Woodward - Fieser Rule for Dienes

These are the rules given by Robert B Woodward and Louis Fieser to predict the wavelength of absorption maxima (λ_max) of the compounds theoretically.

For the calculation of λ_max , following general terms are important to learn.

Must read: UV spectroscopy principle and instrumentation 

Cisoid diene (s-cis) 

In a conjugated diene, viewing from the single bond, if the connected double bonds are in same side of single bond, then it is termed as cisoid diene or homoannular diene ( in case of cyclic compound).

Transoid diene(s-trans)

In a conjugated diene, observing from single bond, if the connected double bonds are on opposite side, the diene is known as transoid diene or heteroannular diene (if cyclic diene).

Conjugation

The word conjugation simply refers to the alternate double-single-double bond system in a compound.

Extended double bond conjugation

Generally, the conjugation is considered to be limited up to double-single-double bond level. If the conjugation further exceeds, then it is called as extended double bond conjugation.

Ring residues/ alkyl residues

The atoms or bonds connected to the carbons of conjugated system are counted as ring residues. It excludes the polar group(Cl, NH2 etc) residues connected to conjugated system.

In the example diagram, compound is given first, followed by the conjugation system which is indicated by bold black bonds. Then, ring residues are indicated by blue colour.

Exocyclic double bond

As the name indicates it is the double bond  present outside the ring . It should be connected with one corner (atom) of the ring. 

Note that, a double bond can be exocyclic double bond for one ring and might not for other. As in example (b)in above diagram, both A and B rings has one exocyclic double bond each making a total of two. So ensure during calculating these bonds by considering each ring. 

Also read : Woodward's rules for enones and alpha beta unsaturated compounds

Rules for the calculation of λ_max

•     Firstly, ascertain whether the parent compound compound is cisoid or transoid. The base value of cisoid is 254nm while that of transoid is 214 nm.  The base value for acyclic conjugated diene is 217nm.

           

• Check for the presence of alkyl residues, halogen substitution, exocyclic double bonds, amine and other substitutions mentioned in the table below. Simply, add the values of substituents to the base value of parent compound. The total will be the λ_max  of the compound.

Empirical values to calculate λ_max

	Homoannular (Cisoid)	Heteroannular(Transoid)
·         Parent Compound	254                  214
·         Ring residue/ Alkyl substituent	5 nm
·         Exocyclic double bonds	5 nm
·         Double bond extended conjugation	30 nm
·         Polar groups:
o   -Cl, Br	5 nm
o   -OR	6 nm
o   –OCOCH3	0
o   –NR2	60 nm