RULE FOR CONJUGATED DIENES
RULE FOR CONJUGATED CARBONYL
RULE FOR ACYL BENZENES
In 1945 Robert Burns Woodward gave certain
rules for correlating λmax with molecular
In 1959 Louis Frederick Fieser modified these
rules with more experimental data, and the
modified rule is known as Woodward-Fieser
It is used to calculate the position and λmax for a
given structure by relating the position and
degree of substitution of chromophore.
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.
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-
This may be differs from observed value by
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.
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
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.
the base value depends upon whether the
diene is a linear or hetero-annular or
transoid diene, or whether it is a cyclic or
The sum of all substituent contributions are
added to the base value to obtain the
wavelength of maximum absorption of the
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
Transoid / Heteroannular Diene With an
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
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
Cisoid / Homo-annular Diene With An
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
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
This is known as RING RESIDUE.
Ring residue & alkyl substituent
these are the influences of a neighbouring
saturated carbon on a double bond or at a site
of un staturation.
EXTENDED CONJUGATED DOUBLE BOND:
two double bonds separated by the single bond is
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
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.
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.
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 +
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
-Cl, -Br +5mμ
OCOCH3 0 mμ
1. Calculate the absorption maxima in the uv spectrum of
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
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 .
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
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
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
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
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.
2] If the core is a part of a cyclic ring. For
example, cyclopentenone contribution is
202 nm whilecyclohexenone is 215 nm.
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
Substituents can be located on either α, β
If the conjugation is extended to γ and δ
positions, then substitutions at these position
also play a vital role in determining the practical
1] Exocyclic Double Bonds
In general exocyclic double bonds add an
additional + 5 nm to the base value.
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
Water = – 8 nm
Methanol/Ethanol = – 1 nm
Ether = + 6 nm
Hexane / Cyclohexane = + 7 nm
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
In such a case you must add an additional 35 nm to
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
→ 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
chromophore Increment in mυ for a position w.r.t carbonyl group
α β γ δ
+35 +30 - +50
+6 +6 +6 +6
+15 +12 - -
+25 +35 - -
+35 +30 17 31
- +85 - -
- +95 - -
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
2.Calculated absorption maximum in UV Spectra for
Basic value = 215 mμ
2 β ring residue(2X12nm) =24 mμ
1Exocylic double bonds =5 mμ
calculated value =244 mμ
Observed value =241 mμ
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
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
Rules for calculating absorption maximum for derivatives of
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
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
+3 +3 +10
+7 +7 +25
0 0 +10
+2 +2 +15
+13 +13 +58
+20 +20 +45
+20 +20 +85
+11 +20 +75
Calculating the absorption maximum for the following
Basic value =246mμ
OH- substation at para position = 25mμ
OH - substation at meta position =7mμ
Calculated value =278 mμ
Observed value =281mμ
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.
Detection of functional group.
It is possible to detect the functional group with the help of uv
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
Chatwal GR, Anand SK,; Instrumental
methods of chemical analysis-5th edition.PG
YR. sharma, elimentary organic spectroscopy