thumb|An sp<sup>2</sup>-hybridized [[carbon atom, with re and si faces]]
In stereochemistry, prochiral molecules are those that can be converted from achiral to chiral in a single step, such as changing one atom. An achiral species which can be converted to a chiral in two steps is called proprochiral.
The concept of prochirality is necessary for understanding some aspects of enzyme stereospecificity. Alexander Ogston pointed out that when a symmetrical molecule is placed in an asymmetric environment, such as the surface of an enzyme, supposedly identically placed groups become distinguishable. In this way he showed that earlier exclusion of non-chiral citrate as a possible intermediate in the tricarboxylate cycle was mistaken.
Another biochemical example of prochirality is glycerol. It is achiral, but when it is phosphorylated (at carbon number 3 in stereospecific numbering) the molecule becomes the chiral glycerol 3-phosphate, also called L-α-glycerophosphoric acid. A triacylglycerol having the same fatty acid at carbon 1 and carbon 3 is achiral, but when one of these two is released by hydrolysis, the resulting diacylglyerol is chiral.