(S)-α-Methylserine (C₄H₉NO₃) - R or S Configuration

Chiral Center Analysis

(S)-α-Methylserine (C₄H₉NO₃) contains 1 chiral center. A quaternary stereocenter - the chiral carbon has NO hydrogen. Four different non-H groups: NH₂, COOH, CH₂OH, and CH₃. Students must identify CH₃ as lowest priority.. 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: -NH₂ - Nitrogen directly bonded (atomic number 7) - highest first-atom priority
• Priority 2: -COOH - Carboxyl: C bonded to O, O, O at second level - highest among carbon substituents
• Priority 3: -CH₂OH - Hydroxymethyl: C bonded to O, H, H at second level - O beats C but only one O vs COOH's three
• Priority 4: -CH₃ - Methyl: C bonded to H, H, H - lowest priority carbon substituent (no hydrogen on this chiral center!)

R/S Configuration

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

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