It is an analytical technique used in which the deconstruction or fragmentation of targeted organic molecule is done to produce starting material, generally called as “synthon”.

Fragments generated via a particular pattern of break down. It is called as retro synthesis because it is a reversible process of chemical synthesis. E.J Corey gave this concept in his book The logic of chemical synthesis.

It involves some terminologies such as:

Disconnection which means the breakdown of bonds between atoms.

Reagents are those moieties utilized in chemical synthesis.

Synthon ion or radical formed after a disconnection.

FGI or FGE was written as a double arrow, Functional group interconnection, or equivalence.

Rules for retro synthesis:

  • All the obtained fragment ions should be in their stable forms.
  • After the deconstruction of a bond number of fragmented ions should be less as possible.
  • Breakdown of bond always takes place through a carbon hetero atom and they accommodate their lone pair due to high electronegativity than carbon.
  • It is not always possible to get stabilized ions after a disconnection; in that case, FGI (Functional group interconnection) can be used to get sufficiently stabilized ions.
  • Fragmented ions can be positively or negatively charged and hence are replaced by required chemical moieties.
  • The relationship between the present groups needs to be known. If a proper directing group is present then it should be disconnected in the last to get the target molecule.
  • Firstly, the electron-withdrawing group should be disconnected.
  • In some cases FGI is required then it must be done at the proper stage to have the desired orientation.
  • The sequence that requires more steps or additional reactions must be avoided.
  • If a compound contains two parts by heteroatom removal should be next to heteroatoms.


  • To get a clear idea about the structure of naturally available compounds.
  • It is a useful technique to prepare a compound to analyze the mechanism of the reaction and biological metabolism. One good example is the labeled compound.
  • To prepare pharmaceutical compounds, dyes, polymers, etc. which are useful to mankind.
  • Novel problems required modern solutions that lead to the development of modern chemistry.