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Woodward-Fieser Rules

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					Introduction to Woodward-Fieser Rules

In the middle of the last century, R. B. Woodward studied the UV spectra of conjugated
dienes and developed a set of rules for predicting the wavelength of maximum UV
absorption based on the structure of the diene. Later L. M. Fieser extended the rules to
conjugated aldehydes and ketones.

In order to apply the rules to specific structures, we need to learn how certain structures
are described, so we may apply numeric wavelength values to the structure.

Conjugated Dienes

We learned in lecture that conjugated dienes must lie in an s-cis conformation in order to
undergo a Diels-Alder reaction. Thus, two fundamental conformations of conjugated
dienes are the s-cis conformation and the s-trans conformation. Figure 1 shows that an s-
trans conjugated diene is one in which the two double bonds lie on opposite sides of the
single bond that joins them. Whereas, a double bond in the s-cis conformation is one in
which both double bonds lie on the same side of the single bond that joins them.


        s =single bond that connects the two double bonds (dashed, straight line below)




      1,3-butadiene               s-trans                   s-cis
                                  conformation              conformation
                                  (double bonds on          (double bonds on
                                  opposite sides            same side of
                                  of single bond)           single bond)


Figure 1. Conformations of 1,3-butadiene.

The first-step in predicting the wavelength of maximum UV absorption for a conjugated
diene is to determine whether it lies in an s-trans or s-cis conformation. If it lies in the s-
trans conformation, its base wavelength is 217 nm. If it lies in the s-cis conformation, its
base wavelength is 253 nm.

Endocyclic vs Exocyclic Double Bonds

An interesting feature of double bonds is that they may be part of a ring system, in which
case they are called endocyclic double bonds because their  bond lies “within” the ring.
Double bonds may also project from a ring, in which case they are called exocyclic
double bonds because their  bond lies “outside” the ring. If a compound is bicyclic, a
double bond might be endocylic with respect to one ring and exocyclic with respect to the
other ring.
                                     A    B

 endocyclic      exocyclic         endo to A
                                   exo to B

Figure 2. Types of double bonds.

Figure 2 shows that a double bond is endocyclic if its  bond is part of the ring. Whereas,
the double bond is exocyclic if its bond projects from the ring. If a double bond is
exocyclic to a ring, it adds 5 nm to the base wavelength of a conjugated diene.

Extended Conjugated Double Bonds

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




         s-cis               s-cis extended          s-trans        s-trans extended

   max) = 217 nm      max) = 247 nm        max) = 253 nm      max) = 283 nm

Figure 3. Effect on (max) of extending the conjugation.

Each double bond that extends the conjugation adds 30 nm to the wavelength of
maximum absorption.

Effect of Alkyl Groups

Any alkyl group bonded to a carbon atom of the conjugated system (i.e., a carbon sharing
a conjugated  bond) adds 5 nm to the wavelength of maximum absorption.




         s-cis            s-cis extended             s-trans        s-trans extended
   + 2 alkyl groups     + 3 alkyl groups       + 2 alkyl groups     + 3 alkyl groups
   max) = 227 nm      max) = 262 nm        max) = 263 nm      max) = 298 nm

Figure 4. Conjugated systems containing alkyl groups.
The wavelength values shown in figures 3 and 4 are predicted values. The actual values
vary slightly from the predicted values and must be determined by experimentation.

Conjugated Aldehydes and Ketones

A conjugated aldehyde or ketone arises when a double bond is separated by a single bond
from the carbonyl group of an aldehyde or ketone.


               O                         O

                H               
                                   
     conjugated aldehye         conjugated ketone
     (max) = 210               (max) = 215

Figure 5. Conjugated aldehyde and ketone.

The base wavelength for a conjugated aldehyde is 210 nm and for a conjugated ketone is
215 nm. The compounds shown in Figure 5 are also called -unsaturated because their
carbon-carbon double bond lies between the alpha and beta carbon atoms. The Greek
lettering system starts with the carbon atom bonded to the carbonyl carbon atom of the
aldehyde or ketone. An alkyl groups bonded to an -carbon adds 10 nm to (max) and an
alkyl group bonded to a -carbon adds 12 nm to the base value. The effects of an
exocyclic double bond and extended conjugation are the same for aldehydes and ketones
as for dienes. An exocyclic double bond adds 5 nm and an extended double bond adds 30
nm to the base values.

A wavelength predictor has been created in Excel for conjugated dienes and for
conjugated aldehydes and ketones. A diene is characterized as lying in an s-cis or s-trans
conformation and by its number of alkyl groups bonded to the conjugated system, exo
double bonds, and extended double bonds. The analyst enters these values into the
predictor and the predictor calculates (max). The predicted value of (max) is the same
as one obtains by calculating the value as described above. A separate sheet in the Excel
workbook handles aldehydes and ketones in a similar fashion. The structure is identified
as an aldehyde or ketone, and its number of  and  alkyl groups, exo double bonds and
extended double bonds are entered into the appropriate boxes. The predictor calculates
(max).

Use the wavelength predictor to answer the problems in the two exercises found in the
folder with this document.

				
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posted:12/1/2011
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