Tartaric Acid


The most abundant naturally occurring form of tartaric acid is L- or (+)tartaric acid.

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(C4H6O6),basically consist of the repetition of the same unit (a carbon attached to a CO2H group, a H and an OH group), and yet  have different properties. The meso compound melts at 146-148 oC while
D-tartaric and L-tartaric acids melt in the172-174oC range. 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 Pasteur. 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 CO2 for the leavening effect in baking unless the recipe calls for lots of lemon juice. Potassium acid tartrate, KC4H5O6, on the other hand, will react with baking soda in the absence of any acidic ingredient within the recipe:

KC4H5O6 + 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.

Data from:

Merck Index 12th Edition.