See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

These three structures (p), (q), and (r) are resonating structures of the same compound: (E)-pent-2-en-4-ynenitrile or more precisely a molecule with a vinyl group, a CN group, and an acetyl (COCH3) group — specifically they represent resonance contributors of CH2=CH-C(CN)=C(O-)(CH3) type system, i.e., resonance structures of a compound like methyl vinyl ketone with a nitrile substituent. Step 1 – Identify what each structure represents in terms of charge separation and stability criteria: Structure (p): Has O- (negative charge on oxygen) and the CN group is intact (C≡N). The negative charge resides on oxygen. Oxygen is highly electronegative and can stabilize negative charge very well. Additionally, the structure has a C=C double bond between the central carbons (fully conjugated). The negative charge on oxygen is stabilized by electronegativity of oxygen. Structure (q): Has C=O (neutral carbonyl) and the negative charge is on carbon (carbanion on the carbon bearing CN). Carbon is less electronegative than oxygen or nitrogen, so a carbanion is less stable than an oxyanion or nitroanion in general. However, the carbanion is adjacent to CN and C=O groups which provide some stabilization through delocalization. Structure (r): Has C=O (neutral carbonyl) and C=N- (negative charge on nitrogen). Nitrogen is more electronegative than carbon but less electronegative than oxygen, so N- is less stable than O- but more stable than a carbanion (C-). Step 2 – Rank by stability of charge placement: - Negative charge on O (most electronegative, best at stabilizing negative charge) → structure (p) is most stable. - Negative charge on N (intermediate electronegativity) → structure (r) is intermediate. - Negative charge on C (least electronegative, worst at stabilizing negative charge) → structure (q) is least stable. Step 3 – Order: p > r > q Step 4 – Check against options: Option (d) states p > r > q, which matches our analysis. Why other options fail: - (a) p > q > r: Incorrectly places q (C-) above r (N-); nitrogen stabilizes negative charge better than carbon. - (b) q > p > r: Incorrectly ranks carbanion structure highest, which is wrong. - (c) q > r > p: Also incorrectly ranks carbanion highest. Therefore, the correct answer is D.