# Ultraviolet-Visible (UV-Vis) Spectroscopy – Sample Problems Using Woodward-Fieser Rules

## Introduction

In this page we discuss a few examples of how we can utilize the Woodward-Fieser rules to determine the wavelength of maximum absorption for some molecules. We highly recommend that you read up the first two sections on the Woodward rules to calculate the λmax for conjugated dienes and the Woodward rules to calculate the λmax for unsaturated carbonyl compounds, before you read this page.

Note: Numerical values for Woodward-Fieser rules differ slightly from one textbook to another. We have tried to compile an extensive list of numerical values from online resources, textbooks and journal articles based on the popularity of the number. It is recommended that you learn on how to apply the values for the contributors and then follow the values given in a text book recommended by your teacher, or use our values. We believe that learning how to apply the rules is more essential than actually getting the exact answer. Other’s opinions may vary.

In these sample problems you will be shown the structure, then the structure is highlighted to show you key features which would affect the λmax of the molecule. Then the table will show you the solutions on how to solve to get the wavelength of maximum absorption, with a final calculated λmax using the Woodward-Fieser rules. In some cases if we have an observed λmax for comparison, it may be given as well.

Note- If you have your own problems please write the IUPAC name in the comments section and I will attempt to solve it and add it to this list of examples.

## Example/Sample Problem 1

 Name of Compound 2,4-dimethylpenta-1,3-diene Woodward Component Contribution Core- Transoid/Heteroannular Diene + 215 nm Substituents- 3 alkyl groups 3 x 5 = + 15 nm Other Effects 0 Calculated λmax 230 nm Observed λmax 234 nm

## Example/Sample Problem 2

 Name of Compound 1-methylcyclohexa-1,3-diene Woodward Component Contribution Core- Cisoid/Homoannular Diene + 253 nm Substituents- 3 alkyl groups 3 x 5 = + 15 nm Other Effects 0 Calculated λmax 268 nm Observed λmax N/A

## Example/Sample Problem 3

 Name of Compound 3-methoxy-10-methyl-2,7,8,9,10,11,12,13,14,15,16,17-dodecahydro-1H-cyclopenta[a]phenanthrene Woodward Component Contribution Core- Transoid/Heteroannular Diene + 215 nm Substituents- 3 alkyl groups 1 alkoxy group 3 x 5 = + 15 nm + 6 nm Other Effects- Exocyclic Double Bond + 5 nm Calculated λmax 241 nm Observed λmax N/A

## Example/Sample Problem 4

 Name of Compound 10,13-dimethyl-2,3,9,10,11,12,13,15,16,17-decahydro-1H-cyclopenta[a]phenanthrene Woodward Component Contribution Core- Transoid/Heteroannular + 215 nm Substituents- 5 alkyl groups 1 Double bond extending conjugation 5 x 5 = + 25 nm + 30 nm Other Effects- 3 Exocyclic Double Bond + 15 nm Calculated λmax 285 nm Observed λmax 283 nm

## Example/Sample Problem 5

Note- In this example the molecule contains both, a homoannular diene system and a heteroannular diene system. In such a molecule the core chromophore is considered to be the homoannular system and accordingly the calculations are performed.

Homoannular system

 Component Contribution Core- Homoannular/Cisoid diene + 253 nm Substituents– 5 alkyl substituents Double bond extending conjugation 5 x 5 = + 25 nm + 30 nm Other Effects- 3 Exocyclic double bonds 3 x 5 = + 15 nm Calculated λmax 323 nm Observed λmax n/a

## Example/Sample Problem 6

 Name of Compound 3,4-dimethylpent-3-en-2-one Component Contribution Core- α,β-unsaturated ketone + 215 nm Substituents at α-position- 1 alkyl group + 10 nm Substituents at β-position- 2 alkyl groups 2 x 12 = 24 nm Other Effects 0 Calculated λmax 249 nm Observed λmax 249 nm

## Example/Sample Problem 7

 Name of Compound 1-methyl-4,5,6,7,8,8a-hexahydroazulen-2(1H)-one Component Contribution Core- cyclopentenone + 202 nm Substituents at α-position 0 Substituents at β-position- 2 alkyl groups 2 x 12= + 24 nm Other Effects- 1 Exocyclic Double Bond + 5 nm Calculated λmax 231 nm Observed λmax 226 nm

## Example/Sample Problem 8

 Name of Compound 4,4a,5,6,7,8-hexahydrophenanthren-2(3H)-one Component Contribution Core- cyclohexenone + 215 nm Substituents at α-position: 0 Substituents at β-position: 1 alkyl group + 12 nm Substituents at γ-position: 0 Substituents at δ-position: 0 Substituents at ε-position: 1 alkyl group + 18 nm Substituents at ζ-position: 2 alkyl group 2 x 18 = + 36 nm Other Effects: 2 Double bonds extending conjugation 2 x 30 = + 60 nm Homoannular Diene system in ring B + 35 nm 1 Exocyclic double bond + 5 nm Calculated λmax 381 nm Observed λmax 388 nm

Note- If you have your own problems please write the IUPAC name in the comments section and I will attempt to solve it and add it to this list of examples.

## Example/Sample Problems For β-Carotene and all-trans-lycophene

β-Carotene and lycophene are polyenes with more than 4 double bonds, and hence their calculations cannot be done using the Woodward rules. In order to calculate their λmax you must use the Fieser-Kuhn rules. Check out this post on the Fieser-Kuhn Rules to Calculate Wavelength of Maximum Absorption (Lambda-max) of Polyenes (with Sample Problems) for the solution of these problems.

## Books on Analytical Chemistry and Spectroscopy

Check out these good books for analytical chemistry and spectroscopy