Chemistry - STEREO ISOMERISM Question with Solution | TestHub

Ans. (a) The dashed H is pro-R and the wedged H is pro-S. (b) The wedged H is pro-R and the dashed H is pro-S.

Sol. Rule: mentally replace the H under test by D (higher priority than H) and assign the configuration of the resulting stereocentre; if it is R the replaced H is pro-R, if S it is pro-S.

(a) (S)-glyceraldehyde, HOCH₂-CH(OH)-CHO: at the CH₂OH carbon the priorities are OH $>$ CH(OH)CHO $>$ D $>$ H. Replacing the dashed H by D gives the R configuration, so the dashed H is pro-R; replacing the wedged H by D gives S, so the wedged H is pro-S.

(b) Phenylalanine, Ph-CH₂-CH(NH₃⁺)-CO₂^-: at the CH₂ carbon the priorities are CH(NH₃⁺)CO₂^- $>$ Ph $>$ D $>$ H. Replacing the wedged H by D gives the R configuration, so the wedged H is pro-R; replacing the dashed H by D gives S, so the dashed H is pro-S.