The most abundant naturally occurring form of tartaric acid is L- or
It has the following 3d structure:
This acid is not only found in grapes but in a wide variety of fruits. It is
fairly acidic: a 0.1 N solution will have a pH of 2.2.
The molecule that constitutes its mirror image is less common in nature: D-
or (-) tartaric acid. The microorganism Penicillium notatum ,among
others, will convert
a mixture of the two images into the latter form.
If the OH and H group on one of the chiral carbons are inverted, then the
molecule will have an internal plane of symmetry. Such a molecule , meso tartaric
acid, no longer exists in two forms; both images would be superimposable.
It's amazing how three molecules can all have the same formula(C4
consist of the repetition of the same unit (a carbon attached to a CO2
group, a H and an OH group), and yet have different properties. The meso
compound melts at 146-148 o
D-tartaric and L-tartaric acids melt in the172-174o
The less natural form of tartaric acid (D-tartaric) has a specific
optical rotation of -12o
, whereas the enantiomer L-tartaric
acid is of course + 12o
. The first to realize this peculiar
behaviour was Louis
. A compound known as cream of tartar precipitates after
grape juice has fermented. This nearly pure substance(99.5%) turns out
to be potassium acid tartrate, basically an acidic salt version of
tartaric acid. This is the stuff that's added to baking soda to create
baking powder. You see, baking powder, NaHCO3
will not create
for the leavening effect in baking unless the recipe
calls for lots of lemon juice. Potassium acid tartrate, KC4
on the other hand, will react with baking soda in the absence of any
acidic ingredient within the recipe:
+ NaHCO3 --> CO2 + H2O + KC4H4O6Na
My wife once tried to substitute baking powder with baking soda in a
recipe that contained very fewacidic ingredients. The result: the most
bitter cookies I've ever tasted. The bitterness was, as you
guessed, due to unreacted NaHCO3, which is a base.
Merck Index 12th Edition.