See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Analyze Pair 1: The first pair shows hexa-1,3,5-triene in two different spatial arrangements (an s-trans extended zigzag versus an all-trans open-chain representation). Both structures have the same connectivity and the same configuration at each double bond; the difference is only in rotation about single bonds (C2-C3 and C4-C5 single bonds). Rotation about single bonds produces conformers, not new stereoisomers. Therefore Pair 1 = Conformers. Step 2 - Analyze Pair 2: The second pair shows hexa-1,3,5-triene in two arrangements where the geometry at the double bonds differs. The first structure shows the s-trans (extended) arrangement and the second shows an s-cis (U-shaped) arrangement. Crucially, when examining the terminal double bonds, the geometry (E/Z) at C1=C2 and C5=C6 double bonds is different between the two structures - one has the terminal =CH2 groups pointing in opposite directions and the other has them pointing in the same direction. Since the double bonds are locked (no free rotation), these represent geometric isomers (GI), which are a type of stereoisomers. Therefore Pair 2 = Stereoisomers (GI). Step 3 - Analyze Pair 3: The third pair shows two Kekule structures of benzene (cyclohexatriene). These are the two classic Kekule resonance structures where the positions of single and double bonds alternate. In terms of actual structures, these two drawings represent the same compound (benzene) viewed in different conformations of the ring, or more precisely, they are conformers of cyclohexatriene (the same connectivity, same geometry, differing only in the arrangement in space due to ring flipping or bond rotation). For the purpose of this question, since the ring geometry and connectivity are the same and only the apparent position of double bonds differs (which in a real cyclic context would be conformational), Pair 3 = Conformers. Step 4 - Match to options: Pair 1 = Conformer, Pair 2 = Stereoisomers (GI), Pair 3 = Conformer. This matches option (c): 1-Conformer ; 2-Stereoisomers (GI) ; 3-Conformer. Why other options fail: - (a) calls Pair 2 a conformer, but locked double bonds prevent free rotation making them geometric isomers. - (b) calls Pair 3 stereoisomers (GI), but both Kekule structures of benzene represent the same molecule and are effectively conformers. - (d) calls Pair 1 stereoisomers (GI), but rotation about single bonds in Pair 1 gives conformers, not geometric isomers. Therefore, the correct answer is C.