Statement I: A pair of enantiomers differ from each other in their melting points and boiling points.
Statement II: The sign of optical rotation of a compound is not necessarily related to the absolute configuration of its molecule.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Only Statement II is correct.
Enantiomers are stereoisomers that are non-superimposable mirror images of each other. The sign of optical rotation, (+) or (−), is found by experiment with a polarimeter; the configuration is the actual arrangement of groups in space around the asymmetric carbon.
Statement I: A pair of enantiomers differ from each other in their melting points and boiling points. — False. Enantiomers have identical physical properties: melting point, boiling point, refractive index. They differ only in the direction in which they rotate plane polarised light; one is dextrorotatory and the other laevorotatory.
Statement II: The sign of optical rotation of a compound is not necessarily related to the absolute configuration of its molecule. — True. Two different compounds with the same configuration can rotate light in opposite directions. When (−)-2-methylbutan-1-ol is heated with concentrated HCl, no bond to the asymmetric carbon breaks, so the configuration is kept, yet the product is (+)-1-chloro-2-methylbutane.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “Enantiomers possess identical physical properties namely, melting point, boiling point, refractive index, etc. They only differ with respect to the rotation of plane polarised light. If one of the enantiomer is dextro rotatory, the other will be laevo rotatory. However, the sign of optical rotation is not necessarily related to the absolute (actual) configuration of the molecule.”
“…the sign of optical rotation has changed in the product. This is so because two different compounds with same configuration at asymmetric centre may have different optical rotation.”