- For other meanings see Substitution.
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The terms substituent, side-chain, group, branch, or pendant group are used almost interchangeably to describe branches from a parent structure, though certain distinctions are made in the context of polymer chemistry. In polymers, side chains extend from a backbone structure. In proteins, side chains are attached to the alpha carbon atoms of the amino acid backbone.
The suffix yl is used when naming organic compounds that contain a single bond replacing one hydrogen; -ylidene and -ylidyne are used with double bonds and triple bonds, respectively. In addition, when naming hydrocarbons that contain a substituent, positional numbers are used to indicate which carbon atom the substituent attaches to when such information is needed to distinguish between 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.
The suffix -yl is used in organic chemistry to form names of radicals, either separate or chemically bonded parts of molecules. It can be traced back to the old name of methanol, "methylene" (coined from Greek words methy = "wine" and hȳlē = "wood"), which became shortened to "methyl" in compound names. Several reforms of chemical chemical nomenclature eventually generalized the use of the suffix to other organic substituents.
- -yl means that one hydrogen is replaced.
- -ylidene means that two hydrogens are replaced by a double bond between parent and substituent.
- -ylidyne means that three hydrogens are replaced by a triple bond between parent and substituent.
The suffix -ylidine (with "ine" instead of "yne" or "ene") is encountered sporadically, and appears to be a variant spelling of "-ylidene". It is not mentioned in IUPAC guidelines.
For multiple bonds of the same type, which link the substituent to the parent group, the prefixes di, tri, tetra, etc.are used: -diyl (two single bonds), -triyl (three single bonds), -tetrayl (four single bonds), -diylidene (two double bonds)
For multiple bonds of different bond types, multiple suffixes are added: -ylylidene (one single and one double), -ylylidyne (one single and one triple), -diylylidene (two single and one double)
The parent compound name can be altered in two different ways.
- For many common compounds the substituent is linked at one end (the 1 position), which is therefore not explicitly numbered in the formula.The substituent name is modified by stripping ane (see Alkane) and adding the appropriate suffix.This is "recommended only for saturated acyclic and monocyclic hydrocarbon substituent groups and for the mononuclear parent hydrides of silicon, germanium, tin, lead, and boron". Thus, if there is a carboxylic acid called "X-ic acid", an alcohol ending "X-anol" (or "X-yl alcohol"), or an alkane called "X-ane", then "X-yl" typically denotes the same carbon chain lacking these groups but modified by attachment to some other parent molecule.
- The more general method omits only the terminal "e" of the substituent name, but requires explicit numbering of each yl prefix, even at position 1 (except for -ylidyne, which as a triple bond must terminate the substituent carbon chain). Pentan-1-yl is an example of a name by this method, and is synonymous with Pentyl from the previous guideline.
Methane substituents 
According to the above rules, a carbon atom in a molecule, considered as a substituent, has the following names depending on the number of hydrogens bound to it, and the type of bonds formed with the remainder of the molecule:
|−CH3||methyl group||one single bound to a non-hydrogen atom|
|=CH2||methylene group or methylidene||one double bond|
|−CH2−||methylene bridge or methanediyl||two single bonds|
|≡CH||methylidyne group||one triple bond|
|=CH−||methine group, methanylylidene, methylylidene||one single bond and one double bond|
|>CH−||methanetriyl group||three single bonds|
|≡C−||methylylidyne group||one triple bond and one single bond|
|=C=||methanediylidene group||two double bonds|
|>C=||methanediylylidene group||two single bonds and one double bond|
|>C<||methanetetrayl group||four single bonds|
In a chemical structural formula, an organic substituent such as methyl, ethyl, or aryl can be written as R (or R1, R2, etc.) This is a generic placeholder, the R derived from radical or rest, which may replace any portion of the formula as the author finds convenient. The first to use this symbol was Charles Frédéric Gerhardt in 1844.
Statistical distribution 
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. Fifty 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. An infinite number of substituents can be obtained simply by increasing carbon chain length. For instance, the substituents methyl (-CH3) and pentyl (C5H11).
See also 
- Functional groups are a subset of substituents
- D.R. Bloch (2006). Organic Chemistry Demystified. ISBN 978-0-07-145920-4.
- "PAC, 1996, 68, 2287. Glossary of basic terms in polymer science (IUPAC Recommendations 1996)". IUPAC Gold Book. doi:10.1351/pac199668122287. This distinguishes a pendant group as neither oligomeric nor polymeric, whereas a pendant chain must be oligomeric or polymeric.
- acdlabs. com/iupac/nomenclature/93/r93_271. htm "R-2. 5 Substituent Prefix Names Derived from Parent Hydrides". IUPAC. 1993.
- The PubChem database lists 740,110 results for "-ylidene", of which 14 have synonyms where the suffix is replaced by "-ylidine". Another 4 results contain "-ylidine" without listing "-ylidene" as a synonym.
- Jensen, W. B. (2010). "Why Is “R” Used To Symbolize Hydrocarbon Substituents?". Journal of Chemical Education 87 (4): 360–361. doi:10.1021/ed800139p.
- Ertl, P. (2003). "Cheminformatics Analysis of Organic Substituents: Identification of the Most Common Substituents, Calculation of Substituent Properties, and Automatic Identification of Drug-like Bioisosteric Groups". Journal of Chemical Information and Modeling 43 (2): 374. doi:10.1021/ci0255782.