Understanding the Configuration of a Molecule

The given structure is a chiral molecule with two stereocenters (carbon atoms at positions 2 and 3). To assign R/S configuration, we use the Cahn-Ingold-Prelog (CIP) rules.

Step 1: Identify the Stereocenters

Carbon 2 is bonded to: H, CH₃, Cl, and the rest of the chain (which includes carbon 3).

Carbon 3 is bonded to: H, CH₃, Br, and the rest of the chain (which includes carbon 2).

Step 2: Assign Priorities at Each Stereocenter

For carbon 2:

Cl (atomic number 17) has highest priority (1).
The chain to carbon 3 (C-C-Br) is next (2).
CH₃ (C-H-H) is next (3).
H (atomic number 1) is lowest (4).

For carbon 3:

Br (atomic number 35) has highest priority (1).
The chain to carbon 2 (C-C-Cl) is next (2).
CH₃ (C-H-H) is next (3).
H (atomic number 1) is lowest (4).

Step 3: Orient the Molecule and Determine R/S

For carbon 2: Priorities are 1(Cl), 2(C3 chain), 3(CH₃), 4(H). The lowest priority (H) is on a dash (away from us). The sequence 1→2→3 is clockwise, which corresponds to R configuration.

For carbon 3: Priorities are 1(Br), 2(C2 chain), 3(CH₃), 4(H). The lowest priority (H) is on a wedge (toward us). We reverse the direction: the sequence 1→2→3 is clockwise, but since H is toward us, this becomes S configuration.

Final Answer

The configuration is 2R, 3S.

Related Topics

Stereochemistry
Chirality and Stereocenters
Cahn-Ingold-Prelog (CIP) Rules
Enantiomers and Diastereomers
Fischer Projections

Key Formulae and Theory

The R/S system is based on the CIP rules:

Assign priorities based on atomic number (higher atomic number = higher priority).
If two atoms are identical, look at the next set of atoms.
Orient the molecule so the lowest priority group is away from the viewer.
Trace the path from priority 1 to 2 to 3:
- Clockwise = R (from Latin rectus, meaning right)
- Counterclockwise = S (from Latin sinister, meaning left)

If the lowest priority is toward the viewer, reverse the assignment.