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) 2,3-dibromobutane (B) 2-bromo-3-chloropentane (C) 1,2-dimethylcyclopropane (D) 2,2-dichlorobutane
(E) 2,4-dibromopentane (F) 3,4-dimethylhexane (G) 1,2-dichlorocyclopropane (H) 2-methylbutane
Answer:
Solution:
A meso stereoisomer needs two stereocentres carrying identical sets of groups and an internal plane of symmetry.
(A) CH3-CHBr-CHBr-CH3 : two identical centres - the (2R,3S) form is meso. Yes.
(B) CH3-CHBr-CHCl-CH2-CH3 : Br and Cl differ, the two centres are not equivalent - no meso form.
(C) 1,2-Dimethylcyclopropane : the cis form is meso. Yes.
(D) 2,2-Dichlorobutane : no stereocentre. No.
(E) CH3-CHBr-CH2-CHBr-CH3 : the (2R,4S) form is meso. Yes.
(F) CH3CH2-CH(CH3)-CH(CH3)-CH2CH3 : two identical centres - the (3R,4S) form is meso. Yes.
(G) 1,2-Dichlorocyclopropane : the cis form is meso. Yes.
(H) 2-Methylbutane : no stereocentre. No.
x = 5, so x + 7 = 12.
Ans. 12
