Chapter 1:- Structure and Reactivity
B.sc 1st year Book
Organic Chemistry
(Page 9)
Tautomerism
Canard L. in 1885-86 published a paper in which he explained why some compounds such as ethyl acetoacetate, p-nitrophenol, isatin, di-isobutylene etc. give properties of two groups. He described that such compounds exist in two isomeric forms which differ only in the position of atomic nuclei, generally hydrogen and called this phenomenon as” tautomerism” (Greek, tauto = the same; meso = part). So, tautomerism may be defined as “a phenomenon in which a single compound exists in two readily interconvertible structures that differ markedly in the relative position of at least one atomic nucleus”, generally hydrogen. Two different/unique structures are known as tautomers of each other. Tautomerism is also known as “Desmotropism” (Greek desmos = bond; tropos = turn) since the interconversion of two forms involves a change of bond, or “Dynamic isomerism” as the 2 forms are in dynamic equilibrium with each other. Other common names for tautomerism are Kryptomerism, Allelotropism or Merotropy. However, tautomerism is the most widely accepted term.
What is tautomerism ?
Compounds existed in two isomeric forms which differ only in the position/location of atomic nuclei, generally hydrogen and now known as the phenomenon ” tautomerism“.
keto-enol tautomerism
There are many types of tautomerism of which keto-enol tautomerism is the most important. In this type, one tautomer exists as a ketone while the other exists as an enol.
In general, the above-mentioned equilibrium between the enolic and keto structures is mostly in favour of the keto structure so that enols change into keto forms as soon as they are formed in the reaction.
Such rearrangements of enols to keto structures are obviously due to the polarity of the O−H bond. The hydrogen in the enol may separate as hydrogen ion, H and anion A, which is a resonance hybrid of structures I and II.
Such rearrangements of enols to keto structures are obviously due to the polarity of the O−H bond. The hydrogen in the enol may separate as hydrogen ion, H and anion A, which is a resonance hybrid of structures I and II.
It may take up a new hydrogen ion either at the oxygen or at the carbon. Of course, reattachment of the hydrogen ion at oxygen will regenerate that enol undergoing ionisation again but the attachment of the H ion at carbon will give rise to a keto structure where the hydrogen may stay on.
Tautomerism Examples:
The two simplest examples are acetone and phenol in which Keto-enol tautomerism takes place.
The two forms are readily interconvertible by acid or base catalysts. At ordinary conditions surface of the glass is sufficient to catalyse the interconversion. The exact composition of the equilibrium depends upon the nature of the compounds, solvents, temperatures etc. The conversion of a Keto-form into an enol-form is known as “enolisation”. The tautomeric forms are quite chemically distinct entities and can be separated and characterised, as evidenced by the following :
Evidence in favour of the ketonic form :
(i) Acetoacetic ester forms the additional product with HCN and Sodium bisulphite.
(ii) It reacts with hydroxylamine and phenylhydrazine to form oxime and phenylhydrazone respectively.
(iii) On hydrolysis it gives acetone (Which shows the presence of >C=O group).
(iv) It forms mono and dialkyl derivatives indicating the presence of active methylene (−CH2−) group.
(ii) It reacts with hydroxylamine and phenylhydrazine to form oxime and phenylhydrazone respectively.
(iii) On hydrolysis it gives acetone (Which shows the presence of >C=O group).
(iv) It forms mono and dialkyl derivatives indicating the presence of active methylene (−CH2−) group.
Evidence in favour of the enolic form :
(i) Acetoacetic ester gives reddish-violet colour with FeCl3 solution which is a characteristic reaction of enols.
(ii) It reacts with sodium to form sodium derivative with the evolution of H2 gas again indicating the presence of the −OH group.
(iii) It discharges the colour of ethanolic Br2 solution indicating the presence of a carbon-carbon, double bond.
(ii) It reacts with sodium to form sodium derivative with the evolution of H2 gas again indicating the presence of the −OH group.
(iii) It discharges the colour of ethanolic Br2 solution indicating the presence of a carbon-carbon, double bond.
It is important to remember that Keto-enol tautomerism is possible only in those aldehydes and ketones which have at least one α-hydrogen atom which can convert the ketonic group to the enolic group.
Distinction from Resonance:
(i) The tautomeric forms are quite chemically distinct entities and can be separated and characterised. On the other hand, resonating forms differ only in the distribution of electrons and can never be separated from one another since neither of them has any real existence.
(ii) Tautomerism involves a change in the position of the H-atom, while resonance involves a change in the position of the unshared p-electrons only.
(iii) Tautomers are in dynamic equilibrium with each other while the resonating structures are not in dynamic equilibrium.
(ii) Tautomerism involves a change in the position of the H-atom, while resonance involves a change in the position of the unshared p-electrons only.
(iii) Tautomers are in dynamic equilibrium with each other while the resonating structures are not in dynamic equilibrium.
(iv) Tautomerism has no effect on bond lengths, while resonance affects the bond lengths.
(v) Tautomerism does not lower the energy of the molecule and hence does not play any role in stabilising the molecule while resonance decreases the energy and hence increases the stability of the molecule.
(vi) Tautomerism may occur in planar or non-planar molecules, while resonance occurs only in planar molecules.
(vii) Resonance is represented by inserting a double-headed arrow (⟷) between the contributing structures while tautomerism is represented by inserting a reversible arrow (⇌) between the two tautomers.
(viii) Examples :
(v) Tautomerism does not lower the energy of the molecule and hence does not play any role in stabilising the molecule while resonance decreases the energy and hence increases the stability of the molecule.
(vi) Tautomerism may occur in planar or non-planar molecules, while resonance occurs only in planar molecules.
(vii) Resonance is represented by inserting a double-headed arrow (⟷) between the contributing structures while tautomerism is represented by inserting a reversible arrow (⇌) between the two tautomers.
(viii) Examples :
