4. INTRODUCTION
In 1945 Robert Burns Woodward gave certain rules for correlating wavelength of maximum
absorption (λmax) with molecular structure.
In 1959 Louis Fredrick Fieser modified these rules with more experimental data and the
modified rule is known as Woodward_Fieser Rules.
Woodward rules work well for dienes and polyenes with upto 4-double bonds or less.
These sets of rules to calculate the wavelength of maximum absorption (λmax) of a compound in
the ultraviolet-visible spectrum, based empirically have been called the Woodward_Fieser Rules
or Woodward’s Rules.
This may be differs from observed value by 4-5nm.
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6. A) WOODWARD_FIESER RULE FOR CONJUGATED
DIENES AND TRIENES
Conjugated diene: Organic compound containing two or more double bonds each separated from
other by a single bond.
Base/Parent value: It means that each type of diene or triene system is having a certain fixed
values 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 base value to obtain λmax for a
particular compound.
Longer the conjugated system greater the wavelength of absorption maximum.
According to Woodward’s rules the λmax of the molecule can be calculated using a formula:
λmax = Base value + Σ Substituent Contributions + Σ Other Contributions
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7. Table A: Parent values and increments for different substituents /groups for
calculating maximum wavelength
Conjugated dienes correlations Standard (nm)
(A) Parent value
Homo-annular conjugated dienes 253
Hetero-annular conjugated dienes 214
Butadiene system or a cyclic conjugated dienes 217
Acyclic trienes 245
7
8. (B) Increment for each substituent Standard (nm)
Alkyl substituent or ring residue 5
Exocyclic double bond 5
Double bond extending conjugation 30
(C) Auxochrome Standard (nm)
–OCOCH3 0
–Cl,-Br 5
–OR 6
–SR 30
–NR2 60
8
9. Example 1:
Solution:
Base value for Heteroannular conjugated diene = 214 nm
3 Ring residues (3×5) = 15 nm
1 Exocyclic double bond = 5 nm
λmax = 234 nm
9
10. Example 2:
Solution:
Base value for Homoannular conjugated Diene = 253 nm
3 Ring residues (3×5) = 15 nm
1 Exocylic double bond = 5 nm
λmax = 273 nm
10
11. Example 3:
Solution:
Base value for Heteroannular conjugated diene = 214 nm
3 Ring residues (3×5) = 15 nm
1 Exocyclic double bond = 5 nm
λmax = 234 nm
11
12. Example 4:
Solution:
Base value for Heteroannular conjugated diene = 214 nm
3 Ring residues (3×5) = 15 nm
1 Exocyclic double bond = 5 nm
-OR (Alkoxy group) = 6 nm
λmax = 240 nm
12
13. Example 5:
Solution:
Base value for Homoannular conjugated diene = 253 nm
1 Alkyl substituent = 5 nm
3 Ring residues (3×5) = 15 nm
1 Exocyclic double bond = 5 nm
λmax = 278 nm
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14. Example 6:
Solution:
Base value for Homoannular conjugated diene = 253 nm
5 Ring residues (5×5) = 25 nm
2 Double bond extending conjugation (2×30) = 60 nm
3 Exocyclic double bond (3×5) = 15 nm
-OCOCH3 = 0 nm
λmax = 353 nm
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15. B) WOODWARD_FIESER RULE FOR CYCLIC
DIENES (or AROMATIC COMPOUNDS)
Like Woodward fieser rules, Scott devised a set of rules for calculating the absorption
maximum for the derivatives of Acyl benzenes. These rules help in estimating the position of
absorption maximum (λmax) in ethanol in a number of mono substituted aromatic ketones,
aldehydes, acids, and esters.
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16. Basic values Standard (nm)
Ar COR 246
Ar CHO 250
Ar CO2H or Ar CO2R 230
Increments
Alkyl groups or ring residues at ortho
or meta positions
3
Alkyl groups or ring residues at Para
positions
10
Table B: Parent values and increments for different substituents/groups
for calculating maximum wavelength
16
18. Example 1:
Solution:
Base value for Aromatic ketone = 246 nm
Hydroxyl group at meta position = 7 nm
Hydroxyl group at para position = 25 nm
λmax = 278 nm
18
19. Example 2:
Solution:
Base value for Ar-COOH = 230 nm
Hydroxyl groups at meta positions (2×7) = 14 nm
Hydroxyl group at para position = 25 nm
λmax = 269 nm
19
21. C) WOODWARD-FIESER RULE FOR α,β -
UNSATURATED CARBONYL COMPOUNDS
(ENONES)
Woodward and Fieser framed certain empirical rules for estimating the
absorption maximum for α,β-unsaturated carbonyl compounds. The rules
later modified by Scott.
21
22. Table C: Parent values and
increments for different
substituents/groups for
calculating maximum
wavelength
22
25. Example 2:
Solution:
Base value for α, β-unsaturated six membered ring ketone = 214 nm
β- ring residue (1×12) = 12 nm
δ- ring residue (1×18) = 18 nm
1 Double bond extending Conjugation (1×30) = 30 nm
1 Exocyclic double bond = 5 nm
λmax = 279 nm
25
26. Example 3:
Solution:
Base value for α, β-unsaturated six membered ring ketone = 215 nm
Substitution of alkyl groups at α-position = 10 nm
Ring residue at β-position = 12 nm
λmax = 237 nm
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31. FIESER-KUHN RULE
If the number of conjugated double bonds is more than 4, the Woodward and Fieser rules may
not be applicable and hence Fieser with Kuhn has derived an equation for predicting the λmax.
M = No. of alkyl substitutents
N = No. of conjugated double bonds
Rendo = No. of rings with endocyclic double bonds
Rexo = No. of rings with exocyclic double bonds
λmax = 114 + 5(M) + n(48-1.7n) - 16.5 Rendo - 10 Rexo
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33. POINTS TO REMEMBER
In case for which both types of diene systems are present then the one with the
longer wavelength is designated as a parent system.
Whenever there is an increasing conjugation leads to increase in wavelength and
requires less amount of energy.
Up to four conjugations, Woodward_Fieser rule is applied.
> Four conjugations, Fieser_Kuhn rule is applied.
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34. REFRENCES
1) Donald L. Pavia, Gary M. Lampman, George S. Kriz, “ Spectroscopy”, published by Cengage
Learning, 2010. Pp. (379-387)
2) SHARMA Y.R., “Elementary Organic Spectroscopy”, published by S. Chand & Company
Ltd. 1st edition, 2006. Pp. (31-49)
3) CHATWAL G.R., ANAND S.K., “Instrumental Methods Of Chemical Analysis”, published
by Himalaya Publishing House, 5th edition, 2008. Pp. ( 2.162- 2.167)
4) KALSI P.S., “A Text Book Of Organic Spectroscopy”, published by NEW AGE
INTERNATIONAL PUBLISHERS, 6th edition. 2007. Pp. (40)
5) RAJASHEKARAN, “A Text Book Of UV-Visible and Infrared Spectroscopy”, Pp. (88-100)
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