Question

Which of the following statement is correct?

• A. Two tautomers have different functional groups.
• B. Tautomerism may occur in planar or non planar molecules.
• C. Tautomerism is possible in benzaldehyde (C₆H₅CHO).
• D. Tautomerism is not possible in benzophenone (C₆H₅COC₆H₅).

Answer 1

Answer: (A), (B), (D)

The correct options are
A

Two tautomers have different functional groups.

B

Tautomerism may occur in planar or non planar molecules.

D

Tautomerism is not possible in benzophenone (C₆H₅COC₆H₅).

It is a phenomenon in which a single compound exists in two readily interconvertible structures that differ in the relative position of atleast one atomic nucleus,generally hydrogen.Tautomeric structures are obtained by the simultaneous shift of H atom and double bond at1 and 3 position.That is why it is also called 1,3-shift.It is further called desmotropism or dynamic isomerism or kryptomerism .

$-\stackrel{\underset{||}{O}}{C}-\stackrel{\underset{|}{H}}{\underset{\stackrel{|}{\left(Keto\right)}}{C}}\leftrightharpoons -\stackrel{\underset{|}{OH}}{C}=\underset{\stackrel{|}{\left(Enol\right)}}{C}-$

$\to$ One of the major conditions for tautomerism is presence of α-hydrogen

$\to$ $\alpha$-hydrogen is present on $\alpha$- carbon

The different characteristics of tautomerism are:

1. Tautomerism (cationotropy) is caused by the oscillation of hydrogen atom between two polyvalent atoms present in the molecules. The change is accompanied by the necessary rearrangement of single and double bonds.
2. It is a reversible intramolecular change.
3. The tautomeric forms remain in dynamic equilibrium. Hence, their separation is a bit difficult although their separation can be done by special methods, yet they form a separate series of stable derivatives.
4. The two tautomeric forms differ in their stability. The less stable form is called labile form. The relative proportion of two forms varies from compound to compound and also with temperature, solvent etc. The change of one form into another is also catalysed acids and bases.
5. Tautomers are in dynamic equilibrium with each other and inter-convertible.
6. Two tautomers have different functional group.
7. Tautomerism has no effect on bond length.
8. Tautomerism has no contribution in stabilizing the molecule and does not lower its energy.
9. Tautomerism may occur in planar of non planar molecules.