Woodward-Fieser rules

Woodward-Fieser rules , named after Robert Burns Woodward , Louis Frederick Fieser and Mary Fieser  - also known as Woodward rules  - are empirically determined rules which predict the wavelength at the absorption maximum λ _{max of} the π → π * band of α, β-unsaturated carbonyl compounds and of dienes and polyenes  - especially of terpenes and steroids  - allow. They have proven to be useful for the analysis of UV spectra for the structure elucidation of many organic chemical compounds, especially of natural products. The Woodward-Fieser rules are an incremental system. This means that depending on the substitution, substitution pattern and other structural parameters, as well as the solvent, specific increments are added to a base value of the stem chromophore. The absorption maximum λ _{max of} the π → π * band, theoretically calculated according to the Woodward-Fieser rules, generally agrees with the experimentally determined value with an accuracy of approximately ± 10  nm .

Woodward rules

Robert Burns Woodward researched α, β-unsaturated carbonyl compounds and established the original rules:

Stem chromophore λ max / nm α, β-unsaturated aldehyde 210 α, β-unsaturated ketone 215 α, β-unsaturated carboxylic acid ester 295 Cyclopentenone 202 Cyclohexanone 215

structural features Δλ max / nm each additional conjugated double bond + 30 each exocyclic double bond (away from one ring, can also be part of a neighboring ring) +  05 each homoannular cyclodiene partial structure in the conjugated system +  05

solvent Δλ max / nm water -  08 Methanol -  01 Ethanol -  01 chloroform ±  00 1,4-dioxane +  05 Diethyl ether +  07 n -hexane + 11 Cyclohexane + 11

The influence of the substituents, and thus the increments, is position-dependent:

α-substituents Δλ max / nm per alkoxy group + 35 per alkyl group + 10 per bromine group + 25 per carboxylic acid ester +  06 per chlorine group + 15 per hydroxyl group + 35

β-substituents Δλ max / nm per alkoxy group + 30 per alkyl group + 12 per bromine group + 30 per carboxylic acid ester +  06 per chlorine group + 12 per dialkyl amine residue + 95 per hydroxyl group + 30 per thioether group + 85

γ substituents Δλ max / nm per alkoxy group + 17 per alkyl group + 18 per bromine group + 30 per carboxylic acid ester +  06 per chlorine group + 12 per hydroxyl group + 50

higher substituents Δλ max / nm per alkyl group + 18 per bromine group + 30 per carboxylic acid ester +  06 per chlorine group + 12

Nasty rules

Louis Frederick Fieser and Mary Fieser studied diosterols and determined rules to make predictions for the wavelength at the absorption maximum λ _max . The following table can be used to calculate the wavelength at the absorption maximum of dienes and polyenes:

Stem chromophore λ max / nm acyclic diene 217 homoannular, cyclic diene (two conjugated double bonds in the same ring) 253 heteroannular, cyclic diene (two conjugated double bonds distributed over two rings) 214

structural features Δλ max / nm each additional conjugated, acyclic double bond + 30 each additionally conjugated, endocyclic double bond + 30 each additional conjugated, exocyclic double bond (away from one ring, can also be part of a neighboring ring) + 35

Substituents Δλ max / nm per alkoxy group +  06 per alkyl group +  05 Carboxylic acid esters have no influence ±  00 per dialkyl amine residue + 60 per halogen ( chlorine , bromine ) + 10 per phenoxy group + 18 per thioether group + 30

Solvents have practically no influence on the wavelength and can therefore be neglected.

Sample calculations

Calculation example for conjugated dienes

	Stem chromophore 1,3-butadiene: diene with two conjugated C = C double bonds	Heteroannular diene - two conjugated C = C double bonds distributed over two rings	Homoannular diene - two conjugated C = C double bonds in one ring	Heteroannular triene - three conjugated C = C double bonds distributed over three rings
Stem chromophore (diene): λ max	217 nm	214 nm	253 nm	214 nm
additional conjugation through a C = C double bond in the C-ring of the steroid : 30 nm each	0 nm	0 nm	0 nm	+ 1 x 30 nm = 30 nm
exocyclic double bond on the * -marked carbon atom: 5 nm each	0 nm	+ 1 x 5 nm = 5 nm	+ 1 x 5 nm = 5 nm	+ 3 x 5 nm = 15 nm
Alkyl substituents (marked with ---- ): 5 nm each	0 nm	+ 3 x 5 nm = 15 nm	+ 3 x 5 nm = 15 nm	+ 5 x 5 nm = 25 nm
Sum = λ max (calculated)	217 nm	234 nm	273 nm	284 nm
λ max (measured, steroids)	290 nm	235 nm	275 nm	283 nm

Fieser-Kuhn rule

The Fieser rule only applies to short polyenes. In the case of longer π systems, such as those found in carotenes , the theoretical values ​​deviate more from the actual values. The Fieser-Kuhn rule can correct this error. According to it, the wavelength at the absorption maximum λ _max and the absorption ε _max can be calculated as follows:

${\displaystyle m}$… Number of alkyl groups

${\displaystyle n}$... length of the π system

${\displaystyle R_{\text{endo}}}$… Number of endocyclic double bonds

${\displaystyle R_{\text{exo}}}$… Number of exocyclic double bonds

${\displaystyle \lambda _{\text{max}}=\left(114{,}0+5{,}0\cdot m+48{,}0\cdot n-1{,}7\cdot n^{2}-16{,}5\cdot R_{\text{endo}}-10{,}0\cdot R_{\text{exo}}\right)\,{\text{nm}}}$

${\displaystyle \varepsilon _{\text{max}}=1{,}74\cdot 10^{4}\cdot n}$

Individual evidence

1. ^ ^{A b} Robert Burns Woodward: Structure and the Absorption Spectra of α, β- Unsaturated Ketones . In: J. Am. Chem. Soc. tape 63 , no. 4 , 1941, pp. 1123 , doi : 10.1021 / ja01849a066 (English).
2. ↑ ^{a b} Akul Mehta: Woodward-Fieser rules to calculate wavelength of maximum absorption of conjugated carbonyl compounds. Accessed August 31, 2017 (English). 
3. ↑ Neil Glagovich: Woodward's Rules for Conjugated carbonyl compounds. Central Connecticut State University , July 19, 2007, archived from the original April 10, 2008 ; accessed on August 31, 2017 (English). 
4. ^ ^{A b} Louis F. Fieser, Mary Fieser, Srinivasa Rajagopalan: Absorption Spectroscopy and the Structures of the Diosterols . In: J. Org. Chem. Band 13 , no. 6 , 1948, pp. 800-6 , doi : 10.1021 / jo01164a003 , PMID 18106021 (English).
5. ↑ ^{a b} Akul Mehta: Woodward-Fieser rules to calculate wavelength of maximum absorption for conjugated dienes. Accessed August 31, 2017 (English). 
6. ↑ ^{a b} Neil Glagovich: Woodward-Fieser Rules for Dienes. Central Connecticut State University , July 19, 2007, archived from the original April 10, 2008 ; accessed on August 31, 2017 (English). 
7. ↑ Entry on 1,3-butadiene (UV / Visible spectrum). In: P. J. Linstrom, W. G. Mallard (Eds.): NIST Chemistry WebBook, NIST Standard Reference Database Number 69 . National Institute of Standards and Technology , Gaithersburg MD, accessed November 17, 2019.
8. ↑ Akul Mehta: Ultraviolet-Visible (UV-Vis) Spectroscopy - Sample Problems Using Woodward-Fieser Rules. August 5, 2012, accessed August 31, 2017 . 
9. ↑ Akul Mehta: Ultraviolet-Visible (UV-Vis) Spectroscopy - Fieser-Kuhn Rules to Calculate Wavelength of Maximum Absorption (Lambda-max) of Polyenes (with Sample Problems). Accessed August 31, 2017 (English).