Hammett rho

The inhibited unimolecular decomposition of symmetrically di-substituted benzoyl peroxides into radicals also obeys the Hammett rho-sigma relationship. Unfortunately, no extensive activation parameter data are available. The effect of the substituent changes on the rates at the single temperature has been explained in terms of dipole-dipole repulsion in the peroxide.122... 

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... 

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. 

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. 

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. 

Hammett originally demonstrated that the sigma-rho relation might be expected to hold if entropies of activation (or entropy changes) were constant in a series. It has since been shown that a sufficient condition is a linear relation between enthalpies and entropies of activation (or reaction), and such hnear relations are frequently encountered. Although the existence of such linear relations has always appeared somewhat mysterious, some rationale for this relationship has recently been given. 

Among the most extensive studies of monomer reactivity have been those involving the copolymerization of various meta- and para-substituted styrenes with other styrene monomers (styrene, a-methylstyrene, and p-chlorostyrene) as the reference monomer [Kennedy and Marechal, 1983], The relative reactivities of the various substituted styrenes have been correlated by the Hammett sigma-rho relationship ... 

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. 

Perhaps the most convincing evidence for the play of polar forces comes from copolymerization of a series of ring-substituted styrenes here relative reactivities toward a variety of monomers not only fall into a pattern consistent with the familiar electronic effects of the substituents, but show the same quantitative relationships (the Hammett sigma-rho relationship. Sec. 18.11) as do ionic reactions dissociation of carboxylic acids, for example, or hydrolysis of esters. 

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