The compound with molecular formula C6H6, which gives only one monobromo derivative and takes up four moles of hydrogen per mole for complete hydrogenation has ____ π electrons.
Answer: 8
Solution
A molecular formula of C6H6 with 4 degrees of unsaturation that is fully removed by exactly 4 mol of H2 points to unsaturation carried entirely in multiple bonds, not rings — consistent with two carbon–carbon triple bonds, since each triple bond consumes 2 mol H2 on complete hydrogenation.
1,5-Hexadiyne, HC≡C−CH2−CH2−C≡CH, fits exactly: it is C6H6, has two triple bonds (so 4 mol H2 is needed to saturate it fully to hexane), and its structure is symmetric about the central CH2−CH2 bond, so bromination gives only one monobromo derivative regardless of which terminal alkyne carbon reacts.
Each carbon–carbon triple bond is made of one σ bond and two π bonds, so it carries 4 π electrons; with two triple bonds in the molecule, the total is 2×4=8π electrons.
JEE Main · 2025 · 22 Jan 2025 · Shift 2 · Integer answer
