Ashwini somayaji
1st M.pharm
pharmacology
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 INTRODUCTION
 RULE FOR CONJUGATED DIENES
 RULE FOR CONJUGATED CARBONYL
COMPOUND
 RULE FOR ACYL BENZENES
 APPLICATIONS
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 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.
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 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.

 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.

 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.

 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.
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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.

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).

 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 30mμ
Auxochromes
 -OR +6mμ
 -SR +30mμ
 -Cl, -Br +5mμ
 -NR2 +60mμ
 OCOCH3 0 mμ
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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 =217mμ
2 alkylsubstituents =2X5 =10 mμ
Calculated value =227 mμ
The observed value =227mμ
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Name of Compound
3-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 groups
1 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 Compound
10,13-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 groups
1 Double bond extending conjugation
5 x 5 = + 25 nm
+ 30 nm
Other Effects- 3 Exocyclic Double
Bond
+ 15 nm
Calculated λmax 285 nm
Observed 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 nm
Substituents– 5 alkyl substituents
Double bond extending conjugation
5 x 5 = + 25 nm
+ 30 nm
Other Effects- 3 Exocyclic double
bonds
3 x 5 = + 15 nm
Calculated λmax 323 nm
Observed λmax n/a

3. Calculated absorption maximum for the compound
.Homoannular conjugated Diene =253 mμ
3 Ring residues(3X5nm) =15 mμ
1 Exocylic double bond =5 mμ
Calculated value =273 mμ
Observed value found to be =274 mμ
4.Calculated the λ max for compound
Hetroannular conjugated Diene = 215 mμ
2 Ring residues(2X5nm) =10 mμ
1 Exocylic double bond =5mμ
Calculated value =230 mμ
Observed 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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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.

 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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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

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 mμ
Cyclo-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
C
H
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, OR
Cl
Br
NH2
NHAc
NR2
O-
+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.
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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
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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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