Hammett rho values

However, the effects on the rates in nonpolar solvent systems is dramatically larger and often of apparent contradiction. For example, the Hammett rho value for the oxidation of substituted methyl cinnamates and cinnamic acids by tetrabutylammonium permanganate in methylene chloride solutions is positive (33,49). See Figure lb. However, a rho value of converse sign (-0.6) is obtained from a Taft plot (Figure Ic) for the oxidation of vinyl ethers in aqueous tetrahydrofuran (33,50). For many other compounds the Hammett relationships are no longer linear, but concave upward ... 

Three important and interrelated observations were made Firstly, it was found that the Hammett rho value is substantially larger when the solvent is acetone (1.43), than when it is methylene chloride (0.95). Secondly, the rates of reaction are faster in methylene chloride and, thirdly, the rates of reaction in acetone solutions are independent of the identity of the... 

Similar results were reported by Platz et al. [75] Interestingly, positive p values were observed not only for reaetions of ArCCl with (e.g.) 1-pentene (-1-1.3), and 1-hexene (+1.3), but also with eleetron defieient alkenes, (e.g.) diethyl fumarate (+ 0.45) and ethyl aerylate (+0.75). The Hammett rho values thus suggest that these latter earbene additions are eleetrophilie, even though the selectivity and reactivity of PhCCl toward the set of alkenes in Table 7 is clearly parabolic and ambiphilic. One does note, however, that the electrophilicity of the additions, expressed by p, deereases as the alkenes become less electron-rieh. Thus p deereases from +2.49 for butyl vinyl ether, to +1.3 for 1-pentene, to +0.45 for diethyl fumarate. [75] A more detailed explanation, based on the Houk enthalpy/entropy analysis, [97] has been offered. [75]... 

There is evidence that the contribution of carbenoid resonance forms (cf. 7) in certain heterocyclic ylids is of minor importance at best °. In particular, Hammett rho values for deuterium exchange in a series of l-aryl-4-ethyltetrazolium salts (17) were found to be rather small +0-76 in 3-8 n DCl, +1-38 in the less polar medium 9-0 n... 

The applications of cross-interaction constants pij in mechanistic studies, largely the work of Ikchoon Lee and his group, have been reviewed. The cross-interaction constant is a second derivative parameter (the ordinary Hammett rho value, p, or p , is a first derivative) representing a change in the intensity of interaction between the two reacting partners (or fragments) i and j in a reaction. It may be either an activation parameter or a reaction parameter. 

Alkyllithium-transition metal halide catalysis is completely different from the sodium ketyl and alfin catalysis. Natta, Danusso, Scanesi and Macchi (36) have found that the polymerization of styrene and substituted styrenes by titanium tetrachloride-triethyl aluminum catalysts was different from the above anionic systems. A plot of the log of the rate of the polymerization against Hammett s sigma constant produced a straight line with a rho value of —1.0. Electron releasing groups facilitated this polymerization. 

The fastest and easiest way to determine the pKa is to look it up in tables of pKa values stored within the CHESS system. If no value is found there, it can be calculated using LFER1s such as the Hammett equation. Hammett rho and sigma values are stored in tables inside the CHESS system. Situations that deviate from linear additivity are noted in an exceptions table to prevent erroneous calculations. If a calculation is not possible, the user would be asked to determine the value experimentally. 

Hammett s sigma and rho values (also Taft s sigma values for aliphatic compounds) are discussed in Section 17.2. The most frequently needed values, experimentally determined, are in Table 17.4 (p. 647) and a great many others will be found in Appendix A.l of Perrin, Dempsey and Serjeant s book (1981), in which will also be found Appendix A.6, a useful list of sigma constants for heteroatoms in heterocyclic rings (note also the help available on p. 87 for obtaining a rho constant when the nucleus is not benzene). Sigma and rho constants are also listed by Hansch and Leo (1979). 

The very small secondary isotope effect arising from replacement cf p-CHs by p-CDs is instructive largely because it shows that full development of carbonium ion character is not necessarily associated with a substantial benzylic deuterium secondary isotope effect. It must be recalled in this connection that the Hammett rho for Ki value is —4.410 and that the effect of one p-methyl group on the equilibrium corresponds to a typical cr+ value, i.e., the reaction is quite sensitive to stabilization of charge and p-methyl shows enhanced participation in stabilization. The observed effect is consistent with the analogous very small effects which have been observed in aromatic substitution (105), including bromination, for which p is —12, but smaller than those which have been reported for solvolyses of benzylic (106) and benzhydryl (107) compounds. It does not appear to be possible to draw a general conclusion from these data. 

See also Hammett equation rho (p) value Taft equation. 

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