See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Bredt's rule and ring strain at bridgehead junctures. When three rings meet at a single carbon atom (a bridgehead), the geometry imposed by the rings determines how far the bonds at that atom deviate from the ideal tetrahedral arrangement (planarity here refers to the tendency of the bridgehead carbon to be forced toward a planar, sp2-like geometry, or more generally, the degree of angle strain and distortion). Part A — Greatest deviation from planarity: Step 1: Identify ring sizes at each juncture. - Atom A: junction of three six-membered rings [6-6-6]. This is a relatively relaxed system; six-membered rings are flexible and can adopt chair conformations. The bridgehead is not highly strained. - Atom B: junction of three six-membered rings [6-6-6]. Similar analysis to A; the system is relatively unstrained. - Atom C: junction of a six-membered ring, a four-membered ring, and a five-membered ring [6-5-4]. The four-membered ring introduces severe angle strain (internal angles ~90°), forcing the bonds at atom C into highly compressed geometry. The combination of a strained four-membered ring with larger rings at the same bridgehead creates the greatest geometric distortion. Step 2: Compare strains. - [6-6-6] junctures (A and B) are comparatively strain-free because six-membered rings are low-strain. - [6-5-4] juncture (C) is the most strained because the four-membered ring demands ~90° bond angles, severely deviating from the ideal tetrahedral ~109.5°. This causes the greatest deviation from planarity at the bridgehead. Step 3: Why A and B fail. - Both A and B are [6-6-6] systems with similar, low strain; neither has a small ring to impose severe distortion. Answer to A: (c) C Part B — Most favourable site for H2 addition: Step 1: H2 addition to a C–C bond in a polycyclic system (hydrogenation) is most favourable at the bond under the greatest strain, because relief of strain provides the greatest thermodynamic driving force. Step 2: Identify the bonds. - Bond 1: a bond in the [6-6-6] system at juncture A (part of six-membered rings — low strain). - Bond 2: another bond in the [6-6-6] system at juncture A (similarly low strain). - Bond 3: a bond in the [6-6-6] system at juncture B (low strain). - Bond 4: a bond at juncture C, specifically the bond shared between the four-membered ring and the adjacent ring. This bond is part of the highly strained [6-5-4] system. Breaking/adding across bond 4 (the bond of the four-membered ring) relieves the maximum ring strain. Step 3: The four-membered ring bond (bond 4) is the most strained. H2 addition (hydrogenolysis/hydrogenation of the strained C–C bond) across bond 4 would open the four-membered ring, providing the greatest relief of strain and thus is the most thermodynamically favourable. Step 4: Why 1, 2, 3 fail. - Bonds 1, 2, and 3 are all in six-membered ring systems with minimal strain; H2 addition there provides little strain relief and is less favourable. Answer to B: (d) 4 Therefore, the correct answer is {"A": "c", "B": "d"}.