See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Solution: Identifying the Diad Tautomeric System **Step 1: Understand "Diad System"** A diad tautomeric system requires two tautomeric forms that can interconvert, typically involving: - Migration of a hydrogen atom - Shift of a double bond between adjacent atoms - Both forms must have comparable stability **Step 2: Analyze Each Option** **(A) $CH_3COCH_3$ (Acetone)** - Keto form is highly stable; enol form ($CH_2=C(OH)CH_3$) exists only in trace amounts - Not a true diad system (unequal stability) **(B) $CH_3CH_2NO_2$ (Nitroethane)** - Cannot exhibit tautomerism; nitrogen is too electronegative to form stable imine/amine equivalents - No mobile hydrogen on adjacent carbon to the $NO_2$ group **(C) Cyclohexanone** - Similar to acetone: keto form dominates heavily over enol form - Not an effective diad system **(D) $HCN$ (Hydrogen Cyanide)** $$HCN \rightleftharpoons HNC$$ - Both forms (isocyanide) have comparable stability - H can migrate between C and N atoms - Both tautomers are **significantly populated** and can interconvert - This is a true diad tautomeric system with two distinguishable, energetically comparable forms **Answer: (D) $HCN$** is the only true diad system where both tautomeric forms exist in appreciable concentrations.