A technique for planning the series of reactions required to synthesize a given molecule. It involves working backwards from the known structure. The molecule to be synthesized (the target molecule) is considered to be split into two parts (a process called disconnection). The parts are not themselves molecules, but are fragments (called synthons) that correspond to actual reagents. A trivial example would be the synthesis of the target molecule

This is an ester. A suitable disconnection is at the O-CO bond, giving two synthons,

The convention is to write this in the form

Here the symbol is a logic symbol for ‘implies’; it is called the retrosynthetic arrow. In this example, suitable reagents corresponding to these synthons would be an alcohol R¹OH and an acyl chloride R²COCl. Note that the disconnection is always made at a point corresponding to a known reaction. In actual schemes, the target molecules are complex and a number of retrosynthetic steps are needed.