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Joined 2026-09-21 Message
Optical rotation 2026-10-14
Optical rotation is the signed angle measured in polarimetry. Positive rotation is clockwise for an observer facing the incoming light; negative rotation is counterclockwise. State the viewing convention, wavelength, temperature, solvent, concentration and path length when comparing measurements. R and S configuration specifies spatial arrangement, and does not predict the sign of optical rotation.[ref]
5.5 Sequence Rules for Specifying Configuration. McMurry, Organic Chemistry, §5.5, discussion after Figure 5.10: no simple relation between R/S and rotation sign
R and S configuration 2026-10-14
For a tetrahedral stereocenter, rank its four substituents using stereochemical sequence rules. With lowest-priority group pointing away, a clockwise sequence is and a counterclockwise sequence is . Looking from the other side reverses the apparent sequence, so the viewing condition is essential. The geometric letters A/B/C/D in the tetrahedral mirror-image test carry no assigned priority. Neither nor determines positive or negative optical rotation.[ref]
5.5 Sequence Rules for Specifying Configuration. McMurry, Organic Chemistry, §5.5, Figure 5.8 and discussion after Figure 5.10
Stereochemical sequence rule 2026-10-14
Stereochemical sequence rules rank substituents for R and S configuration. Compare the directly attached chemical atoms by atomic number first, then resolve ties by comparing successive shells at the first difference. Isotopes and multiple chemical bonds require additional rules. This ranking convention does not involve optical rotation.[ref]