(R)-2-Mercaptopropan-1-ol (C₃H₈OS) - R or S Configuration

Chiral Center Analysis

(R)-2-Mercaptopropan-1-ol (C₃H₈OS) contains 1 chiral center. Tests S vs O priority. Students see -OH as "more important" but sulfur (Z=16) has a much higher atomic number than oxygen (Z=8) and wins outright at the first atom.. Each chiral center is analyzed below using CIP (Cahn-Ingold-Prelog) priority rules to determine R or S configuration.

CIP Priority Assignment

The four substituents on the chiral center are ranked by atomic number of the directly attached atom, with ties broken by exploring outward along each branch.

• Priority 1: -SH - Sulfur (Z=16) directly bonded - much higher than oxygen (Z=8)
• Priority 2: -CH₂OH - C at 1st → {O, H, H} at 2nd - oxygen at second level beats methyl's H's
• Priority 3: -CH₃ - C at 1st → {H, H, H} at 2nd - all hydrogens, lowest among carbons
• Priority 4: H - Hydrogen (Z=1) - always lowest

R/S Configuration

With the lowest priority group (priority 4) oriented away from the viewer, the remaining three substituents trace a clockwise path from priority 1 to 2 to 3. This gives the R configuration.

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