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Which of the following is a meso compound? (A) HOOC-CH(Cl)-CH(Cl)-COOH, with the two Cl atoms on opposite sides (anti configuration in Fischer projection) (B) A methyl-substituted alkane (structure not specified here) (C) HOOC-CH(CH3)-CH(Cl)-COOH with CH3 and Cl as substituents (D) cis-1,3-dibromocyclobutane

1. HOOC-CH(Cl)-CH(Cl)-COOH, with the two Cl atoms on opposite sides as shown in option (A)
2. Me-substituted alkane shown in option (B)
3. HOOC-CH(CH3)-CH(Cl)-COOH, with CH3 and Cl as shown in option (C)
4. 1,3-dibromocyclobutane shown in option (D)

Correct answer: HOOC-CH(Cl)-CH(Cl)-COOH, with the two Cl atoms on opposite sides as shown in option (A)

Solution

Option A is meso-2,3-dichlorosuccinic acid: the molecule has two identical chiral centres in R,S (anti) configuration, giving an internal plane of symmetry and hence no net optical activity. Option C has two different substituents (CH3 and Cl) so no such internal symmetry exists.

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