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Name:  Chitta
Status: educator
Grade: 9-12
Location:N/A
Country: N/A
Date: 7/28/2005


Question:
Though the C-H bond energy is higher that a C-C bond energy yet in many organic reactions preferentially C-H bond breaks in organic compounds. Why and how this happens?


Replies:
Two factors come into play here:

1. Too much emphasis is given to "bond energies" in introductory chemistry courses. These are only "rules of thumb" not to be take very rigorously. For example a single C--C bond can vary from ~ 100 to ~ 500 kJ/mol depending upon what is attached to the two carbons. The bond dissociation energy of a C--H bond can vary from ~350 to ~ 550 kJ/mol depending upon the other substituents on the carbon atom. Both texts and teachers place too much unwarranted importance on small differences in bond energies. They are just ball park estimates. 2. The mechanisms of carbon chemistry are far more complicated than "sawing" C--C and/or C--H bonds and hooking them back together in some other configuration. This point is also not well taught in introductory courses in chemistry. So the relative bond energies are more often than not pretty irrelevant to the course of the reaction, which depends upon many other factors: temperature, solvent, specific chemical structure, etc.

Vince Calder



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