Woodward's rules
Woodward's rules are a set of rules about how organic chemical compounds absorb ultraviolet light.
They give information about the wavelength of the absorption maximum (symbol λmax ) in an ultraviolet-visible (UV) spectrum of a compound. The rules are named after Robert Burns Woodward. He was a Harvard University professor who won the 1965 Nobel Prize in chemistry. The rules are sometimes called the Woodward-Fieser rules, to also honor Louis Fieser.
The rules build the prediction on the type of chromophores present, the substituents on the chromophores, and changes due to the solvent.[1][2] Examples are conjugated carbonyl compounds,[3][4] conjugated dienes,[5] and polyenes.[4]
Implementation
One set of Woodward-Fieser rules for dienes is shown in table one. A diene is either homoannular with both double bonds contained in one ring or heteroannular with two double bonds distributed between two rings.
| Base value for heteroannular diene | 214 |
| Base value for homoannular diene | 253 |
| Increments | |
| Double bond extending conjugation | + 30 |
| Alkyl substituent or ring residue | + 5 |
| Exocyclic double bond | + 5 |
| acetate group | + 0 |
| Ether group | + 6 |
| Thioether group | + 30 |
| bromine, chlorine | + 5 |
| secondary amine group | + 60 |
| Table 1. Rules for wavelength of maximum diene absorption (in nanometers) [6] | |
These rules predict the UV absorption maximum of compounds. Here are two examples:[7]
In the compound on the left the base value is 214 nm (a heteroannular diene). This diene group has 4 alkyl substituents (labeled 1,2,3,4) and the double bond in one ring is exocyclic to the other (adding 5 nm for an exocyclic double bond). In the compound on the right, the diene is homoannular with 4 alkyl substituents. Both double bonds in the central B ring are exocyclic with respect to rings A and C.
References
- ↑ Woodward, Robert Burns (1941). "Structure and the Absorption Spectra of α,β-Unsaturated Ketones". J. Am. Chem. Soc. 63 (4): 1123–1126. doi:10.1021/ja01849a066.
- ↑ Louis F. Fieser, Mary Fieser, Srinivasa Rajagopalan (1948). "Absorption Spectroscopy and the Structures of the Diosterols". J. Org. Chem. 13 (6): 800–806. doi:10.1021/jo01164a003. PMID 18106021.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ↑ Neil Glagovich (2007-07-19). "Woodward's Rules for Conjugated Carbonyl Compounds". Central Connecticut State University. Archived from the original on 2008-04-10. Retrieved 2008-05-05.
- ↑ 4.0 4.1 William Reusch. "UV-Visible Spectroscopy". VirtualText of Organic Chemistry. Michigan State University. Archived from the original on 2007-10-16. Retrieved 2011-07-27.
- ↑ Neil Glagovich (2007-07-19). "Woodward-Fieser Rules for Dienes". Central Connecticut State University. Archived from the original on 2008-04-10. Retrieved 2008-05-05.
- ↑ Spectroscopic Determination of Organic Compounds, 5th Edition, Silverstein, Bassler, Morrill 1991
- ↑ Organic spectroscopy William Kemp