Using BDEs to Predict Exothermicity and Endothermicity

Since bond strengths are measured in enthalpy units, we can use bond strength differences between reactants and products to predict the exothermicity or endothermicity of a reaction. This is an exercise routinely done in introductory organic chemistry classes, so we only briefly review it here with one example. Consider the reaction given in Eq. 2.6. We only need to look at the differences in the BDEs for those bonds that change between reactants and products. The reactants possess the stronger bonds, which means they are the more stable structures, and hence the reaction is endothermic. The important point is that we can 

One of the strongest bonds in chemistry is the O-H bond of water, with a BDE of 119 kcal / mol. Since thermodynamics often presages reactivity in radical chemistry, it should not be surprising to hear that the hydroxyl radical, HO, is extremely reactive. That is, a reaction such as 

The implications of this observation for biological chemistry are profound. Water is plentiful in a biological system, but the generation of HO is a high energy process. If HO were to form, it would rapidly abstract a hydrogen from almost any organic molecule. This includes carbohydrates, proteins, and, perhaps most 

Valentine, J. S., Greenberg, A., and Liebman, J. F. (1995). Active Oxygen in Chemistry. Structure Energctioi and Reactivity in Chemistry Vol. 2. Chapman Hall, New York. 

A variety of experimental techniques has been employed to measure BDEs. Often, different techniques produce slightly different values, and over time, different workers have produced detailed analyses that choose the best value for a given BDE. We present the results of two such analyses in Table 2.2, one from 1982 and one from 2003. Where comparisons are available, there are small but significant differences. Certainly, the trends in BDEs are the same for the two. In general, though, when comparing two different BDEs it is best to use values from the same data set. 

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