For the audio technique known as side-chaining, see Dynamic range compression § Side-chaining. For other meanings, see Substitution (disambiguation).

In organic chemistry and biochemistry, a substituent is an atom or group of atoms which replaces hydrogen atom on the parent chain of a hydrocarbon, becoming a moiety of the resultant new molecule. The terms substituent, side chain, group, branch, or pendant group are used almost interchangeably to describe branches from a parent structure,[1] though certain distinctions are made in the context of polymer chemistry.[2] 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. In this terminology, methane is used as a reference of comparison. Using methane as a reference, for each hydrogen atom that is replaced or "substituted" by something else, the molecule can be said to be more highly substituted. For example:


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 nomenclature eventually generalized the use of the suffix to other organic substituents.

The use of the suffix is determined by the number of hydrogen atoms that the substituent replaces on a parent compound (and also, usually, on the substituent). According to 1993 IUPAC guidelines:[3]

The suffix -ylidine (with "ine" instead of "yne" or "ene") is encountered sporadically, and appears to be a variant spelling of "-ylidene".[4] 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 ways.

Note that some popular terms such as "vinyl" (when used to mean "polyvinyl") represent only a portion of the full chemical name.

The suffix "-yl" arose by extracting it from the word "methyl".

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:

methane no bonds
methyl group one single bond to a non-hydrogen atom
methylene group or methylidene one double bond
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.) T 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.[5]

The symbol X is often used to denote electronegative substituents such as the halides.[6][7]

Statistical distribution

One cheminformatics study identified 849,574 unique substituents up to 12 non-hydrogen atoms large and containing only carbon, hydrogen, nitrogen, oxygen, sulfur, phosphorus, selenium, and the halogens in a set of 3,043,941 molecules. Fifty substituents can be considered common as they are found in more than 1% of this set, and 438 are found in more than 0.1%. 64% of the substituents are found in only one molecule. The top 5 most common are the methyl, phenyl, chlorine, methoxy, and hydroxyl substituents. The total number of organic substituents in organic chemistry is estimated at 3.1 million, creating a total of 6.7×1023 molecules.[8] 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


  1. D.R. Bloch (2006). Organic Chemistry Demystified. ISBN 978-0-07-145920-4.
  2. "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.
  3. "R-2. 5 Substituent Prefix Names Derived from Parent Hydrides". IUPAC. 1993.
  4. 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.
  5. See:
    • Charles Gerhardt, Précis de chimie organique (Summary of organic chemistry), vol. 1 (Paris, France: Fortin et Masson, 1844), page 29. From page 29: "En désignant, par conséquent, les éléments combustibles par R, sans tenir comptes des proportions atomiques de carbone et d'hydrogène, on peut exprimer d'une manière générale: Par R. — Les hydrogènes carbonés." (Consequently, by designating combustible components by R, without considering the atomic proportions of carbon and hydrogen, one can express in a general way: By R — hydrocarbons.)
    • William B. Jensen (2010) "Ask the Historian: Why is R Used for Hydrocarbon Substituents?," Journal of Chemical Education, 87: 360-361. Available at: University of Cincinnati.
  6. Jensen, W. B. (2010). "Why Is "R" Used To Symbolize Hydrocarbon Substituents?". Journal of Chemical Education. 87 (4): 360–361. doi:10.1021/ed800139p.
  7. The first use of the letter X to denote univalent electronegative groups appeared in:
  8. 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.
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