TABLE A3-2 The Woodward-Fieser Rules for Conjugated Dienes Values for Auxochromic Groups. Heteroannular diene (transoid or s-trans) More intense, e = 12,000-28,000 X shorter (234 nm)
Homoannular diene (cisoid or s-cis) Less intense, e = 5,000-15,000 X longer (273 nm) Typical absorption spectra follow the expected pattern In cyclic dienes, where the central bond is a part of the ring system, the diene chromophore is usually held rigidly in either the s-trans (transoid) or the s-cis (cisoid) orientation. The position of this band appears to be quite insensitive to the nature of the solvent. In general, conjugated dienes exhibit an intense band ( = 20,000 to 26,000) in the region from 217 to 245 mn, owing to a - r transition. Therefore, the 175-nm band is not usually detected. Furthermore, the s- trans conformation is more favorable for butadiene than is the s-cis conformation. Ī transition at 175 nm lies below the cutoff points of the common solvents used to determine UV spectra (Table 10.1) and therefore is not easily detectable. May not be copied, scanned, or duplicated, in whole or in part. The transi- Ĭopyright 2013 Cengage Learning.
In butadiene, two possible transitions can occur 2 and 2 We have already discussed the easily observable Y2 transition (see Fig.
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