Double vs Single Bond Stability
Name: Bushra A.
Why is a double bond more stable than a single bond? It is strange that in
organic compounds the compounds having double or triple bonds are less stable or strong
than those having single bond?
Bond strength and stability are not the same thing. If you will check the bond
dissociation energies listed in your text, you will find that the C to C triple bond
is stronger than the C to C double bond -- and it is in turn stronger than the C to C
single bond. However, the triple bond represents a region of greater inter-atomic
electron density than that of the double bond who's electron density is likewise greater
than that of a single bond. This means that the electrons binding multiple bonded atoms
are more vulnerable to attack by electron-seeking species. Attack by an elecrtophile is
a very different process than that of simply pulling the atoms apart -- dissociation.
You have to distinguish "bond strength" and "reactivity". They are not the same thing. The
bond strengths in HC=-CH triple bond, H2C=CH2 double bond compared to a H3C-CH3 single
bond (230 kcal/mol, 172 kcal/mol, 88 kcal/mol) is approximately in the ratio of 3~ 2~ 1.
This refers to the cleavage of the respective carbon-to-carbon bonds.
This is in contrast to the reactivity of the respective bond types to such
reaction as the addition of Cl2, or H2, or some other reactants. The reactivity
of double bonds and triple bonds is much greater than the reactivity of a single
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Update: June 2012