See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: A meso compound is an achiral molecule that contains stereocenters but has an internal plane of symmetry, making it optically inactive despite having chiral centers. Step 1: Determine possible structural isomers of C4H8Cl2 that can have stereocenters. The molecular formula C4H8Cl2 has degree of unsaturation = (2×4 + 2 - 8)/2 = 1, but Cl counts as H equivalents, so DoU = (2×4 + 2 - 8 - 0)/2 = 1. Wait, recalculating: DoU = (2C + 2 + N - H - X)/2 = (2×4 + 2 - 8 - 2)/2 = (8 + 2 - 8 - 2)/2 = 0/2 = 0. So no rings or double bonds; it is a saturated dichlorobutane. Step 2: Enumerate the structural isomers of dichlorobutane (C4H8Cl2): - 1,1-dichlorobutane: CH2Cl2-CH2-CH2-CH3 — no stereocenters - 1,2-dichlorobutane: CHCl2 placement... CHClCH(Cl) — C2 and C3 could be stereocenters depending on substitution - 1,3-dichlorobutane: CH2Cl-CH2-CHCl-CH3 — one stereocenter at C3 - 1,4-dichlorobutane: ClCH2CH2CH2CH2Cl — no stereocenters - 2,2-dichlorobutane: CH3-CCl2-CH2-CH3 — no stereocenters - 2,3-dichlorobutane: CH3-CHCl-CHCl-CH3 — two stereocenters at C2 and C3 - 3,3-dichlorobutane: same as 2,2 by renaming - 1,2-dichlorobutane: CH3CH2-CHCl-CH2Cl — one stereocenter Step 3: Identify which structural isomers can have a meso form. For a meso compound, we need at least two stereocenters with an internal plane of symmetry. 2,3-dichlorobutane (CH3-CHCl-CHCl-CH3) is the key candidate: - C2 and C3 are both stereocenters. - The molecule is symmetric: CH3 on both ends. - The (R,S) or (S,R) configuration gives the meso form because C2 and C3 bear the same four groups (H, Cl, CH3, and the other CHClCH3 group), and the internal plane of symmetry between C2 and C3 makes the molecule achiral. - The (R,R) and (S,S) forms are enantiomers (not meso). Step 4: Check other isomers for meso possibility. - 1,3-dichlorobutane: CH2Cl-CH2-CHCl-CH3 — only one stereocenter at C3, cannot be meso. - 1,2-dichlorobutane: CH3CH2-CHCl-CH2Cl — only one stereocenter at C3 (C2 is CH2Cl, not a stereocenter), cannot be meso. - All other isomers have no stereocenters. Step 5: Count meso isomers. Only 2,3-dichlorobutane yields exactly 1 meso isomer (the R,S form). Why other options fail: - (a) 0 is wrong because 2,3-dichlorobutane does have a meso form. - (c) 2 and (d) 3 overcount; no other structural isomer of C4H8Cl2 produces a meso compound. Therefore, the correct answer is B.