2WOODWARD-FEISER RULES FOR CALCULATING λmax INTRODUCTION From the study of the ultraviolet absorption spectra of a large number of compounds, Woodward (1941) gave certain rules for correlating λ max with molecular structure.Scott-Feiser (1959): modified rule with more experimental data.The modified rules known as Woodward-fieser rules, can be used to calculate the position of λ max for a given structure by relating the position of λ max with the position and degree of substitution of chromophore.longer the conjugated system, greater is the wavelength of absorption a maximum.The intensity of the absorption also increases with the increase in length of the chromophore.
3Different TermsAlicylic dienes or dienes contained in an open chain systemi.e, where basic unit is butadiene system.2. Homo-annular conjugated double bondsAre the conjugated double bond present in the same ring. it is also called as Homodiene. some examples of this type are:
4Different Terms 3. Hetero-annular conjugated double bonds Are the double bonds which are present in different ring. for example.4. Endocyclic double bondThe double bond present in a ring only (inside).5. Exocyclic double bondThe double bond is a part of the ring (outside).
5Woodward – Fieser’s rules for conjugated dienes, trienes, polyenes etc According to this rules each type of dienes or triens system is having a certain fixed value at which absorption takes place.This constitutes the basic value or parent value.This value of absorption maximum depends upon ……(I) The number of alkyl substitution or ring residues(II) The number of double bonds which extending conjugation(III) The presence s of polar groups such as Cl, Br, OR, are added to the basic value to obtain λ max for a particular compound.
6The parent values and increment for different substituents / groups (A) Parent value :-1) Homo annular conjugated diens λ 253 nm2) Hetero annular conjugated diens Acyclic conjugated diens λ 214 nm3)Acyclic conjugated diens λ 214 nm4) Butadiene system open chain (Acyclic) λ 217 nm5) Acyclic triens λ 245 nm(B) Increment :-1) Each alkyl substitution or ring residue λ + 5 nm2) Exocyclic double bond λ +5 nm3) Double bond extending conjugation λ +30 nm(C) Auxochromes :--O- alkyl λ +6 nm-S-alkyl λ nm-Cl, -Br λ +5 nm-N(alkyl)2 λ nm-OCOCH3 λ nm
7EXAMPLES1. Calculate absorption maximum in UV Spectrum of 2,4 Hexadiene.Butadiene system; basic value = 217nm2 Alkyl substituent(2X5nm) = 10nmCalculated value = 227 nmObserved value = 227 nm
82. Calculate λ max for 1,4- dimethylcyclohex-1,3-diene Parent value for homoannular ring : = 253 nmTwo alkyl substituents : 2 x 5 = 10 nmTwo ring residue : x 5 = 10 nmcalculated value : = 273 nmobserved value : = 263 nm
93. Calculate λ max of following compound Parent value for heteroannular diene : = 214 nmFour ring residue : X5 = 20 nmCalculated value : = 234 nmObserved value : = 236 nm
10Parent value for heteroannular diene : = 214 nm 3 ring residue : X5 = 15 nmExocyclic double bond = 5 nmThiomethyl substituent =30 nmCalculated value : = 264 nmObserved value : = 268 nm