IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry - Resonance-- basics

punnima

t is to be emphasized that the different contributing structures of a molecule differ only in the distribution of electrons and not in the arrangement of atoms, i.e., the position of atoms remains the same (different from tautomerism). Moreover, each canonical form must have same number of paired electrons, only their positions may differ. Usually, more are the number of canonical forms in a compound, more is its resonance energy, so more is the stability.

(i) Difference in stability of various canonical forms of a compound : All the canonical forms of a compound are not necessarily equally stable. One form may be more stable than the other and hence contributes more to the actual resonance hybrid. Following are the main factors governing the stability of a canonical structure :

1. Nonpolar structure is more stable than a polar structure.

2. More are the number of covalent bonds in a canonical structure, more is its stability.

3. In polar structures, that structure is more stable in which the negative and positive charges exist on the most electronegative and the most electropositive atoms respectively.

4. Out of the two canonical structures, the one with completed octet of various atoms (or duplet of hydrogen) is more stable even if the more electronegative atom has positive charge.

5. A canonical structure having an electron deficient positively charged atom is much less stable.

6. Also a structure in which like charge are on the atoms close to each other, is least stable.

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