Question

Which of the following is/are correct about tautomers?
[A] They possess different atomic and electronic arrangements.
[B] They possess different electronics but the same atomic arrangement.
[C] They have different atomic arrangements but the same electronic arrangement.
[D] They exist in equilibrium.

Answer 1

HINT: Tautomerism is a kind of isomerism. In this, there is an interconversion between the two possible chemical forms of the same species. There are sometimes tautomers which are more stable than the other one but an equilibrium is maintained between the forms.

COMPLETE STEP BY STEP SOLUTION: We can define tautomers as the structural isomers of chemical compounds which they readily interconvert into each other. Generally, it’s the proton that relocates and thus there exists equilibrium between the two forms.
We call this process of interconverting of protons as tautomerization and the species undergoing the process are called the tautomers.
There are different kinds of tautomerism between different chemical species. Now let us discuss some of the types of tautomerism.
- Keto – enol tautomerism – In this tautomerism, a compounds containing ketone groups i.e. –CO can undergo enol formation i.e. –CO converts to –COH even though Keto forms are considered more stable.
Whether a species will exist as a tautomer or not depends upon many factors like the solvent, hydrogen bonding etc. In some solvents keto form is stable whereas in others enol form is stable.
     \[H-O-C=C\rightleftharpoons O=C-C-H\]
- Enamine – imine tautomerism – Here also, there exists an equilibrium between the two groups.
     \[H-N-C=C\rightleftharpoons N=C-C-H\]
Similarly, there are other tautomerisms like amide – imidic acid, lactam – lactim etc.
Now, let us try answering the question.
As we can see from the above discussion that the atomic arrangement is the same throughout its just a proton transfer. Therefore, option [A] and [C] cannot be correct.
Also, we can understand that between tautomers exists equilibrium.

Therefore, the correct answers are options [B] they possess different electronic but same atomic arrangement and [D] they exist in equilibrium.

NOTE: We should remember that tautomers are not similar to the canonical forms of resonance stabilised species. We can chemically distinguish tautomers and they can be identified by their different spectroscopic data. On the other hand resonance canonical structures do not physically exist and are just a hypothetical concept to understand theory of chemical compounds better.