Specific rotation conventions

As pointed out on page 276, the nomenclature of tetraacetyl-/S-D-glucopyranosylbenzene (IV), its anomer,66 and related compounds was based on their specific rotations in accordance with Hudson s conventions,63 and not on the actual stereochemical configurations of the phenyl group about the anomeric centers. Thus, for example, it has not been known if the rotationally designated /3-D-xylopyranosylbenzene (LX), actually possesses the configurational tram /3-structure (LXI), or cis a-structure (LXII). 

Some important aspects of topochemical polymerizations can be understood by inspection of Eq. (1), All reactivity comes about by very specific rotations of the monomers and by 1,4-addition of adjacent units and an extended, fully conjugated polymer chain is formed. The unique feature of the topochemical polymerization of diacetylenes is the fact that in many cases the reaction can be carried out as a single phase process. This leads to macroscopic, defect-free polymer single crystals which cannot be obtained, in principle, by crystallization of ready-made polymers by conventional methods. Thus, polydiacetylenes are ideal models for the investigation of the behaviour of macromolecules in their perfect three dimensional crystal lattice. 

By convention we always define as positive, whatever the absolute configuration and the sign of the specific rotation of the product. 

Enantiomers rotate plane-polarised light in the opposite directions. The (-l-)-enantiomer (or dextrorotatory enantiomer) rotates the light to the right, while the (—)-enantiomer (or laevorotatory enantiomer) rotates the light to the left. The amount of rotation is called the specific rotation, [a]D, and this is measured using a polarimeter. (By convention, the units of [a]D are often not quoted.)... 

Note that the units of a measured optical rotation a are degrees, but, by convention, specific rotation [a] is quoted without units. 

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