Tautomerism

Tautomerism

It is a special type of functional isomerism in which an a-hydrogen atom is shifted from one position to another. This shift is referred as 1,3 -shift . Such shifts are common between a carbonyl compound containing an a-hydrogen atom and its enol form.

In most cases, the equilibrium lies towards the left. Thus, the keto form is thermodynamically more sable than enol form by about 12 kcal/mol.

The term tautomerism is used for isomers that are fairly readily interconvertible and that differ from each other only (a) in electron distribution and (b) in the position of a relatively mobile atom or group, The mobile atom is generally hydrogen and the phenomenon is then called as prototropy .

Both acids and bases catalyse such interconversions. Possible limiting mechanisms are those (a) in which proton removal and proton acceptance (from the solvent) are separate operations and a carbanion intermediate is involved. i.e. an intermolecular pathway and
(b) in which one and the same proton is transferred intramolecularly.

Mostly the keto form is more stable than enol form but in certain cases, enol form can become the predominant form. The enol form is predominant in following cases:

1. Molecules in which the enolic double bond is in conjugation with another double bond/phenyl ring. In such cases, sometimes intramolecular hydrogen bonding also stabilizes the enol.

2. Molecules, which contain two bulky aryl groups.

3. Highly fluorinated enols.

Because of the greater acidity of a-hydrogen atom (due to the presence of strongly electron withdrawing fluorines), the conversion to its enol form is high.The extent of enolization is also affected by the solvent, concentration and temperature. Thus, acetoacetic ester has an enol content of 0.4% in water and 19.8% in toluene. This is because water reduces the enol content by hydrogen bonding with the carbonyl group, making this group less available for intramolecular hydrogen bonding. The effectiveness of intramolecular hydrogen-bonding in stabilizing the enol, with respect to the keto form is seen on varying the solvent and particularly on transfer to a hydroxylic solvent with MeCOCH ₂ COMe.

Also, the enol content of pentan-2,4-dione (CH ₃ COCH ₂ COCH ₃ ) is found to be 95% and 45% at 27.5° and 275°C respectively.

When a strong base is added to a solution of a ketone with a-hydrogen atom, both the enol and keto form can lose a proton. The resulting anion is same in both the cases as they differ only in the placement of electrons. They are not tautomers but canonical forms.

Other types of tautomerism

(i)  Phenol-Keto tautomerism:

(ii)        Nitroso-Oxime tautomerism:

Difference between Tautomerism & Resonance

a)   In tautomerism, an atom changes place but resonance involves a change of position of pi-electrons or unshared electrons.

b)   Tautomers are different compounds and they can be separated by suitable methods but resonating structures cannot be separated as they are imaginary structures of the same compound.

c)   Two tautomers have different functional groups but there is same functional group in all canonical structures of a resonance hybrid.

d)   Two tautomers are in dynamic equilibrium but in resonance only one compound exists.

e)   Resonance in a molecule lowers the energy and thus stabilises a compound and decreases its reactivity. But no such effects occur in tautomerism.

f)    In resonance, bond length of single bond decreases and that of double bond increases e.g. all six C—C bonds in benzene are equal and length is in between the length of a single and a double bond.

g)   Resonance occurs in planar molecules but atoms of tautomers may remain in different planes as well.

h)   Tautomers are indicated by double arrow in between the two isomers but double headed single arrow ¬¾® is put between the canonical (resonating) structures of a resonating molecules.