Woodward fieser rule

© Ramaiah University of Applied Sciences
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Faculty of Pharmacy © Ramaiah University of Applied Sciences
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Faculty of Pharmacy
Woodward – Fieser rule for dienes & cyclic dienes
By
Burhanuddin Madriwala
M.Pharm – Sem II
Department of Pharmaceutical Chemistry
M.S Ramaiah University of Applied Sciences

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Contents
• Introduction
• What are Dienes ?
• Woodward – Fieser rules for conjugated dienes
• Base values for parent conjugated dienes & substituents
• Calculation of 𝜆max with examples.
• Summary
• References

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Introduction
• System of rules used for calculating 𝜆max of compounds with the help of parent
values.
• Developed by Robert Burns Woodward in 1945 & modified in 1959 by Louis Fredrich
Fieser. Therefore, called as Woodward – Fieser rule.
• 𝜆max is calculated based on parent moieties present in a compound.
• Rules are used to correlate the wavelength with the molecular structure.

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Dienes
• Structure consists of two carbon – carbon double bonds.
• Dienes can be classified as –
1. based on structure : acyclic & cyclic dienes

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2. Based on conjugation : conjugated & non – conjugated dienes

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Woodward – Fieser rules for conjugated dienes
• Parent system having fixed 𝜆max value at which absorption takes place.
• Different types of conjugated dienes have different 𝜆max value.
1. Homoannular dienes – cyclic dienes having conjugated bonds in same ring.
2. Heteroannular dienes – cyclic dienes having double bonds in different rings.
3. Acyclic conjugated dienes – conjugated bonds present in acyclic structure.
base value (𝜆max ) = 253 nm
• Homoannular diene exist in s-cis or cisoid form requires less energy for transition &
thus have longer wavelength.

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• Acyclic & Heteroannular dienes are mostly in s-trans or transoid form & so highly
stable require more energy for transition. Thus have shorter wavelength as base
value.
the base value increases as the number of double bonds increase in conjugated
form. This is called conjugation effect.
Ex. – ethene has base value = 175
Base value 𝜆max = 214 nm Base value 𝜆max = 217 nm

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Total 𝜆max of a compound is given as,
λmax = Base value + Σ Substituent Contributions + Σ Other Contributions
Other substituents include
1. Endocyclic double bond : double bond present inside the ring.
2. Exocyclic double bond : double bond present outside or as part of ring.
3. Double bond extending conjugation: bond present outside conjugation
4. Functional groups

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• Carbon atoms or substituents directly attached to the conjugated system forming a
ring are called ring residues.
All the substituents discussed above including functional groups gives additional
value called as increment to the base value.

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Base values for parent conjugated dienes & substituents
• Base value for acyclic dienes = 217 nm
• Base value for Homoannular (cisoid) diene = 253 nm
• Base value for Heteroannular (transoid) diene = 214 nm
• Increment for :-
a) Alkyl substituent or Ring residue attached to the parent diene = 5 nm
b) Double bond extending conjugation = 30 nm
c) Exocyclic double bonds = 5 nm
d) Polar substituents :-
I) -OCOCH3 = +0
II) -OR = +6
III) -SR = +30
IV) -Cl, -Br = +5
V) -NR2 = +60

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Calculation of 𝜆max with examples
• Base value=214 nm
• 2 Alkyl groups=2X5=10 nm
• 1 exocyclic bond=1X5=5 nm
• Thus λmax = 229 nm
• Base value=253 (Homoannular)
• 3 ring residues =3X5=15
• 1 exocyclic bond=1X5=5

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• Base value=214 (Heteroannular)
• Base value = 253 (Homoannular)
• 1 double bond exocyclic to two rings=
2X5=10
• 1 double bond extending
conjugation=1X30=30

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Base value = 253 nm Base value = 214 nm
4 ring residues = 4 x 5 = 20 nm 4 ring residues = 4 x 5 = 20 nm
2 exocyclic double bonds = 2 x 5 = 10 nm 1 exocyclic double bond = 5 nm
Thus λmax = 283 nm Thus λmax = 239 nm

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Summary
• Woodward – Fieser rule is used to calculate λmax of compounds theoretically.
• Different structural moieties present in a compounds called as base or parent
moiety.
• Woodward – Fieser rule for conjugated dienes is discussed.
• Conjugated dienes have fixed λmax values based on their structural correlation with
wavelength & are called as base values.
• Different substituents directly attached to conjugated system gives additional value
called increments.

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References
• Pavia, D., 2009. Introduction to spectroscopy. 4th ed. Australia: Brooks/Cole Cengage
Learning, p.394.

Woodward fieser rule

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