In the Classroom
SOS: A Mnemonic for the Stereochemistry of Glucose Ronald Starkey Department of Chemistry, University of Wisconsin–Green Bay, Green Bay, WI 54311; [email protected]
The mnemonic SOS (Same, Opposite, Same) can be helpful to recall the stereochemistry in either D-glucose or L-glucose. In carbohydrate chemistry the term gluco, such as in the name D-glucose, specifies the configuration of the stereocenters at C-2, C-3, and C-4 relative to that of C-5 (the highest-numbered stereocenter) in an aldohexose; gluco indicates that C-2 has the same configuration, C-3 has the opposite configuration, and C-4 has the same configuration as C-5. Same and opposite can be interpreted either in reference to the absolute configurations specified by R or S, or as relative positions of the OH groups on the Fischer projection structural formula in Figure 1. The SOS mnemonic applies to either D-glucose or L-glucose because the configuration specification is relative to C-5, which is indicated by the Dor L- in the carbohydrate’s name. The term gluco does not specify an absolute configuration of any stereocenter in an aldohexose, but only defines the Same–Opposite–Same con-
R S R R
HO HO H H
2 3 4 5
CH2OH Figure 1. Fischer projection structural formula of D-glucose.
figuration of three stereocenters relative to that of the C-5 reference carbon. The SOS mnemonic can help to reinforce that sugar root names do not provide absolute stereocenter configurations.
Journal of Chemical Education • Vol. 77 No. 6 June 2000 • JChemEd.chem.wisc.edu