🚀 go-pugleaf

In article <1993Jan27.190213.27488@cs.sfu.ca> jones@cs.sfu.ca (John Jones)
writes:
>I've been checking the values given for the energies of some common
>chemical bonds in various textbooks, and have been surprised to find wide
>variations in the values given.  For example, take the C-C, C=C and
>C[triple bond]C energies; I found the following values (in kJ/mol)
>
[Table showing a 10-20% variances in textbook-reported bond energies deleted.]
>
>What is the cause of these discrepancies?

     Bond energies vary a lot from molecule to molecule.  The figures
shown in typical textbook tables are averages over several different
kinds of molecules containing a certain "type" of bond.  I put the word
"type" in quotation marks in the previous sentence because, for
instance, the C-C bond in an aldehyde is quite a different thing than
that in a normal hydrocarbon.  As you can imagine, the average value you
arrive at will depend crucially on your sample.

>Suppose I actually needed to know the bond energies for some practical
>purpose, how would I come up with a reliable figure?

     If you really needed an estimate of a bond energy, your best
strategy would be to try to find a more detailed table and to use a bond
energy from a closely related molecule.  For a small example of what I
mean, look at the table on p. 1064 of Kemp and Vellaccio's Organic
Chemistry.  There are better bond energy tables around, but I don't
happen to have such a thing in my personal library.
     Usually, it's best to try to avoid using bond energies.  They are
sometimes useful for ballpark arguments, but usually standard
thermodynamics tables can be made to yield the same information and
because the latter tables are almost always more comprehensive, it's
easier to gauge the variability of reaction energies from them.

				Marc R. Roussel
                                mroussel@alchemy.chem.utoronto.ca