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WOODWARD FIESER RULE AND APPLICATION

Ashwini somayaji1st M.pharm

pharmacology

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CONTENT INTRODUCTION

RULE FOR CONJUGATED DIENES

RULE FOR CONJUGATED CARBONYL COMPOUND

RULE FOR ACYL BENZENES

APPLICATIONS

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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.

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Woodward rules work well for dienes and polyenes with upto 4-double bonds or less.

The fieser rule must be applied for certain plant pigments such as cartenoids have even more than 4-conjugated double bonds and extended rules to conjugated aldehydes and ketones.

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 These sets of rules to calculate the wavelength of maximum absorption or λmax of a compound in the ultraviolet-visible spectrum, based empirically have been called the Woodward-Fieser rules or Woodward’s-rules.

This may be differs from observed value by 4-5nm.

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Conjugated dienes and polyenes are found in most organic compounds.

Longer the conjugated system,greater the wavelength of absorption maximum.

According to Woodward’s rules the λmax of the molecule can be calculated using a formula:

λmax = Base value + Σ Substituent Contributions + Σ Other Contributions.

 

WOODWARD FIESER RULE FOR CONJUGATED DIENE:

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BASE VALUE:

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 first step in predicting the wavelength of maximum UV absorption for conjugated dienes is to determine wheather it lies in an s-trans or s-cis conformation.

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If it lies in the s-trans conformation, its base wavelength is 217nm.

If it lies in the s-cis conformation,its base wavelength is 253nm.

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the base value depends upon whether the diene is a linear or hetero-annular or transoid diene, or whether it is a cyclic or homo-annular diene.

The sum of all substituent contributions are added to the base value to obtain the wavelength of maximum absorption of the molecule.

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TRANSOID/HETEROANNULAR DIENE / LINEAR DIENE: This type of diene generally involves the

attachment of trans dienes .

Since the two double bonds attached are trans, it leads to a linear diene which is also called a hetero-annular diene.

The base value for a hetero-annular diene system is 215 nm according to the Woodward-Fieser rules.

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Transoid / Heteroannular Diene With an Example

In the above example, it can be seen that one of the double bonds belongs to ring A while the other double bond belongs to ring B, hence making the double bond hetero-annular.

Since both double bonds are trans with respect to substituents making the diene a transoid diene. In general, hetero-annular dienes are transoid. If the diene is not a part of a ring then it is just transoid.

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2] CISOID DIENE / HOMO-ANNULAR DIENE / CYCLIC DIENE: This type of diene involves the

conjugation of two cis dienes.

Since the double bonds are cis to each other, the molecule often tends to form a closed ring system and therefore also called a cyclic or homo-annular diene.

The base value for homo-annular diene system is 253 nm according to the Woodward-Fieser rules.

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Cisoid / Homo-annular Diene With An Example

In the above example, it can be seen that both the double bonds belong to ring B making this type of diene a homo-annular diene.Since both double bonds are cis with respect to substituents making the diene a cisoid diene. In general, homo-annular dienes are cisoid. If the diene is not a part of a ring (i.e. the green bonds do not exist) then it is just a cisoide. If a molecule has both a homo-annular diene and a hetero-annular diene , then the homoannular diene as the core chromophore.

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Only the substituents attached directly to the double bond diene systems can influence the ultraviolet visible absorption of the molecules.

If the substituents are not directly attached to the carbons of the diene system, it will not affect the UV-Visible absorption spectrum of the molecule.

SUBSTITUENT EFFECTS:

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Transoid and Cisoid Dienes with substituents highlighted in red.

The figure above highlights possible substituents in red given by the different -R groups. In the above examples 1 and 2, assignment of substituents must be given to all the atoms which are directly connected to the diene. Hence even though the structure has no substituents, the core carbon atoms have yet to be considered as alkyl-substituents.This is known as RING RESIDUE.

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Ring residue & alkyl substituent

these are the influences of a neighbouring saturated carbon on a double bond or at a site of un staturation.                                         

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EXTENDED CONJUGATED DOUBLE BOND: two double bonds separated by the single bond is

conjugated.

If a third double bond is separated from one of the original pair of a double bonds by a single bond, the three double bonds represents an extended conjugated double bonds.

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OTHER CONTRIBUTIONS:1] Exocylcic Double Bonds: Exocyclic doube bond by definition is a double

bond where one of the participating carbon atoms is a part of a ring, while the other carbon atom is not part of the same ring.

From the name we can understand that exo-cyclic would stand for a double bond outside the ring and endo-cyclic would stand for a double bond within the ring.

For each exocyclic double bond, we must add +5 nm to obtain the λmax.

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In example 1, the double bond present within ring A is exocyclic to ring B as it is attached to an atom which is shared between ring A and ring B, while the double bond present in ring B is not connected to any ring A atoms and is within just one ring, hence making it endocyclic.

In example 2, both double bonds are present within ring B with connections to shared carbon atoms with ring A, making both the double bonds exocyclic.

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In example 3, there is a single double bond which is exocyclic at two points to two different rings. In such a case, the influence would be 2 times + 5 nm (i.e + 10 nm).

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Parent value for buta diene system or a cyclic conjugated diens 217mμ Acyclic triene 245mμ

Homo annular conjugated dienes 253mμ Heteroannular conjugated diene 215mμ

Increment for each substituents alkyl substituent or ring residue 5mμ Exocyclic double bond 5mμ Double bond extending conjugation 30mμAuxochromes -OR +6mμ -SR +30mμ

-Cl, -Br +5mμ -NR2 +60mμ OCOCH3 0 mμ

SOLVENT ETHANOL TRANSITION INVOLVED -Π→Π*

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Example: 1. Calculate the absorption maxima in the uv spectrum of 2,4-hexadiene.

CH₃-CH=CH-CH=CH-CH₃

Solution: The basic unit in 2,4 hexadiene is butadiene .

Basic value =217mμ2 alkylsubstituents =2X5 =10 mμCalculated value =227 mμThe observed value =227mμ

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PROBLEMES:

Name of Compound3-methoxy-10-methyl-2,7,8,9,10,11,12,13,14,15,16,17-dodecahydro-1H-cyclopenta[a]phenanthrene

Woodward Component Contribution

Core- Transoid/Heteroannular Diene + 215 nm

Substituents- 3 alkyl groups1 alkoxy group

3 x 5 = + 15 nm+ 6 nm

Other Effects- Exocyclic Double Bond + 5 nm

Calculated λmax 241 nm

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name of Compound10,13-dimethyl-2,3,9,10,11,12,13,15,16,17-decahydro-1H-cyclopenta[a]phenanthrene

Woodward Component Contribution

Core-Transoid/Heteroannular + 215 nm

Substituents- 5 alkyl groups1 Double bond extending conjugation

5 x 5 = + 25 nm+ 30 nm

Other Effects- 3 Exocyclic Double Bond + 15 nm

Calculated λmax 285 nmObserved λmax 283 nm

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Note- In this example the molecule contains both, a homoannular diene system and a heteroannular diene system. In such a molecule the core chromophore is considered to be the homoannular system .

Component Contribution

Core- Homoannular/Cisoid diene + 253 nmSubstituents– 5 alkyl substituentsDouble bond extending conjugation

5 x 5 = + 25 nm+ 30 nm

Other Effects- 3 Exocyclic double bonds 3 x 5 = + 15 nm

Calculated λmax 323 nmObserved λmax n/a

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3. Calculated absorption maximum for the compound .Homoannular conjugated Diene =253 mμ3 Ring residues(3X5nm) =15 mμ1 Exocylic double bond =5 mμCalculated value =273 mμObserved value found to be =274 mμ

4.Calculated the λ max for compound Hetroannular conjugated Diene = 215 mμ2 Ring residues(2X5nm) =10 mμ1 Exocylic double bond =5mμCalculated value =230 mμObserved value =232 mμ

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Bicylic system

If a Diene system is present in a bicylic compound,then due to strain correction , the value of absorption maximum is-

For the 2,3 dimethylene bicylo[2,2,1] heptane. Basic value =217mμ 2Ring residue(2X5nm) =10mμ 2Exocylic double bonds(2X5nm ) =10mμ Bicylo system(strain correction) =15mμ Calculated value =252mμ Observed value =254mμ

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FOR CONJUGATED CARBONYL COMPOUND:Core Chromophores With Base

Values: As Woodward and Fieser have listed, α,β-

unsaturated carbonyl compounds have a range of influence on the λmax of the molecule depending upon:

1] The type of carbonyl functionality present. For example, α,β-unsaturated aldehyde contribute 210 nm while α,β-unsaturated ketones contribute 215 nm and α,β-unsaturated esters contribute 195 nm.

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2] If the core is a part of a cyclic ring. For example, cyclopentenone contribution is 202 nm whilecyclohexenone is 215 nm.

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3] If the conjugation is extended to γ,δ-positions to form dienes. For example, in such cases, a simple addition of 30 nm to the base value of the α,β-unsaturated carbonyl compound gives appropriate estimates to the observed influences.

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SUBSTITUENT EFFECT: According to Woodward, in case of α,β-

unsaturated carbonyl compounds, the location of the substituent is significant in determining the influence on the wavelength of maximum absorption.

Substituents can be located on either α, β positions.

If the conjugation is extended to γ and δ positions, then substitutions at these position also play a vital role in determining the practical λmax.

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OTHER CONTRIBUTION:1] Exocyclic Double Bonds

In general exocyclic double bonds add an additional + 5 nm to the base value.

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2] Solvent Effects Since carbonyl functional groups have polarity,

solvents play an important role in how the electronics of the structure play out. The rules are simple and straight forward:

Solvent Influence: Water = – 8 nm Methanol/Ethanol = – 1 nm Ether = + 6 nm Hexane / Cyclohexane = + 7 nm

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3] Homoannular Cyclohexadiene

In a special case where you have α,β-γ,δ-diene carbonyl compound and both the double bonds are present within one ring system you get a homoannular or homocyclic cyclohexadiene carbonyl compound.

In such a case you must add an additional 35 nm to the system.

O

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The structure increments for estimating λmax for a given αβ- unsaturated compound are as follows:

→ For each exocyclic double bond : +5 mμ→ For each double bond endocyclic

In 5 or7 numbered ring except : +5 mμCyclo-pent-2enone .→ For each alkyl substituent or ring Residue at the, α-position :+10 mμ β-position :+12 mμ γ or δ- or higher position :+18 mμ→ For each double bond extending conjugation :+30 mυ → For homoannular conjugated diene :+39 mμ

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chromophore Increment in mυ for a position w.r.t carbonyl group α β γ δ

-OH

-Oac

-Cl

-Br

-OR

-SR

-NR2

+35 +30 - +50

+6 +6 +6 +6

+15 +12 - -

+25 +35 - -

+35 +30 17 31

- +85 - -

- +95 - -

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1.Calculate λ max for given compound

ά-β unsaturated ketone =215 mμ 2 β alkyl substituents(2X12 =24 mμ Calculated value = 239 mμ Observed value =237 mμ

CH3

CH3

CH

O

CH3

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2.Calculated absorption maximum in UV Spectra for following compound.

Basic value = 215 mμ 2 β ring residue(2X12nm) =24 mμ 1Exocylic double bonds =5 mμ calculated value =244 mμ Observed value =241 mμ

O

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4. Calculated the λ max for compound

ά-β unsaturated Cyclopentane =202 mμ 1 β alkyl substitution =12 mμ 1 Exocylic double bond =5 mμ double bond with extending conjugation =30 mμ 1γ ring residue =18 mμ 1δ ring residue =18 mμ Calculated value =285 mμ Observed value =285 mμ

O

CH3

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5.Calculated the λ max for the following compound

Basic value =215mμ ά-ring residue =10mμ δ-ring residue =18mμ 1 Exocylic double bond =5mμ 1Homoannular conjugated diene =39mμ 1 Double bond with extended conjugation =30mμ Calculated value =317mμ Observed value =319mμ

O

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Rules for calculating absorption maximum for derivatives of acyl benzenes

Like Woodward fieser rules, Scott devised a set of rules for calculating the absorption maximum for the derivatives of Acyl benzenes, These rules help in estimating the position of absorption maximum in ethanol in a number of mono substituted aromatic ketones aldehydes and acid esters.

For the compound of the type 1.The basic value is 246 mμ if X is an alkyl group acyclic residue.

2. If X is hydrogen atom, the basic value becomes 250 mμ 3. The basic value is 230 mμ if, X is OH and 245 mμ if X is

OR

C

O

X

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The structural increments in nm for further substation on the aromatic ring in the ortho meta & para position are given in the table

Auxochrome Incriment in mυ position of substituent

Ortho meta para

alkyl OH, ORClBrNH2NHAc NR2O-

+3 +3 +10 +7 +7 +25 0 0 +10 +2 +2 +15 +13 +13 +58 +20 +20 +45 +20 +20 +85 +11 +20 +75

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Calculating the absorption maximum for the following compound

Basic value =246mμ OH- substation at para position = 25mμ OH - substation at meta position =7mμ Calculated value =278 mμ Observed value =281mμ

C

OH

OH

CH3

O

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Detection of conjugation: It helps to show the relationship between the different

groups particularly with respect to conjugation; it may be between two or more carbon- carbon double bond , between cabon-carbon and carbon-oxygen double bond or between double bond or aromatic ring.

Determination of geometrical isomers. Trans isomers exhibit λmax at slightly longer wavelength. And have larger extinction coefficient than cis isomers.

APPLICATION:

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Detection of functional group.It is possible to detect the functional group with the help of uv

spectrum.

Identification of unknown compound. A unknown compound can be identified by comparing the

spectrum with the known spectra. Examination of polynuclear hydrocarbons Identification of compound in different solvent. Determination of strength of hydrogen bonding. Elucidation of the structure of vitamin A and K

REFERENCES: Chatwal GR, Anand SK,; Instrumental

methods of chemical analysis-5th edition.PG .NO:2.163-2.167

YR. sharma, elimentary organic spectroscopy PG.NO:36-48

Internet source

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