Stereoisomerism And Chirality

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A molecule of 2,3-dibromobutane contains two similarly substituted stereocentres; how many optically active stereoisomers does this compound actually have?

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Counting stereoisomers begins from the 2^n maximum, where n is the number of stereocentres, but this count must be corrected when the molecule has internal symmetry. The compound 2,3-dibromobutane has two equivalent stereocentres, so the naive maximum is 2^2 = 4. However, because the two ends of the molecule are identically substituted, one of the apparent stereoisomers is a meso compound, which possesses an internal plane of symmetry and is therefore optically inactive despite containing stereocentres. The remaining two structures form a pair of enantiomers that are genuinely chiral and optically active, so the number of optically active stereoisomers is two. The answer 4 wrongly counts the meso form and double-counts it as two distinct active isomers. The answer 3 forgets that the (R,R) and (S,S) forms are the only active species while meso is single and inactive. The answer 1 ignores that enantiomers come in a pair. This meso correction is a standard NCERT and JEE Advanced subtlety. Plausibility check: subtracting the one inactive meso form from the corrected set leaves exactly the enantiomeric pair, giving two optically active isomers.

Stereoisomerism And Chirality

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