Calculating absorption maximum using Woodward-Fieser rules - answers to quiz

Dear Students,

In this posting, I take the opportunity to show you how to calculate absorption maximum of UV-absorbing compounds using the Woodward-Fieser rules. I will use the recent quiz question: Calculate absorption maximum for this molecule:

First, you should identify the parent UV-absorbing conjugate system: is this a diene, an α,β-unsaturated carbonyl, or a benzene/aromatic system? It is an α,β-unsaturated carbonyl system (see below, coloured blue):

So, you should use the rule for this system. This is a six-membered cyclic (ring) α,β-unsaturated ketone, so the base value is 215 nm.

Next, look for extensions to this conjugated system, meaning here the C=C double bonds that extend from this parent system as show below, in green) There are two double bonds, so add two values (30 x 2 nm):

Now, treat this extended conjugated system as a whole chain with the carbonyl being the head of the chain. Going along this chain from alpha to delta, and beyond, look for ring residue/alkyl/methyl:

Then, look for any other types of auxochromes. None here. Next on the list, look for any exocyclic C=C. There are two (coloured red), exocyclic to the ring labelled A:

Lastly, is there any homodiene element (two C=C bonds inside the same ring)? None.  So, total this up and you get your calculated maximum absorption (λ max), 361 nm. 

Best regards, Dr Lee.