Laidler scheme

The so-called Laidler scheme was developed as a tool to estimate standard enthalpies of formation of organic compounds [90], It relies on the bond-additivity concept, that is, it assumes that the standard enthalpy of atomization of a given molecule in the gas phase (Aat//°, defined as the standard enthalpy of the reaction where all the chemical bonds are cleaved, yielding the gaseous ground-state atoms) can be evaluated by adding the relevant bond enthalpy terms. For instance, in the case of phenol, its standard enthalpy of atomization, or simply its enthalpy of atomization, refers to reaction 5.28 at 298.15 K ... 

Despite some lack of physical meaning, the empirical Laidler terms have a rather successful predictive power. That justified their use in the past as replacements for Es values [85,86,88], when computational methods were not readily available. It also justifies recent efforts to develop the Laidler scheme by using a larger number of parameters (accounting, e.g., for intramolecular repulsions) [92],... 

In order to develop a quantitative interpretation of the effects contributing to heats of atomization, we will introduce other schemes that have been advocated for estimating heats of formation and heats of atomization. We will discuss two schemes and illustrate them with the example of alkanes. Laidler [11] modified a bond additivity scheme by using different bond contributions for C-H bonds, depending on whether hydrogen is bonded to a primary (F(C-H)p), secondary ( (C-H)g), or tertiary ( (C-H)t) carbon atom. Thus, in effect, Laidler also used four different kinds of structure elements to estimate heats of formation of alkanes, in agreement with the four different groups used by Benson. 

All three schemes, the Benson, the Laidler, and the Allen scheme, use four structure contributions for the estimation of thermochemical data of alkanes. As might be guessed, they are numerically equivalent all three schemes provide the same accuracy. This is shown below by Eqs. (7)-(10) for the interconversion of the various contributions. 

A number of workers (Rice et Rice and Rice Echols and Pease Steacie ° Semenov Benson Purnell and Quinn Sagert and Laidler " ) have proposed free-radical chain mechanisms for the butane pyrolysis, and most of the suggested schemes have many features in common. In the light of present knowledge of the overall kinetics and of the elementary reactions involved there seems little doubt that the following mechanism is close to the truth. 

The reaction mechanism will be discussed on the basis of the scheme put forward by Laidler and Eusuf °°, viz. 

Even though there are a variety of mechanisms leading to catalysis, many catalyzed reactions for a single substrate and a single catalyst can be considered according to the following general scheme (Laidler, 1987) ... 

The resonance energy of 1,3,5-triazine has been calculated from experimental heats of formation by a Laidler bond energy scheme which gave a value for 1,3,5-triazine of 44.9 Kcal compared with... 

Reference 21 Chap. 7 shows the equivalence of the group additivity approach and the schemes proposed by Laidler and by Allen. 

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