Question

$d - $ and $l - $ form of an optically active compound differ in:
A. boiling point
B. melting point
C. specific rotation
D. specific gravity

Answer 1

Hint: An absolute configuration refers to the spatial arrangement of the atoms of a chiral molecular entity (or group) and its stereochemical description e.g. $d - $ or $l - $forms. Absolute configurations for a chiral molecule (in pure form) are most often obtained by X-ray crystallography.

Complete answer:
An enantiomer can be named by the direction in which it rotates the plane of polarized light. Clockwise rotation of the light traveling toward the viewer is labeled (+) enantiomer. Its mirror-image is labeled (−). The (+) and (−) isomers have been also termed as $d - $ and $l - $(for dextrorotatory and levorotatory). But, naming with $d - $ and $l - $ is easy to confuse with $d - $ and $l - $ labeling and is therefore discouraged by IUPAC. An optical isomer can be named by the spatial configuration of its atoms. The $d - $/$l - $ system (named after Latin dexter and laevus, right and left), not to be confused with the d- and l-system, see above, does this by relating the molecule to glyceraldehyde. Glyceraldehyde is chiral itself, and its two isomers are labeled $d - $ and $l - $ (typically typeset in small caps in published work). Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration, and its historical use for this purpose (possibly combined with its convenience as one of the smallest commonly used chiral molecules) has resulted in its use for nomenclature. In this system, compounds are named by analogy to glyceraldehyde, which, in general, produces unambiguous designations, but is easiest to see in the small biomolecules similar to glyceraldehyde.

Thus option C is the correct answer.

Note:
One example of the compounds based on their specific rotation is the chiral amino acid alanine, which has two optical isomers, and they are labeled according to which isomer of glyceraldehyde they come from. On the other hand, glycine, the amino acid derived from glyceraldehyde, has no optical activity, as it is not chiral (achiral).