See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The acidity of a C-H bond (and thus the pKa) is related to the s-character of the carbon orbital used in the C-H bond. Greater s-character means the electrons are held closer to the nucleus, making the conjugate base (carbanion) more stable, which means higher acidity (lower pKa). The s-character can be inferred from the H-C-H bond angle: larger bond angles correlate with greater s-character. However, these hydrocarbons are not all simple cases. We must also consider the effect of ring strain and hybridization: - sp3 carbon: ~25% s-character, bond angle ~109.5°, pKa ~50 - sp2 carbon: ~33% s-character, bond angle ~120°, pKa ~44 - sp carbon: ~50% s-character, bond angle ~180°, pKa ~25 - Strained rings increase s-character of exocyclic bonds beyond nominal hybridization Now analyzing each compound: (I) Cyclopentadiene - the sp3 CH2 carbon shows 109.5°. This is a typical sp3 carbon, but cyclopentadiene is famously acidic (pKa ~16) due to aromaticity of the cyclopentadienyl anion upon deprotonation. This is the most acidic, giving the LOWEST pKa. (V) Acetylene - bond angle 180°, sp hybridized carbon, 50% s-character, pKa ~25. Second most acidic. (IV) Adamantane-type compound - shows 108° (slightly compressed from tetrahedral 109.5°), indicating slightly increased p-character at the C-H bond, meaning slightly less acidic than normal sp3. The 128° angle elsewhere indicates distortion. The bridgehead C-H in adamantane-like systems can have slightly different acidity, pKa slightly above normal alkane (~50 or slightly different). Positioned third lowest pKa. (III) Alkene with 117° H-C-H angle - sp2-like carbon, pKa ~44 for vinyl C-H. Third or fourth in order. (II) Strained bicyclic with 115° - between sp3 and sp2, the compressed angle (larger than 109.5°) suggests increased s-character at the C-H bond, giving somewhat enhanced acidity compared to normal sp3 but less than sp2 alkene. (VI) Cyclopropene with 120° H-C-H - the sp3 carbon of cyclopropene ring has significant s-character due to ring strain (bent bonds), cyclopropyl C-H bonds are more acidic than typical sp3. The shown angle is 120° at the CH2 of cyclopropene, reflecting increased s-character, pKa around 46 but modified by ring strain effects. Arranging from lowest pKa (most acidic) to highest pKa (least acidic): - I (cyclopentadiene, aromatic anion stabilization, pKa ~16) - LOWEST pKa - V (acetylene, sp, 50% s-character, pKa ~25) - IV (adamantane-type, slightly strained, pKa slightly modified ~50 region but placed here) - III (alkene, sp2-like 117°, pKa ~44) - II (strained bicyclic, 115°, between sp3 and sp2) - VI (cyclopropene CH2, 120° with ring strain effects, highest pKa in this ordering) This gives the order: I < V < IV < III < II < VI, which corresponds to answer (d). Why other options fail: (a) Places V first (lowest pKa) ignoring cyclopentadiene's exceptional aromaticity-driven acidity. (b) Places IV first which would imply adamantane is most acidic, incorrect. (c) Places II first which is also incorrect. Therefore, the correct answer is D.