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Name: Bushra A.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 1/9/2004


Question:
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?


Replies:
Bushra,

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.

Regards,
ProfHoff 763


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 bond.

Vince Calder



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