INTRODUCTION:

In 1945 Robert Burns Woodward gave certain rules for correlating λmax with molecular structure. In 1959 Louis Frederick Fieser modified these rules with more experimental data, and the modified rule is known as Woodward-Fieser Rules. It is used to calculate the position and λmax for a given structure by relating the position and degree of substitution of chromophore.

WOODWARD–FIESER RULES:

Each type of diene or triene system is having a certain fixed value at which absorption takes place; this constitutes the Base value or Parent value. The contribution made by various alkyl substituents or ring residue, double bond extending conjugation and polar groups such as –Cl, –Br etc. are added to the basic value to obtain λ_max for a particular compound.

a) Homoannular Diene:	Cyclic diene having conjugated double bonds in same ring.
b) Heteroannular Diene:	Cyclic diene having conjugated double bonds in different rings.
c) Endocyclic double bond:	Double bond present in a ring.
d) Exocyclic double bond:	Double bond in which one of the doubly bonded atoms is a part of a ring system. Here Ring A has one exocyclic and endocyclic double bond. Ring B has only one endocyclic double bond.

PARENT VALUES AND INCREMENTS FOR DIFFERENT SUBSTITUENTS/GROUPS:

I) CONJUGATED DIENE CORRELATIONS:

i) Base value for homoannular diene = 253 nm

ii) Base value for heteroannular diene = 214 nm

iii) Alkyl substituent or Ring residue attached to the parent diene = 5 nm

iv) Double bond extending conjugation = 30 nm

v) Exocyclic double bonds = 5 nm

vi) Polar groups: a) -OAc = 0 nm

b) -OAlkyl = 6 nm

c) -Cl, -Br = 5 nm

Eg:

Base value = 214 nm

Ring residue = 3 × 5 = 15 nm

Exocyclic double bond = 1 × 5 = 5 nm

λ_max = 214 + 15 + 5 = 234 nm

II) α, β UNSATURATED CARBONYL COMPOUNDS OR KETONES:

1. Base value:
   • a) Acyclic α, β unsaturated ketones = 214 nm
   • b) 6 membered cyclic α, β unsaturated ketones = 215 nm
   • c) 5 membered cyclic α, β unsaturated ketones = 202 nm
   • d) α, β unsaturated aldehydes = 210 nm
   • e) α, β unsaturated carboxylic acids & esters = 195 nm
2. Alkyl substituent or Ring residue in α position = 10 nm
3. Alkyl substituent or Ring residue in β position = 12 nm
4. Alkyl substituent or Ring residue in γ and higher positions = 18 nm
5. Double bond extending conjugation = 30 nm
6. Exocyclic double bonds = 5 nm
7. Homodiene compound = 39 nm
8. Polar groups:
   • a) –OH
     • in α position = 35 nm
     • in β position = 30 nm
     • in δ position = 50 nm
   • b) –OAc in α, β, γ, δ positions = 6 nm
   • c) –OMe
     • in α position = 35 nm
     • in β position = 30 nm
     • in γ position = 17 nm
     • in δ position = 31 nm
   • d) –Cl
     • in α position = 15 nm
     • in β position = 12 nm
   • e) –Br
     • in α position = 25 nm
     • in β position = 30 nm
   • f) –NR₂ in β position = 95 nm

Eg:

Base value = 214 nm

β-Substituents = 1 × 12 = 12 nm

δ-Substituents = 1 × 18 = 18 nm

Double bond extending conjugation = 1 × 30 = 30 nm

Exocyclic double bond = 5 nm

λ_max = 214 + 12 + 18 + 30 + 5 = 279 nm

III) AROMATIC COMPOUNDS:

1. Base value:
   • a) ArCOR = 246 nm
   • b) ArCHO = 250 nm
   • c) ArCO₂H = 230 nm
   • d) ArCO₂R = 230 nm
2. Alkyl group or ring residue
   • in ortho and meta position = 3 nm
   • in para position = 10 nm
3. Polar groups:
   • a) –OH, –OCH₃, –OAlkyl
     • in ortho, meta position = 7 nm
     • in para position = 25 nm
   • b) –O (oxonium)
     • in ortho position = 11 nm
     • in meta position = 20 nm
     • in para position = 78 nm
   • c) –Cl
     • in ortho, meta position = 0 nm
     • in para position = 10 nm
   • d) –Br
     • in ortho, meta position = 2 nm
     • in para position = 15 nm
   • e) –NH₂
     • in ortho, meta position = 13 nm
     • in para position = 58 nm
   • f) –NHCOCH₃
     • in ortho, meta position = 20 nm
     • in para position = 45 nm
   • g) –NHCH₃ in para position = 73 nm
   • h) –N(CH₃)₂
     • in ortho, meta position = 20 nm
     • in para position = 85 nm

Eg:

Base value = 246 nm

Ring residue in o-position = 1 × 3 = 3 nm

Polar group –OCH₃ in p-position = 25 nm

λ_max = 246 + 3 + 25 = 274 nm