If a chiral atom is in the "R" configuration, what direction does the arc go?

In a molecule with a chiral center, the "R" and "S" nomenclature is used to describe the stereochemistry based on the priority of the groups attached to the chiral atom, according to Cahn-Ingold-Prelog rules. To determine whether the configuration is "R" or "S," you first assign priority to the substituents attached to the chiral center based on atomic number (higher atomic numbers receive higher priority).

When visualizing the chiral center with the lowest priority substituent pointed away from you, you then look at the arrangement of the remaining three groups. If the order of priority from highest to lowest goes in a clockwise direction, the configuration is designated as "R." This clockwise arc represents a positive rotation, which aligns with the "R" designation.

Conversely, if the order were counterclockwise, it would be labeled "S." Therefore, when a chiral atom is confirmed to be in the "R" configuration, the arc of the priority arrangement indeed goes in a clockwise direction. This is an essential concept in stereochemistry, helping chemists to visualize and understand the 3D arrangement of molecules.