atom (group) that replaces one or more hydrogen atoms attached to a parent structure or characteristic group except for hydrogen atoms attached to a chalcogen atom

In organic chemistry and biochemistry, substituent is the name for a single atom or for a group of atoms that take the position of another atom in a molecule. They replace this atom. This reaction is called a substitution reaction. The atom or group of atoms that gets replaced is called the leaving group.

The terms substituent, side chain, group, branch, or pendant group are used in the same way to describe branches from a parent structure.[1] These terms have different meanings in polymer chemistry.[2] In polymers, side chains extend from a backbone structure, while they are attached to the alpha carbon atoms of the amino acid backbone in proteins.

Organic chemistry has rules for naming compounds with substituent groups. The substituent group is listed first with a suffix to describe how it is attached to the main carbon chain. The suffix -yl is used when naming organic compounds that contain a single bond replacing one hydrogen. -ylidene and -ylidyne are used for double bonds and triple bonds respectively. When naming hydrocarbons that contain a substituent, position numbers may be used. These number will indicate which carbon atom the substituent is attached to. One case where this information is needed is that of isomers.

The phrases most-substituted and least-substituted are often 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.


In a structural diagram or formula, the letters R or X are used to represent a substituent group. An organic substituent such as methyl, ethyl, aryl can be written as R (or R1, R2, etc.) This is a generic placeholder. The R derived from radical or rest. The R can represent as many atoms as the author wants. The first to use this symbol was Charles Frédéric Gerhardt in 1844.

The symbol X can represent electronegative substituents such as the halides.[3]

Related pagesEdit


  1. D.R. Bloch (2006). Organic Chemistry Demystified. ISBN 9780071459204.
  2. "PAC, 1996, 68, 2287. Glossary of basic terms in polymer science (IUPAC Recommendations 1996)". IUPAC Gold Book. doi:10.1351/pac199668122287. S2CID 98774337. {{cite journal}}: Cite journal requires |journal= (help) This distinguishes a pendant group as neither oligomeric nor polymeric, whereas a pendant chain must be oligomeric or polymeric.
  3. Jensen, W. B. (2010). "Why Is "R" Used To Symbolize Hydrocarbon Substituents?". Journal of Chemical Education. 87 (4): 360–361. Bibcode:2010JChEd..87..360J. doi:10.1021/ed800139p.