Calculating UV λmax of molecules using the Woodward-Fieser Rules

Dear First Year students,

This is a short tutorial on calculating UV absorption maximum (λmax) using the Woodward-Fieser rules. I hope this would help to clear any doubts you may have, so that you will understand the topic better and do well during the coming exam.

Let's look at the questions in the previous quiz.. They are good examples for us to understand how to arrive at our λmax.

The first question:

Not so difficult,isn't it. The second question:

To sum it up, follow these general steps:

1.     First, determine the parent or base structure be it a diene, α,β -unsaturated ketone, or an aromatic ring. This would tell you which rule to use

2.     Look for any double bonds extending this diene or α,β -unsaturated ketone.

3.     Treating this conjugated system as a whole, go along the chain looking for alkyls or auxochromes directly attached to it, anything far from the chain not counted.

4.     Lastly, look for other elements such as exocyclic double bonds and homodiene element, if there are rings involved.

I hope that this tutorial will help you. Please also study the tutorials for other examples. 

I wish you all the best for exams and happy studying! 

Best wishes, Dr Lee