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Atom set which has replaced hydrogen atoms on a hydrocarbon's parent chain
the term in chemistry

In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms substituent and functional group, as well as side chain and pendant group, are used almost interchangeably to describe those branches from the parent structure, though certain distinctions are made in polymer chemistry. In polymers, side chains extend from the 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. Substituents can be a combination of the inductive effect and the mesomeric effect. Such effects are also described as electron rich and electron withdrawing. Additional steric effects result from the volume occupied by a substituent. The phrases most substituted and least substituted are frequently used to describe or compare molecules that are products of a chemical reaction. 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:

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

In a chemical structural formula, an organic substituent such as methyl, ethyl, or aryl can be written as R (or R1, R2, etc.) It 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. The symbol X is often used to denote electronegative substituents such as the halides.

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. An infinite number of substituents can be obtained simply by increasing carbon chain length. For instance, the substituents methyl ( CH3) and pentyl ( C5H11).