Which structure represents (R)-1-chloro-1-fluoroethane?
A)I
B)II
C)III
D)More than one of these choices.
E)None of these choices.

Question

Which structure represents (R)-1-chloro-1-fluoroethane? 
A)I 
B)II 
C)III 
D)More than one of these choices. 
E)None of these choices.

Quizplus · Accepted Answer

The molecule has two chiral centers, carbon 1 and carbon 2. The priority groups for carbon 1 are Cl, C, and H, while for carbon 2 they are F, C, and H. Using the Cahn-Ingold-Prelog priority rules, the Cl and F groups are assigned priority groups 1 and 2, respectively. The H atoms on each carbon are assigned priority group 4, and the two remaining carbon atoms have priority group 3. To determine the absolute configuration, we need to look at the molecule in 3D and compare it with a reference molecule of known configuration. In this case, the reference molecule is (R)-2-chlorobutane. By aligning the two molecules and comparing them, we can see that the Cl and F atoms are on opposite sides (trans) of the molecule, and that H, C, and C atoms are also in the same order. Therefore, the configuration at carbon 1 must be R. The correct structure is option B.

Which Structure Represents (R)-1-Chloro-1-Fluoroethane