Questions: Enantiomers, Chirality, and R/S Configuration

Question 1

Which statement about a pair of enantiomers is FALSE?

• A. Enantiomers rotate plane-polarized light in opposite directions by equal amounts
• B. Enantiomers have identical boiling points when measured in an achiral environment
• C. The R-configured enantiomer always rotates light clockwise (+)
• D. Enantiomers react at identical rates with achiral reagents

Answer: (C)

R/S configuration and optical rotation (+/−) are completely independent naming systems. CIP R/S is determined by atomic-number priority rules; optical rotation is an experimentally measured physical property. An R-configured compound can be either (+) or (−) depending on its structure. There is no systematic relationship between the two. This is one of the most persistent misconceptions in stereochemistry.

Question 2

A molecule with two stereocenters should be chiral.

• T. True
• F. False

Answer: False

Meso compounds contain two or more stereocenters but are achiral overall because they possess an internal plane of symmetry (or other symmetry element). The classic example is meso-tartaric acid, which has one R and one S stereocenter but whose mirror image is superimposable on itself. Stereocenters are necessary but not sufficient for chirality; the overall molecular symmetry determines chirality.

Question 3

You are assigning R/S configuration and find that the lowest-priority group (group 4) is pointing toward you instead of away. You observe the remaining three groups in clockwise order (1→2→3). What is the correct configuration, and why?

Model answer: The correct configuration is S. When group 4 points toward you, your perspective is inverted relative to the CIP convention. A clockwise reading from this flipped perspective corresponds to counterclockwise (S) from the correct viewpoint, so you must invert the result.

The CIP protocol defines R/S by looking at groups 1→2→3 with group 4 pointing away from the observer. When group 4 points toward you, everything appears mirrored — clockwise becomes what would be counterclockwise from the standard perspective. The rule is: if group 4 points toward you, flip your observed rotation to get the correct assignment.