Other Linear Free Energy Relationships

The Av is also correlated linearly with 19F of p-FC6H4X103. Besides these investigations, other linear free energy relationships for the hydrogen bond of sulphones with several... 

Other linear free-energy relationships are those for the oxidation of a series of Fe(II) phenanthroline complexes by Co(III) and Mn(III). ... 

The Hammett relationship is called a linear free energy relationship since it is based on—and reveals—the fact that a linear relationship exists between free energy change and the effect exerted by a substituent. Other linear free energy relationships arc known, which take into account steric as well as electronic effects, and which apply to ortho substituted phenyl compounds as well as meta and para, and tc aliphatic as well as aromatic compounds. Together they make up what is perhaps the greatest accomplishment of physical-organic chemistry. 

In this chapter it is clearly impossible to do more than sample the extensive literature on the carbon acidity of sulfinyl and sulfonyl compounds, as it illuminates the electronic effects of these groups, particularly in connection with linear free-energy relationships. There are three main areas to cover first, as already indicated, equilibrium acidities (pKa values) second, the kinetics of ionization, usually studied through hydrogen isotopic exchange and finally, the kinetics of other reactions proceeding via carbanionic intermediates. 

Nucleophilic reactivity toward Pt(II) complexes may be conveniently systematized via linear free energy relationships established between reactions of trans Ptpy2Cl2 (py = pyridine) with various nucleophiles and reactions of other Pt(II) complexes with the same nucleophiles. First, each nucleophile is characterized by a nucleophilicity parameter, derived from its reactivity toward the common substrate, trans Ptpy2Cl2. Reactivity toward other Pt(II) substrates is then quite satisfactorily represented by an equation of the form (21), wherein ky is the value of in the reaction with nucleophile Y... 

Quite early on (p. 361) in this discussion of linear free energy relationships consideration was restricted to the side-chain reactions of m- and p-substituted benzene derivatives. The reactions of o-substituted benzene derivatives, and indeed of aliphatic compounds, were excluded because of the operation of steric and other effects, which led to non-linear, or even to apparently random, plots. 

Large numbers of reactions of interest to chemists only take place in strongly acidic or strongly basic media. Many, if not most, of these reactions involve proton transfer processes, and for a complete description of the reaction the acidities or basicities of the proton transfer sites have to be determined or estimated. These quantities are also of interest in their own right, for the information available from the numbers via linear free energy relationships (LFERs), and for other reasons. 

Hansch et al. (1968) established the linear free-energy relationship between aqueous and octanol-water partition of organic liquid. Others, such as Tulp and Hutzinger (1978), Yalkowsky et al. (1979), Mackay et al. (1980), Banerjee et al. (1980), Chiou et al. (1982), Bowman and Sans (1983), Miller et al. (1985), Andren et al. (1987) and Doucette and Andren (1988b) have all presented similar but modified relationships. 

There is also a relationship between p a-values of ethanol, phenol and acetic acid on the one hand, and ethylamine, aniline and acetamide on the other (Liler, 1971c, p. 108), which may be presented as a linear free energy relationship (Figure 6) by regarding Et, Ph and CH3 CO as substituents (R). This excellent relationship leaves no doubt that the acidic group in the R—NH3 series remains unchanged, i.e., that acetylammonium ion is formed in the protonation of acetamide in aqueous acid (half-protonation in ca. 19% sulphuric acid). 

Other examples of linear free energy relationships include the Brpnsted relation, the Grunwald-Winstein relationship, and the Taft equation. See Rho Value... 

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