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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