Benson’s method

Two standard estimation methods for heat of reaction and CART are Chetah 7.2 and NASA CET 89. Chetah Version 7.2 is a computer program capable of predicting both thermochemical properties and certain reactive chemical hazards of pure chemicals, mixtures or reactions. Available from ASTM, Chetah 7.2 uses Benson s method of group additivity to estimate ideal gas heat of formation and heat of decomposition. NASA CET 89 is a computer program that calculates the adiabatic decomposition temperature (maximum attainable temperature in a chemical system) and the equilibrium decomposition products formed at that temperature. It is capable of calculating CART values for any combination of materials, including reactants, products, solvents, etc. Melhem and Shanley (1997) describe the use of CART values in thermal hazard analysis. 

Kinney s method was used In the author s lectures for several years but did not prove to be exploitable since it does not allow estimation of boiling points of many substances such as halogenous derived substances. On the other hand, Benson s method proved to be very useful and will be discussed in Chapter 2 ( Stability ). 

Benson has developed a detailed group contribution method for the estimation of heats of formation see Benson (1976) and Benson et al. (1968). He estimates the accuracy of the method to be from 2.0 kJ/mol for simple compounds, to about 12 kJ/mol for highly substituted compounds. Benson s method, and other group contribution methods for the estimation of heats of formation, are described by Reid et al. (1987). 

It is important to note that Benson s method may require the use of correction terms for molecules having rings, cis/trans or ortho/para isomers, 1,5-repulsions, or gauche interactions. The user is referred to Benson s book162 for a complete description of the method. Some pertinent notation used in this method is as follows ... 

Four worked examples of Benson s method are now given. Example 1. 4-Amino-2-pentanol (g)... 

Most experimentally determined thermochemistry data are for the condensed phase. Yet Benson s method is typically parameterized for the gas phase. Therefore, one usually must convert results from gas to condensed phase and vice versa. 

Benson s method used to calculate the contribution to the AHf provides a convenient strain-free reference value. By subtracting the reference AH from the calculated AHf value, strain energies can be calculated as compiled in Table 16. 

Related Calculations. AG 29X may also be calculated via the relationship AG/gg = A //2°98 — T AS 9g, where S is entropy. Values for S can be found in the literature, or AS 98 can be estimated by Benson s group-contribution method (see example for estimating A f//298 by that method above). However, the deviations for entropy values calculated by Benson s method are high compared with those for A77(29g, so when such group methods are used in the course of calculating AG, only qualitative feasibihty studies are possible, based on these rules-of-thumb (1) if AGj. is less than zero, the reaction is clearly feasible (2) if the value is less than 4.184 x 107 J/(kg mol) [or 1.8 x 104 Btu/(lb mol)], the reaction should be studied further and (3) if the value is greater than 4.184 x 107 J/(kg mol), the reaction is feasible only under exceptional circumstances. (Thus, in the present example, the reaction appears unfavorable at 298 K, but much more promising at 443 K.)... 

A true experimental value for the heat of formation of unsubstituted thiazole is not known. However, a close estimation of an experimental SHf° for thiazole can be obtained by using the experimental SHf° for 4-methylthiazole and the enthalpic additivity value for the methyl substituent from Benson s method. This method gives a value of SHf° (thiazole) = 36.78 kcal mol which has been compared with those calculated through several computational methods (see Section 3.06.2). 

Many of the technical issues involved in computer-aided model-construction have previously been reviewed by Tomlin et al. (1997). Several researchers, most notably Bozzelli, have extended Benson s method for estimating molecular thermochemistry using quantum chemistry (Lay and Bozzelli, 1997a, b Lay et al., 1995). Sumathi and Green (2002) have discussed how quantum chemistry can supplement experiments in developing rate estimation. Matheu et al. have shown how to automate the computation of rates of chemically-activated (pressure-dependent) reactions (Matheu, 2003 Matheu et al., 2003a, b). Here we focus on a few issues which have not been so thoroughly discussed in the literature ... 

This chapter discusses methods of estimating the standard enthalpy of formation, entropy and heat capacity of molecules andfree radicals, using both BENSON s method and those of statistical mechanics. 

Taking the importance of BENSON s method into account, several examples of its application to cyclic and non-cyclic molecules will be treated in this paragraph. 

BENSON s method allows AfH29g and 8 93 to be calculated, as well as Cp from 300 to 1500 K, mainly for molecules. As it happens, the data relating to radical groups are scarce (only about forty groups). BENSON developed a means of calculation based on... 

BENSON S methods allow the pre-exponential factors to be calculated and, in certain rarer cases, the activation energies of other categories of reactions. 

Several arguments can be put forward in favour of a molecular reaction in this case the reaction order is equal to 1, it does not depend on the nature of the walls, the preexponential factor A, calculated by BENSON s methods, is very close to the experimental factor A = 3x10 s L... 

Tables 1.1 and 1.2 contain the data necessary to calculate the thermodynamic properties of molecules in the gas phase by BENSON s methods.

This computer program calculates the kinetic parameters of elementary reactions (unimolecular initiations, radical isomerizations, terminations) using BENSON s methods of thermochemical kinetics. It is part of the EXGAS system. 

This computer program calculates the thermodynamic data of molecules, free radicals and positive ions in the gas phase using BENSON s methods. It also contains a database for nearly 5000 species. 

Using Benson s method of group additivity the program performs several calculations which are useful in evaluating the hazards from a pure chemical, a chemical mixture, or a chemical reaction ... 

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