One reference book lists D -mannose as being dextrorotatory;...

Key Concepts

D/L Nomenclature

The D and L system is a way of designating the absolute configuration of chiral molecules (originally sugars and amino acids) based on their resemblance to the enantiomers of glyceraldehyde. It describes the relative configuration of the molecule's stereocenters and is independent of how the compound rotates plane-polarized light. In this system, 'D' does not necessarily mean dextrorotatory (rotating light to the right) but refers solely to the spatial arrangement of substituents.

Optical Activity

Optical activity describes the ability of a chiral molecule to rotate the plane of polarized light, which can be either to the right (dextrorotatory, denoted by the prefix '+' or the symbol 'd') or to the left (levorotatory, denoted by the prefix '-' or the symbol 'l'). The actual direction of rotation is an empirical property that is measured experimentally and does not have a fixed correlation with the D or L designation. Thus, a D sugar could be either dextrorotatory or levorotatory depending on its molecular environment and specific interactions.

Relationship Between Absolute Configuration and Optical Rotation

The key concept in reconciling the conflicting descriptions of D-mannose is that the absolute configuration (D or L) determined by methods such as Fischer projections is not inherently linked to its optical rotation (dextrorotatory or levorotatory). It is entirely possible for a molecule with a 'D' configuration to exhibit levorotatory behavior due to differences in the spatial arrangement of the substituents around its chiral centers and how these arrangements interact with plane-polarized light. This separation of concepts means both reference books can accurately describe D-mannose with different optical rotation signs while both being correct in their respective contexts.

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