Which of the following will have meso-isomer also?

Meso Compounds

A meso compound is an acyclic or cyclic molecule that contains two or more asymmetric carbon atoms (chiral centers) but is completely achiral (optically inactive) as a whole because it possesses an internal plane of symmetry ($$\sigma$$) or a center of inversion ($$i$$).

Rule of Thumb: For an open-chain molecule to have a meso-isomer, it must be symmetric—meaning both halves of the molecule must have identical structural connectivity and the exact same set of substituents attached to equivalent chiral centers.

• Option A: 2-Chlorobutane

  $$\text{CH}_3-\text{C}^*\text{HCl}-\text{CH}_2-\text{CH}_3$$

  This molecule contains only one chiral center ($$\text{C}_2$$). A compound must contain at least two chiral centers to form an internal plane of symmetry. Therefore, it cannot form a meso-isomer.

  Result: No meso-isomer




• Option B: 2-Hydroxypropanoic acid (Lactic acid)

  $$\text{CH}_3-\text{C}^*\text{H(OH)}-\text{COOH}$$

  This molecule has only one chiral center ($$\text{C}_2$$) bonded to four different groups ($$-\text{H}$$, $$-\text{OH}$$, $$-\text{CH}_3$$, and $$-\text{COOH}$$). Hence, a meso configuration is impossible.

  Result: No meso-isomer




• Option C: 2,3-Dichloropentane

  $$\text{CH}_3-\text{C}^*\text{HCl}-\text{C}^*\text{HCl}-\text{CH}_2-\text{CH}_3$$

  This molecule has two chiral centers ($$\text{C}_2$$ and $$\text{C}_3$$). However, the molecule is structurally unsymmetric because one end terminates in a methyl group ($$-\text{CH}_3$$) while the other terminates in an ethyl group ($$-\text{CH}_2\text{CH}_3$$). Because the two halves are non-identical, no plane of symmetry can exist in any conformation.

  Result: No meso-isomer




• Option D: 2,3-Dichlorobutane

  $$\text{CH}_3-\text{C}^*\text{HCl}-\text{C}^*\text{HCl}-\text{CH}_3$$

  This molecule contains two chiral centers ($$\text{C}_2$$ and $$\text{C}_3$$) and is completely symmetric. Both carbon atoms are bonded to the exact same four groups: $$-\text{H}$$, $$-\text{Cl}$$, $$-\text{CH}_3$$, and a $$-\text{CH(Cl)CH}_3$$ group.

  In its $$(2R, 3S)$$ configuration, an internal plane of symmetry cuts directly through the central $$\text{C}\text{--}\text{C}$$ bond, reflecting one half of the molecule perfectly onto the other.

  Result: Possesses a meso-isomer [Count = 1]

Only 2,3-dichlorobutane satisfies the requirements of having a symmetrical carbon skeleton with equivalent chiral centers to yield a meso-isomer.

Answer: Option D — 2,3-dichlorobutane