preface to fifteenth edition

SPECTROSCOPY 7.21
TABLE 7.11
Absorption Wavelength of Dienes
Heteroannular and acyclic dienes usually display molar absorptivities in the 8000 to 20 000 range, whereas
homoannular dienes are in the 5000 to 8000 range.
Poor correlations are obtained for cross-conjugated polyene systems such as
The correlations presented here are sometimes referred to as Woodward’s rules or the Woodward-Fieser
rules.
Base value for heteroannular or open chain diene, nm214
Base value for homoannular diene, nm 253
Increment (in nm) for
double bond extending conjugation 30
Alkyl substituent or ring residue 5
Exocyclic double bond 5
Polar groupings:
-O-acyl 0
-O-alkyl 6
-S-alkyl 30
-Cl, -Br 5
-N (alkyl) 2 60
Solvent correction (see Table 7.13)
Calculated wavelength total
Ring substitution on the benzene ring affords shifts to longer wavelengths (Table 7.14) and
intensification of the spectrum. With electron-withdrawing substituents, practically no change in the
maximum position is observed. The spectra of heteroaromatics are related to their isocyclic analogs,
but only in the crudest way. As with benzene, the magnitude of substituent shifts can be estimated,
but tautomeric possibilities may invalidate the empirical method.
When electronically complementary groups are situated para to each other in disubstituted benzenes,
there is a more pronounced shift to a longer wavelength than would be expected from the
additive effect due to the extension of the chromophore from the electron-donating group through
the ring to the electron-withdrawing group. When the para groups are not complementary, or when
the groups are situated ortho or meta to each other, disubstituted benzenes show a more or less
additive effect of the two substituents on the wavelength maximum. Calculation of the principal
band of selected substituted benzenes is illustrated in Table 7.15.