Woodward's Rules for Enones and α,β Unsaturated Compounds

 

Carbonyl compounds are the compounds containing carbonyl group in their structure. The compounds having carbonyl group in their structure are carboxylic acids, esters, enones, aldehydes.

ENONES

Enones are basically the compounds having a ketone functionality  and atleast one double bond in conjugation with it in their structure. Enone acts as the chromophoric group of the compound as it absorbs UV radiation. Some few basic examples of the enones is depicted in diagram below.

Must read the general terms first from here for better understanding

Electronic transitions

Generally, the carbonyl groups have the two possible transitions pi to pi*and n to pi*  . The pi to pi* is allowed transition but the n to pi* transition is forbidden.  If the carbonyl group is a part of conjugated system of double bonds, the absorption becomes intense. The more the conjugation is, the more intense pi to pi* transition becomes.

Out of both transitions, the pi to pi* transition varies in a predictable manner on the addition of different substituents to the compound. So, one can predict the wavelength at which the pi to pi* transition could occur. The rules for the calculation of wavelength are given by Woodward.

Empirical rules for Enones

1. Base values for different enones

Parent Compound	Base Value
Acyclic enone	215 nm
Acyclic dienone	245 nm
Five membered ring enone	202 nm
Six membered ring enone	215 nm

2. Increment values.

To be applied for	Increment value
Alkyl or Ring residue	alpha	beta	Gamma and higher
	10	12	18
Extended double bond conjugation	30

 

3. Polar group values.

Polar group	Value
	alpha	beta	gamma	Delta
-OH	35	30	50	-
-OCH3	35	30	17	31
-OCOCH3	6	6	6	-
-Cl	15	12	-	-
-Br	25	30	-	-
-NR2	-	95	-	-

4. Exocyclic double bond: 5 nm

5. Homocyclic diene component: 39 nm

 

Some solved examples are given below:

α,β- unsaturated Aldehydes

These aldehydes have almost same rules as that of enones except that their absorption values are shifted by about 5 to 6 nm towards shorter wavelength. Woodward’s rules for aldehydes are given below.

Base value

•  for parent compound  = 208 nm
• With α or β alkyl residue  = 220 nm
•  With α,β or β,β alkyl  residue  = 230 nm
• With α,β,β alkyl residue    = 245 nm 

    α,β-  Unsaturated Acids and esters

These carboxylic acids and esters also have same rules as that of enones. The values for carboxylic acids and esters given by Nielson are given below. 

Base values

• With α or β alkyl residue   = 208 nm
•  With α,β or β,β alkyl  residue   = 217 nm
• With α,β,β alkyl residue    = 225 nm 

      For endocyclic double bond in 5 or seven membered ring, add 5 nm.

    

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