Substituent effects and linear free-energy relationships

Substituent Effects and Linear Free-Energy Relationships 

Analogous plots for many other reactions of aromatic compounds show a similar linear correlation with the acid dissociation constants of the corresponding benzoic acids. 

SECTION 4.3. SUBSTITUENT EFFECTS AND LINEAR FREE-ENERGY RELATIONSHIPS 

Neither the principles of thermodynamics nor theories of reaction rates require that there should be such linear relationships. There are, in fact, numerous reaction series that fail to show such correlations. Some insight into the origin of die correlation can be gained by considering the relationship between the correlation equation and the free-energy changes involved in the two processes. The line in Fig 4.2 defines an equation in which m is the slope of the line  

Substituting K and k with the appropriate fiee energy of activation  

In Chapter 1, Section 1.2 (p. 16), the effect of substituent groups on the acid strength of acetic acid derivatives was discussed qualitatively. It was noted in particular that the presence of groups more electronegative than hydrogen increased the acid strength relative to acetic acid. A number of important relationships between substituent groups and chemical properties have been developed. In many cases, such relationships can be expressed quantitatively and are useful both for interpretation of reaction mechanisms and for prediction of reaction rates and equilibria. 

The most widely applied of these relationships is the Hammett equation which relates rates and equilibria for many reactions of compounds containing substituted phenyl groups. It was noted in the 1930s that there is a relationship between the acid strengths of substituted benzoic acids and the rates of many other chemical reactions, for instance, the rates of hydrolysis of substituted ethyl benzoates. The correlation is illustrated graphically in Fig. 4.2, which shows log k/ko, where fco is the rate constant for hydrolysis of ethyl benzoate and k is the rate constant for the 

CHAPTER 4 STUDY AND DESCRIPTION OF ORGANIC REACTION MECHANISMS 

The most widely applied of these relationships is the Hammett equation, which 

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Mention must also be made of the use of studies of chemical reactions in the gas phase as a means of determining substituent constants. The investigation of substituent effects and linear free-energy relationships in the gas phase has become an enormous subject with which we can deal only briefly. Part of this subject was established a long time ago and consists in the study of such reactions as the pyrolysis of esters by the techniques of gas... 

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