monosubstitution

Monosubstitution is a term in organic chemistry describing the substitution of a single hydrogen atom in a molecule by a different substituent. The product is called monosubstituted; additional substitutions produce disubstituted or polysubstituted derivatives. The concept applies across different classes of compounds, including arenes, alkanes, and heterocycles, and it is a key consideration in reaction planning and product distribution.

In arenes, monosubstitution typically refers to replacing one hydrogen on a benzene ring, yielding a monosubstituted benzene such as toluene (methylbenzene) or chlorobenzene. These compounds are often represented as Ar–R, where Ar denotes the aryl ring. Electrophilic aromatic substitution is a common pathway to form monosubstituted benzenes, and the nature of the first substituent can influence the rate and directing effects for any subsequent substitutions.

In alkanes, monosubstitution can occur via free-radical or nucleophilic pathways, replacing one hydrogen with a substituent such as a halogen, hydroxyl, or alkyl group. An example is the formation of chloromethane from methane. Under more forcing conditions, further substitutions can take place, yielding dichloro-, trichloro-, and other polysubstituted products.

Factors that influence monosubstitution include the substrate structure, reaction conditions, the presence of directing groups, and steric hindrance, all of which can affect whether a reaction stops at a single substitution or proceeds to multiple substitutions. Regioselectivity on substituted rings determines where, if at all, a second substitution would occur.

Monosubstitution is fundamental in synthesis design, enabling introduction of a single functional group while controlling subsequent transformations. Characterization typically employs NMR, infrared, and mass spectrometry to confirm the presence and position of the single substituent.