If \(\mathrm{D}-(+)\) -glyceraldehyde is treated with \(\mathrm{HCN}\) in aqueous
solution under basic conditions for three days at room temperature,
cyanohydrins are formed (see Chapter 27). The cyanohydrins are not isolated,
but are hydrolyzed to hydroxyacids in the same reaction mixture using dilute
sulfuric acid. In this process, a new stereocenter is formed in the molecule.
The products are diastereomers, formed in unequal amounts, and separable from
each other by recrystallization because of their different physical
properties, including solubilities. The trihydroxybutanoic acids are separated
and then oxidized to tartaric acid with dilute nitric acid, which oxidizes
only the primary alcohol group.
(a) Ignoring stereochemistry, draw the reaction sequence for the
transformations described above and hence deduce the structure of tartaric
acid.
(b) Starting from the Fischer projection of \(\mathrm{D}-(+)-\) glyceraldehyde
and using the reaction scheme from part (a), draw Fischer projections of the
two trihydroxybutanoic acids formed and designate the chiral centers as \(R\) or
\(S\).
(c) Starting from the Fischer projection of \(\mathrm{D}-(+)-\) glyceraldehyde
and using the reaction scheme from part (a), draw Fischer projections of the
two forms of tartaric acid formed and designate the chiral centers as \(R\) or
\(S\).
(d) One form of tartaric acid obtained is optically active, rotating the plane
of polarized light in a negative sense \((-) .\) The other isomer formed, called
meso-tartaric acid, is not optically active. Explain why the other isomer is
not optically active. Draw the dashed-wedged line structure that corresponds
to the Fischer projection of meso-tartaric acid. Can you describe how the two
halves of the molecule are related? Using Fischer projections, write equations
for the conversion of \(L-(-)-\) glyceraldehyde to tartaric acid. Show clearly
the stereochemistry of the tartaric acids that are formed, and indicate
whether you expect them to be optically active.
