Aliphatics linear free energy relationship

Taft RW Jr. Linear free-energy relationships from rates of esterification and hydrolysis of aliphatic and ortho-substituted benzoate esters. / Am Chem Soc 1952 74 2729-32. 

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. 

Gibson, H.W. Linear free energy relationships. VIII. Ionization potentials of aliphatic compounds, Can.J. Chem., 55 (14) 2637-2641,1977. 

Linear Free-Energy Relationships for Aliphatic Systems—a Constants... 

In the 1950s Taft devised a method of extending linear free-energy relationships to aliphatic systems.16 He suggested that, since the electronic nature of substituents has little effect on the rate of acid-catalyzed hydrolysis of meta- or para-substituted benzoates (p values are near 0, see Table 2.3), the electronic nature of substituents will also have littie effect on acid-catalyzed hydrolysis of aliphatic esters. All rate changes due to substituents in the latter reactions are, therefore, probably due to steric factors.17 Taft defined Es, a steric substituent constant, by Equation 2.16... 

Grosjean, D., and E.L. Williams, n. 1992. Environmental persistence of organic compounds estimated from structure-reactivity and linear free-energy relationships. Unsaturated aliphatics. Atmos. Environ. 26A 1395-1405. 

Evidently, the approximately linear Taft correlation obtained for the elimination of / -substituted primary chlorides6 suggested an examination of a possible linear free-energy relationship for aliphatic secondary alkyl chlorides pyrolyses, that is, for a-substituted ethyl chlorides ZCH(C1)CH370 (Table 6). 

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. 

Ponec, R., Girones, X. and Carbo-Dorca, R. (2002) Molecular basis of linear free energy relationships. The nature of inductive effect in aliphatic series. J. Chem. Inf. Comput. Sci., 42, 564-570. 

The development of linear free-energy relationships in aliphatic molecules is complicated because steric and conformation factors come into play along with electronic effects. A number of successful treatments of aliphatic systems have been developed by separating electronic effects from steric effects. We do not discuss these methods in the present work, but there are reviews available that can be consulted for information about this area. ... 

The Hammett relation does run into difficulties in some instances, however. A prominent example of this is provided by ortho-substituted compounds because steric hinderance is often important in their reactions this is a factor not accounted for in the linear free-energy relationship. A similar procedure, though, has been devised by Taft [R.W. Taft, J. Amer. Chem. Soc., 74,2729, 3120 (1952)] to overcome some of these difficulties. For aliphatic compounds we have... 

We can use this equation for example to explore linear free-energy relationships in chromatography. An example is the relationship between the retention I factor of analytes of the same structure but with different aliphatic chains in. the molecule and the number n of methylene groups in the chains in reversed-phase chromatography ... 

Although the preceding measures of the goodness of the linear fit are highly satisfactory by the standards usually applied to linear free energy relationships, inspection of a plot of the results (Fig. 22a) shows systematic displacements of the data points for polychloroaliphatic and aromatic solvents relative to those for nonchlorinated aliphatic solvents. Correspondingly, significant improvements in r and aliphatic solvents, r = 0.994, u = 0.16 for 11 polychlorinated aliphatics, r = 0.993, O = 0.13 for 18 aromatic solvents, r = 0.989, o = 0.12. 

The development of linear free-energy relationships in aliphatic molecules is complicated because steric and conformational factors come into play along with... 

A linear free energy relationship was found between the activation energy of hete-rolytic dissociation of the C —O bond in the dehydration of aliphatic alcohols over SiOi—AI2O3, which is indicative of the 1 mechanism. 

The occurrence of steric as well as polar substituent effects in aliphatic systems and orf/io-substituted aromatic systems complicates the devising of correlation equations. A typical situation is shown in Figure 2. There is clearly no simple relationship between the rates of alkaline hydrolysis of the ethyl esters of the alkanoic acids and the strengths of the acids themselves. Little progress was made until the early 1950s, when Taft made an excellent start in developing linear free energy relationships in this area. This topic is sometimes referred to as the separation of polar, steric, and resonance effects and is the subject of Section 3. 

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