Part two.

Guide for Estimating
the Maximum Absorption Wavelength in
Conjugated Polyene Systems

D.R © UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO 2025

Los derechos patrimoniales pertenecen a la Universidad Nacional Autónoma de México. Ciudad Universitaria, Coyoacán, C.P. 04510, Ciudad de México, México.

Entidad editora: Facultad de Estudios Superiores Cuautitlán.
Este contenido digital se elaboró con el apoyo de la DGAPA- PAPIME PE211025 Integración, validación e implementación de un manual de prácticas digital basado en la técnica de UV-Visible, para el apoyo de la práctica docente en la FES Cuautitlán.

Autores: Benjamín Velasco Bejarano, Anuar Gómez Tagle González, Víctor Manuel Díaz Sánchez, Lourdes Aguilera Arreola y José Félix Olivares Landín.

Excepto donde se indique lo contrario, este contenido digital está bajo una licencia Creative Commons Atribución-No comercial Compartir Igual (CC-BY-NC-SA) 4.0 Internacional https://creativecommons.org/licenses/by-nc-sa/4. 0/deed.es. Fecha de asignación de la licencia 2025-10. Para un uso diferente escribir a: unidadjuridicafesc@cuautitlan.unam.mx
Con la licencia usted es libre de compartir: copiar y redistribuir el material en cualquier medio o formato Adaptar: remezclar, transformar y construir a partir del material. La licenciante no puede revocar estas libertades en tanto usted siga los términos de la licencia.

Forma sugerida de citar: Velasco-Bejarano B., Gómez-Tagle A., Diaz Sánchez V. M., Aguilera Arreola L. y Olivares Landín J. F., Facultad de Estudios Superiores Cuautitlán (2025). Guía para la estimación de la longitud de onda máxima en polienos. [Recuperado: fecha de consulta, URL del recurso en el RU-FES Cuautitlán].


Part two.

Guide for Estimating the Maximum Absorption Wavelength in Conjugated Polyene Systems

Once the fundamental concepts have been established, this section focuses directly on the Fieser–Kuhn rule. The central equation and the definition of its structural variables are presented in order to explain the methodology used to estimate the maximum absorption wavelength, λmax of conjugated polyene systems.


The section concludes with worked examples that illustrate the application of the method and demonstrate its effectiveness for rapidly evaluating structural hypotheses by comparing calculated values (λmax, calc) with experimentally observed data (λmax, exp).


The section concludes with worked examples that illustrate the application of the method and demonstrate its effectiveness for rapidly evaluating structural hypotheses by comparing calculated values (λmax, calc) with experimentally observed data (λmax, exp).

Objective

Applying the Fieser–Kuhn rule to calculate the maximum absorption wavelength, λmax (nm) of conjugated polyene systems, showing the relationship between molecular structure and UV–Vis absorption.

5.

Fieser-Kuhn Rule: Definition and Scope

Origen

Overview of the Fieser-Kuhn rule

The Fieser-Kuhn rule is a set of empirical and semi-empirical correlations that were originally proposed by Louis F. Fieser and Mary Fieser and later refined by Werner Kuhn.

Scope

Application

The Fieser–Kuhn rule enables the prediction of the principal characteristics of the most intense UV–Vis absorption band, associated with the π → π* transition, in open-chain conjugated polyene systems, including many naturally occurring carotenoids.

Utility

Predictive Value

Its value lies on its ability to hypothesize the potential molecular structure of a polyene, based on the comparison between the calculated and the observed maximum absorption maxima.

6.

La Ecuación de Fieser-Kuhn y sus parámetros estructurales

Fórmula General

La regla correlaciona consideraciones estructurales en la siguiente ecuación:

\(\lambda_{max} =\)
114
\(+\)
\(5M\)
\(+\)
\(n(48.0 - 1.7n)\)
\(-\)
\(16.5R_{endo}\)
\(-\)
\(10R_{exo}\)
*Touch or hover the cursor for illuminate the description of the colorized terms.

Variables

M Número de sustituyentes alquilo o derivados alquilo unidos directamente al sistema conjugado.
n Número de dobles enlaces conjugados presentes en la estructura.
Rendo Número de anillos con dobles enlaces endocíclicos en el sistema conjugado.
Rexo Número de anillos con dobles enlaces exocíclicos en el sistema conjugado.
7.

Scope of the method

Plausibility Criterion

It has been observed that if there is a difference between \(\lambda_{max}\), calc and \(\lambda_{max}\), exp within approximately ±5 nm, then the proposed structure of the conjugated polyene system is likely the analyzed polyene

Limitations

The predictive accuracy of the Fieser–Kuhn rule is highest for conjugated polyene systems containing more than four conjugated double bonds. Its reliability decreases for very long polyene chains or for molecules bearing strongly electron-donating or electron-withdrawing substituents that significantly perturb the electronic structure of the chromophore.

The following three worked examples are solved according to the methodology described in Introduction to the Application of the Fieser–Kuhn Rule. There are four additional problems for practicing. In each case, the corresponding answers are provided.

This written material promotes active learning and critical thinking. It also helps to understand the use of the Fieser-Kuhn rule in the correlation of UV-Vis and chemical structure.

8.

Examples

9. Exercises

Fieser-Kuhn Simulator

5
Calculated Wavelength
0.0 nm
300nmVisible800nm

Exercises designed to practice and master the method.

Para reafirmar el conocimiento adquirido, se propone la resolución de los siguientes ejemplos de polienos con base en la secuencia didáctica de su aplicación:

A. Determine the λmax of Violaxanthin using the Fieser-Kuhn rule.

Challenge 1

Data: \(n=9\), \(M=6\), \(R_{endo}=2\). Calculate \(\lambda_{max}\).

B. Determine the λmax of Phytofluene using the Fieser-Kuhn rule.

Challenge 2

Data: \(n=5\), \(M=5\) (Acrylic). Calculate \(\lambda_{max}\).

C. Determine the λmax of Astaxanthin using the Fieser-Kuhn rule.

Challenge 3

Data: \(n=11\), \(M=10\), \(R_{endo}=2\). Calculate \(\lambda_{max}\).


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