No,we can't. In hammett equation, there are 3 kinds of ground state or static electrical influences predominate ,Resonance effect and Direct electrostatic (field) effect: electrical influence of a polar or dipolar substituent which is transmitted primarily to the reactive group through space (including solvent, if any) according to the laws of classical electrostatics and Inductive effect: electrical influence of a group which is transmitted primarily by polarization of the bonding electrons from one atom to the next. Hammet equation is not true anywhere,we can use that for some conjugated structures.
tnx Moisés Canle López,thats what i want,because i obtained hammett constant for tethiophene dyes,tnx for books.
Sarah; the hammet equation should works well. However, some other sets of constant exist too, it depends what kind of physical properties do you plan to rationalize withy them. I suggestthe reading of diffrent papers:
1. Jaffé, H. H., A Rexamination of the Hammett Equation. Chemical Reviews 1953, 53 (2), 191-261.
2. Brown, H. C.; Okamoto, Y., Electrophilic Substituent Constants. J Am Chem Soc 1958, 80 (18), 4979-4987.
3. McDaniel, D. H.; Brown, H. C., An Extended Table of Hammett Substitutent Constants Based on the Ionization of Substituted Benzoic Acids. The Journal of Organic Chemistry 1958, 23 (3), 420-427.
See for example of use on (polyt)hiophene:
1. Garcia, P.; Pernaut, J. M.; Hapiot, P.; Wintgens, V.; Valat, P.; Garnier, F.; Delabouglise, D., Effect of end substitution on electrochemical and optical properties of oligothiophenes. J. Phys. Chem. 1993, 97 (2), 513-16.
i obtained hammet constants for some structures and assume that the sensitivity constant(p) is 1, is it wrong? beacuse the p=1 just defined for banzoic acid reaction
Hammett equation is applicable only to m-& p- substituted benzene derivatives and not for o-substituted benzene derivatives or aliphatic conjugated systems where folding or coiling of the chain occurs.
The review by Hansch Leo, and Taft gives a detailed account of Hammett substituent constants (A Survey of Hammett Substituent Constants and Resonance and Field Parameters, Chem. Rev. 1991, 91, 165-195). The use and limitations of the various scales are well described.
Hammett equation is applicable for m- & p-substituted benzene derivatives and not aliphatic compounds where twisting and folding of carbon chain occurs.
Hammett equation applies to many conjugated systems, not only m- & p-substituted benzene derivatives. For aliphatic compounds there are different treatments using different constants (inductive and field effects, with steric effects in some cases). This is sometimes considered as extension of the Hammett equation, but was developed by Robert W. Taft.
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