraviolet-visible (UV-vis) spectroscopy – Fieser-Kuhn rules to calculate wavelength of maximum absorption (lambda-max) of polyenes (with sample problems): Analytical Chemistry, PharmaXChange.info. Available at: https://pharmaxchange.info/2013/05/ultraviolet-visible-uv-vis-spectroscopy-%E2%80%93-fieser-kuhn-rules-to-calculate-wavelength-of-maximum-absorption-lambda-max-of-polyenes-with-sample-problems/ (Accessed: March 4, 2023).

[2]Gobara, Mohamed & Elsayed, Mohamed. (2016). Preparation and Characterization of PANI/TiO2 Composite for Photocatalytic Degradation of Tartrazine Dye. The International Conference on Chemical and Environmental Engineering. 8. 370-389. 10.21608/iccee.2016.35131.

[3] Ayman A. Ali et al. (2020). Spectrophotometric determination of Tartrazine in soft drink powder and pharmaceutical dosage form. Journal of Basic and Environmental Sciences, 7, https://jbesci.org/published/7.2.7.pdf

[4] Gobara, Mohamed & Baraka, Ahmad. (2014). Tartrazine Solution as Dosimeter for Gamma Radiation Measurement. International Letters of Chemistry, Physics and Astronomy. 33. 106-117. 10.18052/www.scipress.com/ILCPA.33.106.

  • $\begingroup$ References 2 and 4 are identical $\endgroup$
    – Maurice
    Mar 4, 2023 at 12:54
  • $\begingroup$ @Maurice I have now corrected the references. Both the papers 2 and 4 were from the same set of authors so I had pasted same link by mistake. $\endgroup$ Mar 4, 2023 at 13:07

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