Sat February 11, 2012 By:

what is ring chain tautomerism?how valence tautomerism is different from resonanace?

Expert Reply
Mon February 13, 2012
Ring-chain tautomerism occurs when the movement of the proton is accompanied by a change from an open structure to a ring, such as the open chain and pyran forms of glucose and furan form of fructose.

There are many differences :

1)    Tautomers are definite compounds and can be separated and characterized by suitable methods, but resonating structure cannot be separated as they are imaginary structure of the same compound.

2)    Tautomerism involved the migration of a mobile hydrogen atom between two polyvalent atoms present in the molecule and thus tautomers differ a change in the position of atoms. On the other hand, resonating structures have identical atomic arrangement and involves only a change in the position of pi-electrons or unshared electrons.

3)    Tautomers are in dynamic equilibrium with each other and interconvertible but no such equilibrium exits in resonance and there is only form which is intermediate between the many contributing structures.

4)    Two tautomers have different functional groups, but the various resonating structures have same functional group.

5)    Tautomerism has no effect on bond length, while resonance is accompanied by an increase in bond length of a double bond and decrease of a single bond length.

6)    Tautomerism has no contribution in stabilizing the molecule and does not lower its energy, but resonance lowers the internal energy and thus gives rise to extra stability to a molecule.

7)    Tautomerism may occur in planar or nonplanar molecules, while resonatrice occurs only in planar molecules.

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