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Calculation of λmax of Organic Compounds Using Woodward Fieser Rules
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Objectives:

 

  • To familiarize the Woodward-Fieser Rules.
  • To calculate the maximum wave length of organic compounds.

 

 

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.

 

         

Robert Burns Woodward         Louis Frederick Fieser

 

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 λmax for a particular compound.

 

I) CONJUGATED DIENE CORRELATIONS:

 

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 x 5 = 15 nm
 
                                     Exocyclic double bond   = 1 x 5 = 5 nm
 
    λmax = 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

 

–OH in β position = 30 nm

 

–OH in δ position = 50 nm

 

b) –OAc in α, β, γ, δ positions = 6 nm

 

c) –OMe in α position = 35 nm

 

    –OMe in β position = 30 nm

 

    –OMe in γ position = 17 nm

 

    –OMe in δ position = 31 nm

 

d) –Cl in α position = 15 nm

 

    –Cl in β position = 12 nm

 

e) –Br in α position = 25 nm

 

    –Br in β position = 30 nm

 

f) –NR2 in β position = 95 nm

Eg: 

                                                                                                          Base value = 214 nm

            β- Substituents = 1 x 12 = 12 nm

            δ- Substituents = 1 x 18 = 18 nm
            Double bond extending conjugation =1 x 30 = 30 nm
                                                    Exocyclic double bond = 5 nm
                                                     λmax = 279 nm

 

 III) AROMATIC COMPOUNDS:

1) Base value: for  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
 
3) Alkyl group or ring residue in para position =10 nm
 
4) Polar groups:    a) –OH, –OCH3, –OAlkyl in o, m position = 7 nm
 
b) –OH, –OCH3, –OAlkyl p position = 25 nm
 
c) –O (oxonium) in o position = 11 nm
 
d) –O (oxonium) in m position = 20 nm
 
e) –O (oxonium) in p position = 78 nm
 
f) –Cl in o, m position = 0 nm
 
g) –Cl in p position = 10 nm
 
h) –Br in o, m position = 2 nm
 
i) –Br in p position = 15 nm
 
j) –NH2 in o, m position = 13 nm
 
k) –NH2 in p position = 58 nm
 
l) –NHCOCH3 in o, m position = 20 nm
 
m) –NHCOCH3 in p position = 45 nm
 
n) –NHCH3 in p position = 73 nm
 
o) –N(CH3)2 in o, m position = 20 nm
 
p) –N(CH3)2 in p position = 85 nm

 Eg: 

 

Base value =246 nm

Ring residue in o- position = 1 x 3 = 3 nm
Polar group -OCH3 in p- position  = 25 nm
λmax = 274 nm

 

 

 

 

 

 

 

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