Woodward Fieser Rule for Calculating λmax in Dienes
Woodward Fieser Rule for Calculating λmax in Dienes
Woodward in 1941 formulated emperical rules for calculating the wavelength of absorption maximum (λmax) in dienes. These rules were later modified by Fieser in 1948. Acoording to these rules each types of diene has a fixed λmax value and the exact value depends on-1. the number of alkyl substituents or ring residues or the double bond.
2. the number of double bonds which extend conjugation and
3. the presence of polar groups such as- X, -OR, -SR etc.
Terms Used in the Woodward-Fieser Rule
Cyclic dienes
Cyclic dienes can be classified into two classes depending upon whether the double bonds are in the same ring or in different rings.Homoannular Conjugated Diene
If the conjugated double bonds are present in the same ring of cyclic diene, the diene is known as a homoannular conjugated diene. These are also known as Cisoid and according to the Woodward-Fieser rule, their base value is taken as 253 nm.Heteroannular Conjugated Diene
If the conjugated double bonds are present in the two different rings, the diene is known as a heteroannular conjugated diene. These are also known as Transoid and their base value is taken as 214 nm.Endocyclic Double Bond
If both carbon atoms connected by a double bond are members of the same ring, then the double bond is said to be an endocyclic double bond. In other words, it is that double bond, which is present within a cyclic ring.Exocyclic Double Bond
If both carbon atoms connected by a double bond are not the members of the same ring, it means only one carbon is the member of the ring, then the double bond is said to be an exocyclic bond. In other words, it is that double bond, which is present just outside of a cyclic ring.In example 2, the double bond present within ring A is exocyclic to ring B as it is attached to an atom which is shared between ring A and ring B, while the double bond present in ring B is not connected to any atom of ring A and is within just one ring, hence making it endocyclic.
In example 3, both double bonds are present within ring B with connections to shared carbon atoms with ring A, making both the double bonds exocyclic.
In example 4, there is a single double bond which is exocyclic at two points to two different rings. In such a case, the influence would be 2 times + 5 nm (i.e + 10 nm).
Double bonds which are common to two rings are endocyclic. In example 5, a double bond which although has carbon atoms shared between two rings A and B, it is considered endocyclic as at any given time the double bond will only belong to one ring.
Condensed System
The λmax for peri-condensed system appear at longer wavelength as compared to cata-condensed system.
Ring Residues
Ring residues are substituents that are attached to the parent chain of a conjugated diene or the extended chain of a conjugated diene. The parent chain only extends when there is a suitably located double bond participating in the conjugation with the parent chain.Rules for Calculation λmax in Dienes or Polyenes
Solvent: EthanolTransition: π → π*
1. Base Value for Butadiene system or acyclic conjugated diene = 217 nm
2. Base Value for acyclic triene = 245 nm
3. Base value for homoannular diene = 253 nm
4. Base value for heteroannular diene = 214 nm
5. Alkyl substituent or Ring residue attached to the parent diene = 5 nm
6. Double bond extending conjugation = 30 nm
7. Exocyclic double bonds = 5 nm
8. Polar groups:
a. -OR = 6 nm
b. -SR = 30 nm
c. -Cl, -Br = 5 nm
Note: In case of cycic diene or diene contained in a open chain, 17 nm are added in the basic value for Cl as well as Br atom.
d. -NR2 = 60 nm
e. -OCOCH3 = 0 nm
With the increase in the number of double bonds in conjugation, the values of absorption maximum as well as intensity increases. The change from trans to cis configuration at one or more double bonds lower the wavelength as well as the intensity of absorption.
If a conjugated polyene contains more than four double bonds, then Fieser-Kuhn rules are used.
Examples-
2,4-Hexadiene
CH3-CH=CH-CH=CH-CH3
In hexadiene, two alkyl substituents are present.
Base Value: 217 nm
Two alkyl Substituents: 2 x 5 = 10 nm
Calculated Value: 227 nm
λmax Observed Value: 227 nm
In this butadiene system, two alkyl substituents and two ring residues are present.
Base Value: 217 nm
Two alkyl Substituents: 2 x 5 = 10 nm
Two ring residues: 2 x 5 = 10 nm
One exocyclic double bond: 5 nm
Calculated Value: 242 nm
λmax Observed Value: 242 nm
In this heteroannular diene, four ring residues are present.
Base Value: 215 nm
Four ring residues: 4 x 5 = 20 nm
Calculated Value: 235 nm
λmax Observed Value: 234 nm
In this homoannular diene, three ring residues and one exocyxlic double bond are present.
Base Value: 253 nm
Three ring residues: 3 x 5 = 15 nm
One-exocyclic double bond: 5 nm
Calculated Value: 273 nm
λmax Observed Value: 274 nm
In this heteroannular diene, three ring residues and one exocyxlic double bond are present.
Base Value: 215 nm
Three ring residues: 3 x 5 = 15 nm
One-exocyclic double bond: 5 nm
Calculated Value: 235 nm
λmax Observed Value: 235 nm
In this homodiene, five ring residues, one exocyxlic double bond and also two double bonds which extend conjunction are present.
Base Value: 253 nm
Five ring residues: 5 x 5 = 25 nm
One-exocyclic double bond: 5 nm
Two double bonds extending conjugation: 2 x 30 = 60 nm
Calculated Value: 243 nm
λmax Observed Value: 243 nm
In this homodiene, four ring residues, two exocyxlic double bonds and also one double bond which extend conjunction are present.
Base Value: 253 nm
Four ring residues: 4 x 5 = 20 nm
Two-exocyclic double bond: 2 x 5 = 10 nm
One double bonds extending conjugation: 30 nm
Calculated Value: 313 nm
λmax Observed Value: 312 nm
In 2,3 dimethylene bicyclo [2,2,1] heptane, two ring residues, two exocyxlic double bonds and a bicyclic system are present.
Base Value: 217 nm
Two ring residues: 2 x 5 = 10 nm
Two-exocyclic double bond: 2 x 5 = 10 nm
One bicyclic system: 15 nm
Calculated Value: 252 nm
λmax Observed Value: 254 nm
