Chemistry - STEREO ISOMERISM Question with Solution | TestHub
How many compounds have a stereoisomer that is a meso compound : suppose this value is x. So the value of x + 7 is :
(A) 2-bromo-3-chlorobutane (B) 2,2-dibromopropane (C) 
(D) 2,3-dichlorobutane
(E) 
(F) 
(G) CO2H-(CH-OH)2-CO2H (tartaric acid) (H) 1,2-dimethylcyclohexane
(I) 
(J) CH3-CH(OH)-CO2H (lactic acid)
Answer:
Solution:
Ans. 12
Sol. A meso stereoisomer needs at least two stereocentres and an internal plane of symmetry. (A) CH3-CHBr-CHCl-CH3: two centres but Br and Cl differ - no meso form. (B) 2,2-dibromopropane: no stereocentre. (C) 2,3-Dimethylbutane: each central carbon bears two identical methyl groups - no stereocentre. (D) 2,3-Dichlorobutane: meso (2R,3S) exists. (E) 1-Bromo-2-methylcyclohexane: two centres but no internal symmetry - no meso. (F) 3,4-Dimethylhexane: meso (3R,4S) exists. (G) Tartaric acid: meso (2R,3S) exists. (H) 1,2-Dimethylcyclohexane: the cis isomer is meso. (I) 1,3-Dimethylcyclohexane: the cis isomer is meso. (J) Lactic acid: only one stereocentre. So x = 5 [(D), (F), (G), (H), (I)] and x + 7 = 12.
