Substituent

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.

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:

The suffix -yl is used in organic chemistry to form names of radicals, either separate species (called free radicals) or chemically bonded parts of molecules (called moieties). It can be traced back to the old name of methanol, "methylene" (from Ancient Greek: μέθυ méthu, 'wine' and ὕλη húlē, 'wood', 'forest'), which became shortened to "methyl" in compound names, from which -yl was extracted. Several reforms of chemical nomenclature eventually generalized the use of the suffix to other organic substituents. ^{[citation needed]}

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 the 1993 IUPAC recommendations:

The suffix -ylidine is encountered sporadically, and appears to be a variant spelling of "-ylidene"; it is not mentioned in the IUPAC guidelines.

For multiple bonds of the same type, which link the substituent to the parent group, the infixes -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 types, multiple suffixes are concatenated: -ylylidene (one single and one double), -ylylidyne (one single and one triple), -diylylidene (two single and one double).