What are the R/S designations of the two stereocenters of L-threonine?

Question

Quizplus · Accepted Answer

The stereochemistry of amino acids is determined by the position of the amino group (NH2) with respect to the carboxyl group (COOH). In L-threonine, the NH2 group is attached to the second carbon (C2) and the COOH group is attached to the third carbon (C3). Therefore, C2 and C3 are the two stereocenters in L-threonine. To determine the R/S designation at each stereocenter, we need to look at the four substituents attached to each carbon and assign priority based on atomic number. At C2, the priorities are NH2 > COOH > H > CH3 (where H represents the fourth substituent, which is a hydrogen atom). If the lowest priority group (H) is pointing away from us, we assign R or S based on the direction of the remaining three groups. In this case, the configuration is S. At C3, the priorities are COOH > CH3 > NH2 > H, and the configuration is R. Therefore, the correct answer is C: 2S,3R.

What Are the R/S Designations of the Two Stereocenters of L-Threonine