WOODWARD FIESER RULE
IntroductionDefinitionsChoice of solventsRules for dienes and trienes absorptionRules for α,β unsaturated aldehydes and
ketonesRules for conjugated carbonyl compoundsAbsorption for mono & di-substituted
benzene derivatives Rules for polyenes
CONTENTS
The absorption maximum of a dienes system is affected by substitution and WOODWARD (1942) found that the position of the absorption maximum depends on their number and type of dienes.
As a result WOODWARD developed a set of empirical rules (later modified by fieser,1948) for calculating λmax from the molecular structure of the compound.
INTRODUCTION
Longer the conjugate system, greater is the wavelength of absorption.
With the increase in the length of chromophore, the intensity of absorption increases.
The conjugated polyene system appears coloured to the naked eye if there are more than five double bonds are in conjugation and absorption occurs around or above 400nm.
The presence of alkyl group on the double bond also causes bathochromic shift.
SpectroscopyIt is a measurement and interpretation of EMR.
AbsorbanceThe difference between the intensity of incident light
and intensity of transmitted light.
ChromophoreThe group which is responsible for absorption of the
U.V radiation and may or may not impart the colour to the compound.
DEFINITIONS
SOLVENTS WAVELENGTH
Ethanol 210nmHexane 210nmMethanol 210nmCyclohexane 210nmDiethyl ether 210nmWater 205nmBenzene 280nmChloroform 245nmTetrahydrofuran 220nmCarbon tetra chloride 265nm
CHOICE OF SOLVENTS
Parent value for butadiene system or a Cyclic conjugated diene
217nm
Acyclic diene 245nm
Homoannular conjugated diene 253nm
Heteroannular conjugated diene 215nm
RULES FOR DIENES AND TRIENES ABSORPTION
Increment for each substitute-
Alkyl substituent or ring residue 5nm
Exocyclic double bond 5nm
Double bond extending conjugation 30nm
Auxochrome – -OR + 6nm- SR + 30nm- Cl*, - Br* + 5nm- NR2 + 60nm-OCOCH3 0nm
CH2
CH2
Homoannular diene
Heteroannular diene
Acyclic diene
The calculated and experimental values of λ max match with in ±5nm
The double bond exocyclic to a ring
CH2
CH3 CH3
IN CASE OF ACYCLIC DIENES
1.
Observed λ max -224
Calculated λ max base value 214
one alkyl substituent 05 219
2.
CH3 CH3
Observed λ max 232What is the Calculated λ max ?
CH3CH3
CH3
CH3CH3
CH3
Calculate the λ max for the following compounds.
Base value 2532-alkyl substitutions 102- ring residues 10 273
1.
2.
In case of Homoannular diene
IN CASE HETEROANNULAR DIENE
Observed λ max 235Calculated λ maxBase value – 2143-alkyj substituent s - 151-exocyclic double bond - 05
234
Woodward and fieser framed certain emrpical rules for estimating the absorption maximum for α, - unsaturated carbonyl compounds.
These rules were later modified by Scott and are as follows – The basic value α, - unsaturated ketone is taken as 215 nm.
The α, - unsaturated ketone may be a cyclic or six membered. For a compound, = CH – COX, basic value is taken as 215nm If X = alkyl group then the basic value is 215nm If X = H, then the basic value is 207nm If X = OH, then the basic value is 193nm
If the double bond and the carbonyl group are contained in a five membered ring (cyclopentanone), then for such an α, - unsaturated ketone, the basic value becomes 202 nm (The Emax for such compounds are generally above 10,000)
RULES FOR α,β UNSATURATED ALDEHYDES AND KETONES
The structural increament for estimating λmax for a given α, - unsaturated carbonyl compounds are as follows –
• For each exocyclic double bond + 5nm
• For each double bond endocyclic in 5 or 7 membered
• Ring except cyclo-pent-2 enone + 5nm
• For each alkyl substituent or ring residue at the α – position + 10nm – position + 12nm γ or δ – position + 18nm• For each double bond extending conjugation +
30nm• For a Homoannular conjugated diene +
39nm
• Increments for various auxochromes in the various α, , γ – etc., positions are given below –
• Chromophore Increment in nm for position with respect to the carbonyl group
• α- - γ- δ- or higher
• -OH + 35 + 30 - + 50
• -OAc + 6 + 6 + 6 + 6
• -Cl + 15 + 12 - -
• -Br + 25 + 35 - -
• -OR + 35 + 30 17 31
• -SR - + 85 - -
• -NR2 - + 95 - -
O
OH
O
CH3
Observed λmax 247
Calculated base value 2021-β substituent 10α-OH 35 247
Observed λmax 232
Calculated base value 2151-β substituent 121-α-Substituent 10 247
O
CH3CH3
O
CH3
Calculate the λmax for followingstructures
FOR DICARBONYL COMPOUNDS
CH3
CH3 CH3
O
O
CH3
CH3 CH3
OH
O
Examples:
1
Observed λmax – 270 nm
Calculated λmax base value - 215
R-β-alkyl substituent- 14(α-OH)- 35
274
O O OHO
Example 2
Observed λmax - 255nm
What is the calculated λ max ?
ABSORPTION FOR SUBSTITUTED BENZENE DERIVATIVES
R-C6H4-COG Λ max (nm)
Parent chromophore
G-Alkyl or Ring residue 246
G=H 250
G=OH or OR 230
Addition for R
Alkyl or ring residue 0-3 nm
-OH or –Ome ,-O-Alkyl O,M-7 nm
P-25 nm
-O O – 11 nm
M – 20 nm
P – 78 nm
R-C6H4-COG Λ max (nm)
-Cl O,M -0 nm
P – 10 nm
-Br O,M -2 nm
P – 15 nm
-NH2 P – 58 nm
O,M -13 nm
-NHAC O,M -20 nm
P – 45 nm
-NHMe O,M -20 nm
P – 45 nm
-NHMe2 O,M – 20 nm
P – 85 nm
Examples:1. O
O
CH3
Observed λ max – 276 nm
Calculated λ max parent value - 246
para-methoxyl - 25 ortho-ring residue – 03
274
2. OCH3
OCH3
O
Observed λ max – 278
What is the calculated λ max ?
Woodward’s Rules work well only for conjugated system of 4 double bonds or less conjugated polyenes having more than 4 double bonds the fisher-kuhn rules are used.
RULES FOR POLYENES
Gurdeep R. Chatwal, Instrumental methods of chemical analysis , Himalaya publishing house, 5th edition ,2008.
B.K.sharma,Instrumental methods of chemical analysis , Goe publication, 23rd Edition.
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References
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