C-C and C-H Bonding Energy
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
Why and how this happens?
Two factors come into play here:
1. Too much emphasis is given to "bond
energies" in introductory chemistry courses. These are only "rules of
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.
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Update: June 2012