Tartaric acid [HO2CCH(OH)CH(OH)CO2H] was an important compound in the history of stereochemistry. Two naturally occurring forms of tartaric acid are optically inactive. One optically inactive form has a melting point of 210-212oC, the other a melting point of 140oC. The inactive tartaric acid with a melting point of 210-212oC can be separated into two optically active forms of tartaric acid with the same melting point (168-170oC). One optically active tartaric acid has [a]25D = +12, and the other, [a]25D = -12. All attempts to separate the other inactive tartaric acid (melting point 140oC) into optically active compounds fail.
(a) Write the three-dimensional structure of the tartaric acid with melting point 140oC.
(b) Write structures for the optically active tartaric acids with melting points of 168-170oC.
(c) Can you determine from the formulas which tartaric acid in (b) has a positive rotation and which has a negative rotation?
(d) What is the nature of the form of tartaric acid with a melting point of 210-212oC?