UV-VIS spectroscopy and Wood word rules.pptx

UV-VIS spectroscopy and Wood word rules.pptx

1. Woodward-Fieser Rules

2. Nirobenzene Para nitro phenol

3. INTRODUCTION In 1945 Robert Burns Woodward  gave certain rules for correlating λ max   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 λ max  for a given structure by relating the position and degree of substitution of chromophore.

4. 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. Base value :- Each type of diene or triene system is having a certain fixed value at which absorption take place, this value is known as base value.

5. λ max = Base value + ∑substituents- contributes + ∑ other contributes There are three set of Woodward Fischer rule for 1. conjugated diene and polyenes. The term "diene" indicates the presence of two carbon-carbon double bonds, "polyene" describes compounds with many carbon-carbon double bonds. 2. For unsaturated carbonyl compounds 3. For aromatic compound or benzoyl compound.

6. 1. conjugated diene and polyenes A. Homo annular diene : – cyclic diene having conjugated double bond in same ring. B. Hetero annular diene : – cyclic diene having conjugated double bond in different ring. C. Endo cyclic double bond : – double bond present in a ring. D. Exocyclic double bond :-   double bond in which one of the double bond atom is a part of a ring. E. Double bond extending : – when more double bond are present other than conjugated.

7. Acyclic conjugated diene or Hetero annular conjugated diene has 215 nm wavelength. Homo annular conjugated diene :-253 nm Acyclic trienes :- 245 nm. Each alkyl substitution , ring residue or ring residue :- 5 nm Double bond extending conjugation :- 30 nm Auxochrome base value :- – OR = 6 nm – SR = 30 nm – CL,-Br = 5 nm

8. Ring residue: No carbon atoms attached to a conjugated system

9. 2. α, β UNSATURATED CARBONYL COMPOUNDS OR KETONES

10.   2. α,β-unsaturated aldehydes, ketones and nitro compounds

11. 3. AROMATIC/Benzoyl COMPOUNDS Benzoyl-- functional group with the formula −COC₆H₅

12. Base values

13. Base values  a) ArCOR = 246 nm (Ketone) b) ArCHO = 250 nm (Aldehyde) c) ArCO 2 H = 230 nm (Caboxylic acid)  d) ArCO 2 R = 230 nm (Ester)

14. Examples WF rules