Tautomerism is a type of structural isomerism, in which two different structures of same molecular formula can be formed due to oscillation of a monovalent (Hydrogen) between polyvalent atoms. A substance exists in two readily interconvertible different structures leading to a dynamic equilibrium. The term tautomerism (Greek: tauto = same; meros = parts) was used by Laar in 1885.
Conditions for tautomerism
i) The following connection of atoms X, Y and Z can be checked to see that, if tautomerism occur in the compound or not.
X = should have hydrogen, itself can not be hydrogen
Y = can be any atom which can form double bond.
Z= can be Carbon, Oxygen, Nitrogen, Sulfur
ii) Presence of α-hydrogen: Aldehydes and ketones having at least one α-hydrogen atom show keto-enol tautomerism. Thus propanal, butan-2-one and acetophenone having α-Hydrogen atom show keto-enol tautomerism.
iii) Absence of α-hydrogen: Following compounds do not show tautomerism due to the absence of α-hydrogen.
How to draw tautomers?
1,3-Hydrogen migration takes place in tautomerism.
Hydrogen shifted from 1 to 3 and double bond shifted from 2, 3 to 1, 2.
Polyvalent atoms are oxygen and two carbon atoms.
The greater stability of keto form than enol form is due to greater strength of C=O, pi-bond (87 kcal mole-1) as compared to C=C, pi-bond (60 kcal mole-1).
Evidence in favour of Ketonic form
- Keto forms give addition product i.e. a white precipitate with HCN and NaHSO3.
- Keto forms give oxime and phenyl hydrazone on treatment with hydroxyl amine (NH2OH) and Phenyl hydrazine (C6H5NH-NH2) respectively.
Evidence in favour of Enolic form
- Enolic forms give reddish violet colour with neutral FeCl3 in alcoholic solution it is a characteristic test of – (OH)C=C– group as in phenols.
- Enolic forms discharge the colour of Br2 water, indicating the presence of double bond (unsaturation).
Factors affecting the relative amounts of Keto and Enol forms in Keto-Enol tautomerism
The following factors affect the position of equilibrium in keto-enol tautomerism.
If one tautomer formed as aromatic compound that will be more stable than the one which is non aromatic or anti aromatic.
Hydrogen Bonding and Conjugation
If the two keto groups of a compound are separated by methylene group, then enolic form becomes comparatively more stable by hydrogen bonding and conjugation.
In acetyl acetone (1,3-diketone) the amount of enolic form is much higher (76%) and keto form (24%). Hydrogen bonding and conjugation are present in the enol form.
Polar protic solvents such as water, alcohol and acetic acid etc., which form H–bonds with the C=O group of keto form, decrease the enol content. On the other hand, in non-polar (aprotic) solvents such as benzene, hexane etc., or even in absence of solvent, there occurs as increase in the enol content. For example, the enol content of acetoacetic ester is only 0.4% in water, 7% in methanol and 46% in hexane, etc. Similarly, the enol form of acetyl acetone is 76% in ethanol but it is 92% in hexane.
Acteyl actone and Acetoacetic ester
The enol content of α-methyl acetyl acetone (44% in gas phase) is much lower than that of acetyl acetone (92% in gas phase).
Although both the enols are stablized by H–bonding, the enol form of α-methyl acetyl acetone is destabilised by greater internal strain due to the presence of α-methyl group.
Difference between Tautomerism and Resonance
i) Tautomers are definite compounds and can be separated and characterised by suitable methods, but resonating structures cannot be separated as they are imaginary structures of the same compound.
ii) Tautomerism involves the migration of a mobile hydrogen atom between two polyvalent atoms present in the molecule. Thus tautomers differ from each other in a change in the position of atoms. On the other hand, resonating structures have identical atomic arrangement. They involve only a change in the position of pi–electrons or unshared electrons.
iii) Tautomers are in dynamic equilibrium with each other and are interconvertible but no such equilibrium exists in resonance and there is only form which is intermediate between the many contributing structures.
iv) Two tautomers have different functional groups, but the various resonating structures have the same functional group.
v) Tautomerism has no effect on bond length, while resonance is accompanied by an increase in bond length of double bond and decrease of a single bond length.
vi) 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.
vii) Tautomerism may occur in planar or non-planar molecules, while resonance occurs only in planar molecules.