Section 5.11 The R,SSystem of Nomenclature for Isomers with More than One Asymmetric Carbon 205The arrow going from the highest priority group (Br) to the next highest priority group
(the C attached to O, C, H) points counterclockwise, suggesting it has the Sconfigura-
tion. However, because we switched two groups before we drew the arrow, C-3 has the
opposite configuration—it has the R configuration. Thus, the isomer is named
(2S,3R)-3-bromo-2-butanol.
Fischer projections with two asymmetric carbons can be named in a similar man-
ner. Just apply the steps to each asymmetric carbon that you learned for a Fischer pro-
jection with one asymmetric carbon. For C-2, the arrow from the group with the
highest priority to the group with the next highest priority points clockwise, suggest-
ing it has the Rconfiguration. But because the group with the lowest priority is on a
horizontal bond, we can conclude that C-2 has the Sconfiguration (Section 5.6).
By repeating these steps for C-3, you will find that it has the Rconfiguration. Thus, the
isomer is named (2S,3R)-3-bromo-2-butanol.
The four stereoisomers of 3-bromo-2-butanol are named as shown here. Take a few
minutes to verify their names.
CH 3CH 3OH
H BrH(2S,3R)-3-bromo-2-butanol1234RCH 3CH 3OH
H BrH123
4S(2S,3R)-3-bromo-2-butanolH 3 CHHHOBrCH 3C CS RBrH 3 C HH H
HOBrCH 3C C241
3
R(2S,3R)-3-bromo-
2-butanol(2R,3S)-3-bromo-
2-butanol(2S,3S)-3-bromo-
2-butanol(2R,3R)-3-bromo-
2-butanolCH 3CH 3OH
H BrHCH 3CH 3H
Br HHOCH 3CH 3OH
Br HHCH 3CH 3H
H BrHOFischer projections of the stereoisomers of 3-bromo-2-butanolCH 3CH 3OH
H BrHCH 3CH 3H
Br HHOCH 3CH 3OH
Br HHCH 3CH 3H
H BrHO(2S,3R)-3-bromo-
2-butanol(2S,3S)-3-bromo-
2-butanol
perspective formulas of the stereoisomers of 3-bromo-2-butanolH 3 CH
BrOHHCH 3C C(2R,3S)-3-bromo-
2-butanolH
HOH 3 CBrCH 3HC C(2R,3R)-3-bromo-
2-butanolH
HOH 3 CHCH 3BrC CH 3 CBr OH
HHCH 3C C