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### Transcript of Calculation of » using woodward fieser rules

Assignment #2

Calculation of max using Woodward-Fieser rules.

By

To

Prof. Dr. Jamshed Iqbal

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 know 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)

s-trans

conformation

(double bonds on

opposite sides

of single bond)

s-cis

conformation

(double bonds on

same side of

single bond)

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.

endocyclic exocyclic

A B

endo to Aexo 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. 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

max) = 217 nm

s-cis extended

max) = 247 nm

s-trans

max) = 253 nm

s-trans extended

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

+ 2 alkyl groups

max) = 227 nm

s-cis extended

+ 3 alkyl groups

max) = 262 nm

s-trans

+ 2 alkyl groups

max) = 263 nm

s-trans extended

+ 3 alkyl groups

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

H

O

conjugated aldehye

(max) = 210

conjugated ketone

(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.

The max of the p p* transition for compounds with < 4 conjugated double bonds can be

calculated using Woodward-Fieser rules.

i. 30 for each extra conjugated double bond

ii. 5 each time a conjugated double bond is an exocyclic double bond

iii. 36 for each conjugated double bond frozen s-cis

iv. 5 for each alkyl group or halogen bonded to conjugated system of polyene

v. 10 for an -substituent of a conjugated aldehyde or ketone

vi. 12 for a -substituent of a conjugated aldehyde or ketone

Example # 1. Calculate expected max for following

Base = 217

3 alkyl substituents @ 5 each = 15

Example # 2. Calculate expected max for following

i. 30 for each extra conjugated double bond

ii. 5 each time a conjugated double bond is an exocyclic double bond

iii. 36 for each conjugated double bond frozen s-cis

iv. 5 for each alkyl group or halogen bonded to conjugated system of polyene

v. 10 for an -substituent of a conjugated aldehyde or ketone

vi. 12 for a -substituent of a conjugated aldehyde or ketone

Calculated = 232

Base = 215 Base = 217

substituent = 10 Additional = 30

substituent = 12 Homoannular or s-cis = 36

Exocyclic = 5 2 alkyl substituents = 10

Example # 3

(i) and similarly (ii).

Calculated = 242 Calculated = 293

Observed = 241 Observed = 293

(iii) (iv)

The above last compounds were calculated for its max according to the following rules.