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.
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 6.7x10
23 molecules.
*
Functional groups are a subset of substituents
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Cheminformatics Analysis of Organic Substituents: Identification of the Most Common Substituents, Calculation of Substituent Properties, and Automatic Identification of Drug-like Bioisosteric Groups Peter Ertl J. Chem. Inf. Comput. Sci.;
2003; 43(2) pp 374 - 380
Abstract
