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Optical Activity
Concepts tested here
- Racemic mixtures are optically inactive
All Questions
2007 AIPMT-PRE 1 question
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If there is no rotation of plane polarised light by a compound in a specific solvent, thought to be chiral, it may mean thatA chiral compound rotates the plane of plane polarised light.
A racemic mixture contains equal amounts of the two enantiomers (the dextro and the laevo forms).
The rotation caused by one enantiomer is exactly cancelled by the equal and opposite rotation caused by the other (external compensation), so the net rotation is zero.
So if a compound thought to be chiral shows no rotation, it may mean that the sample is a racemic mixture.
It cannot be said that the compound is certainly meso: a meso compound is optically inactive because of a plane of symmetry in its molecule (for example meso-tartaric acid), but it is an achiral molecule, and zero rotation alone does not prove this.
For the same reason the word 'certainly' makes the other statements wrong; zero rotation only allows a possibility.
Hence the best answer is that the compound may be a racemic mixture.