Home >
Community >
Why does the optical rotation prefix differ for glyceraldehyde and glyceric acid when both rotate polarised light in the same direction?
Upvote
20
Downvote
+ Chirality
+ Chemistry
Posted by
Misha Firer
Why does the optical rotation prefix differ for glyceraldehyde and glyceric acid when both rotate polarised light in the same direction?
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.
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.
Thank you Jan......so basically these configurations are only to say the spatial arrangement of groups around a chiral carbon but cannot determine the optical rotation . Am I right ?More
I think as your said, only R and S nomenclature of a compound can be assigned theoretically by looking at the compound. But, + and- can be assigned only after experimentally determining the compounds optical activity using a polarimeter. This is what I have understood. Thank you your answerMore
Upvote
VOTE
Downvote
more replies
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.
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.
Thank u for your answer. But as far as I know D,L is different from d,l. D,L refers to Relative configuration whereas d,l or +,- or R,Srefers to absolute configuration. I want to know why both glyceraldehyde and glyceric acid are assigned different absolute configurations when both rotate plane polarised light in same direction or is there a difference between +,- and R,S. kindly correct me if I am wrongMore
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:
and
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.
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:
and
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.
More
VOTE
VOTE
VOTE
VOTE
VOTE
VOTE
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.
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.
More
VOTE
VOTE