Why does the optical rotation prefix differ for glyceraldehyde and glyceric acid when both rotate polarised light in the same direction?

When both glyceraldehyde and glyceric acid rotate the plane-polarised light in the same direction, why is glyceraldehyde assigned as D(+) and glyceric acid assigned as D(-)?


The first image shows the chemical conversion of D(+)-glyceraldehyde to D(-)-glyceric acid. The second image shows the rule for assigning absolute configuration.


[![enter image description here](https://i.stack.imgur.com/gt5C6.jpg)](https://i.stack.imgur.com/gt5C6.jpg)




D and L are determined by arbitrary rules that you've referenced. (+) and (-) specify the net optical rotation of plane polarized light by a sample. There is no way for a human to determine (+) and (-) just by looking at a molecule. It is fixed, but it is whatever it is.


There is no relationship between the D/L and +/- designation across all substances. However, for a given substance the relationship is fixed.


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The D/L designation is fully irrelevant, as Zhe’s answer points out. The same is true for R/S or any other minor standards that are based on the molecular structure alone. Whether a molecule rotates polarised light to the right ($+$) or to the left ($-$) can be determined only by experiment.


There is a clear discrepancy between your two statements:


* > 
> D-(+)-glyceraldehyde and D-(−)-glyceric acid
> 
> 
> 


and
* > 
> rotate the plane of light in the same direction
> 
> 
>


Both cannot be true; either the naming was incorrect and D-glyceric acid should be called D-(+)-glyceric acid or the measurement result was in fact the opposite, i.e. the polarisation is opposite. The answer by Zhe and Loong’s comment assume the latter. However, checking Sigma-Aldrich’s webpage, I get told that the optical activity of D-glyceric acid is positive by the sign convention. That would mean that it should be correctly labelled as D-(+)-glyceric acid. The caveat being that the disodium salt was measured.


The dispute will only be settled by confirmation experiment or by literature citations. In any case, one statement will fall. Either the designation is correct; then they polarise in different directions. Or the result you state is correct, then glyceric acid’s designation is wrong.


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