See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Acidity of C-H bonds depends on the stability of the conjugate base (carbanion). Key factors are aromaticity of the anion, inductive/electron-withdrawing effects, and hybridization. Step 1: Analyze compound (x) - cyclopentadiene. Cyclopentadiene has an sp3 CH2 group between the diene. Removal of H from the sp3 carbon gives the cyclopentadienyl anion, which is aromatic (6 pi electrons in a 5-membered ring, Huckel's rule: 4n+2 with n=1). This aromaticity provides exceptional stabilization to the conjugate base, making cyclopentadiene a relatively strong C-H acid (pKa ~ 16). Step 2: Analyze compound (z) - 1-cyanocyclopenta-2,4-dien-1-yl (1-cyano-1H-cyclopenta-2,4-diene, i.e., cyclopentadiene with a CN group at C1). The sp3 carbon at C1 bears both H and CN. Removal of the H gives a cyclopentadienyl anion that is: (a) Aromatic (same as cyclopentadienyl anion - 6 pi electrons) (b) Additionally stabilized by the electron-withdrawing CN group through inductive effect and resonance (CN can delocalize the negative charge further) This double stabilization (aromaticity + CN withdrawal) makes compound (z) more acidic than compound (x). pKa of z is lower than that of x. Step 3: Analyze compound (y) - cycloheptadiene (1,3-cycloheptadiene or similar). The seven-membered ring with two double bonds has an sp3 CH2. Removal of H gives the cycloheptadienyl anion. This anion is NOT aromatic (it would be a 7-membered ring with 6 pi electrons, but the geometry and count don't give Huckel aromaticity for a stabilized anion in the same way - actually cycloheptatrienyl cation is aromatic, not the anion). The carbanion from cycloheptadiene is not stabilized by aromaticity and may even be antiaromatic or non-aromatic. Thus compound (y) is much less acidic than cyclopentadiene (x). pKa of y >> pKa of x. Step 4: Establish the order. - z is most acidic (aromatic anion + CN stabilization) - x is next (aromatic anion alone, cyclopentadienyl) - y is least acidic (non-aromatic or destabilized anion) Decreasing order of acidity: z > x > y Why other options fail: (a) x > y > z: Wrong, ignores CN stabilization on z and overestimates y. (b) y > x > z: Wrong, y should be least acidic. (c) z > y > x: Wrong, y should be least acidic, not between z and x. (d) z > x > y: Correct. Therefore, the correct answer is D.