Question

a. In general is there any difference between R and S, and + and -? between d- and l-, and + and -?

b. Discuss the validity of the following statements:

        1. A mixture of (+)- and (--)-carvone can be separated by column chromatography on silica gel using hexanes-acetone as elution.

        2. In general, column chromatography on silica gel is run with a series of solvents of increasing polarity.

        3. Carvone is more polar than limonene.

c. Which compound, limonene or carvone, do you expct would elute first from the silica gel column? Which one would have shorter retention time on the GC? Briefly justify your answer.

Answer 1

(a) (i) R and S are absolute configuration of a chiral centre in stereochemistry, it refers to the spatial arrangement of the atoms of a chiral molecular group or entity and its stereochemical description i.e., R or S.

Or we can say The R / S system is an important nomenclature system for denoting enantiomers. (Enantiomers are non-superimposable mirror images)

Whereas, '+' and '-' signifies direction in which pure enantiomers of any compound rotate plane-polarized light. (+)-indicates clockwise rotation, while (–)-indicates counterclockwise rotation or we can say it depicts the direction of optical rotation.

Both "R" and "S" stereocentres have the ability to be + and -, therefore there is no general correlation between these two labels.

(ii) -d and -l and + and - are same, -d refers to dexorotatory and can be depicted by the sign '+' while -l refers to leavorotatory and can be depicted by the sign '-'.

(Note: "d-" and "l-" prefixes are distinct from the uppercase "D-" and "L-" prefixes)