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D-tartaric acid

Why is the isomer of tartaric acid above labelled as ᴅ and (−)?

  1. it rotates plane-polarized light clockwise
  2. it rotates plane-polarized light anticlockwise
  3. it is an equimolar mixture of the enantiomorphs
  4. its configuration is related to that of ᴅ-glyceraldehyde

If I am not wrong, the ᴅ in ᴅ-glyceraldehyde refers to (+)-glyceraldehyde, and likewise ʟ-glyceraldehyde is (−)-glyceraldehyde. The (+) and (−) labels indicate that these isomers are dextrorotatory and levorotatory, respectively.

So in ᴅ-(−)-tartaric acid, what does the ᴅ mean, if not dextro, and what does the (−) mean, if not levo (the minus sign indicating anticlockwise rotation of plane-polarized light)? How can one molecule be both?

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    $\begingroup$ Wikipedia has a useful section on this topic. $\endgroup$ – Freddy Sep 30 '14 at 9:20
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As far as I remember, small d always means the same as (+), not small cap ᴅ. So, a dextrorotatory compound is prefixed with either (+) or d while a levorotatory compound is prefixed with either (−) or l.

Small caps ᴅ and ʟ prefixes have different meaning: if an enantiomer can in principle be synthesized from naturally occurring (+)-glyceraldehyde without affecting its configuration, it is considered the ᴅ-form. Otherwise, it is considered the the ʟ-form. And, say, the ᴅ-form of a molecule is not necessarily a dextrorotatory one.

So, the answers to your question are options 2 (the (−) sign indicates that it rotates plane-polarized light anticlockwise), and option 4 (the ᴅ label indicates that it is related to ᴅ-glyceraldehyde).

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Dextrorotatory and levorotatory are a set of two terms that designate the direction polarised light is rotated to by a specific compound. Chiral compounds can be identified as being chiral due to their optical activity only. The optical activity is a physically measurable parameter, determined in polarimeters and calculated to a specific optical activity by dividing by concentration and length of the cuvette. Rotation to the right is indicated by a $(+)$ sign, while rotation to the left is indicated by a $(-)$.

The names dextro- and levorotatory derive from the Latin words dexter (right, as in right hand) and laevus (left, as in left hand).


The D/L-nomenclature is entirely different from the above. While the prefixes D and L also derive from the same Latin words, they are used in a different way. They key to D/L-nomenclature is a Fischer projection. If a chiral molecule is drawn in such a (linear) projection, and the group of the highest priority (typically the group with the highest oxidation state) is at the top, a compound is defined as L if the non-hydrogen atom on the lowest asymmetric carbon of the chain points to the left. Likewise, if it points to the right, a compound is termed D.

D-Glyceraldehyde L-Tartaric acid D-Tartaric acid
From left to right: D-Glyceraldehyde, L-tartaric acid, D-tartaric acid.


It is very important to know that there is no connection between D/L and the direction of rotation of polarised light! Likewise, one cannot deduce the direction of optical activity from knowing whether a compound is (R) or (S). D/L and (R)/(S) can be determined on paper or via crystal structures while $(+)/(-)$ can only be determined experimentally in a polarimeter or similar experiments.

That D-glyceraldehyde is dextrorotatory is a mere (but maybe lucky) coincidence.


Therefore, the correct answers to your given question are (2) (it rotates to the left; $(-)$) and (4) (it relates back to D-Glyceraldehyde).

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