See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Tautomerism in hydrocarbons (specifically keto-enol or related tautomerism) requires that a carbon have at least one hydrogen atom alpha (adjacent) to a carbon that can form a double bond, or more generally, that there exist at least one hydrogen on a carbon adjacent to another carbon such that a proton transfer can generate a structural isomer in equilibrium. However, the question in the context of M.S. Chauhan's problem bank refers to a specific type of tautomerism relevant to these alkyl-substituted cyclohexane structures. Step 1 - Concept: For ring-chain tautomerism or for the relevant tautomerism being tested here, each of the three structures (I, II, III) are alkylcyclohexane derivatives. The key point tested is whether each structure possesses the structural prerequisite (presence of alpha-hydrogen atoms or a relevant functional arrangement) to exhibit tautomerism. Step 2 - Structure (I): Methylcyclohexane has a methyl group attached to the cyclohexane ring. The carbon bearing the methyl group has alpha hydrogens available. It can participate in tautomerism. Step 3 - Structure (II): 1,1-Dimethylcyclohexane has two methyl groups on the same ring carbon (quaternary-like, but the ring carbon still has connections to other ring carbons with H). Alpha carbons on the ring adjacent to the gem-dimethyl carbon still bear hydrogens, allowing tautomerism. Step 4 - Structure (III): This structure has methyl substituents on the cyclohexane ring at different positions, and all relevant carbons bear hydrogens that allow tautomeric equilibria. Step 5 - Since all three structures possess the necessary structural features (alpha hydrogens relative to the relevant carbon centers) to exhibit tautomerism, all of them can exhibit tautomerism. Step 6 - Options (a), (b), and (c) are eliminated because they each exclude one or more valid structures. Option (d) correctly identifies that all three structures can exhibit tautomerism. Therefore, the correct answer is D.