Substituent

In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The suffix -yl (meaning "attached to") is used when naming organic compounds that contain a substituent. Additionally, when naming hydrocarbons that contain a substituent, positional numbers are used to indicate which carbon atom the substituent is attached to when such information is needed to distinguish between structural isomers. The polar effect exerted by a substituent is a combination of the inductive effect and the mesomeric effect. Additional Steric effects result from the volume occupied by a substituent.

The phrases most-substituted and least-substituted are frequently used to describe molecules and predict their products. For example:


 * Markovnikov's rule predicts that the hydrogen adds to the carbon of the alkene functional group that has the greater number of hydrogen substituents.
 * Zaitsev's rule predicts that the major reaction product is the alkene with the more highly substituted (more stable) double bond.

Number crunching
One cheminformatics study identified  849,574 unique substituents up to 12 non-hydrogen atoms large and containing only C,H,N,O,S,P,Se and the halogens in a set of 3,043,941 molecules. 50 common substituents are found in only 1% of this set and 438 in 0.1%. 64% of the substituents are unique to just one molecule. The top 5 consists of the phenyl, chlorine, methoxy, hydroxyl and ethyl substituent. The total number of organic substituents in organic chemistry is estimated at 3.1 million creating a total of 6.7×1023 molecules.