Substituent

In organic chemistry and biochemistry, a substituent is an atom or group of atoms which replaces one or more hydrogen atoms on the parent chain of a hydrocarbon, becoming a moiety of the resultant new molecule. The terms substituent and functional group, as well as other ones (e.g. side chain, pendant group) are used almost interchangeably to describe branches from a parent structure, though certain distinctions are made in the context of polymer chemistry. 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. In organic chemistry and biochemistry, a substituent is an atom or group of atoms which replaces one or more hydrogen atoms on the parent chain of a hydrocarbon, becoming a moiety of the resultant new molecule. The terms substituent and functional group, as well as other ones (e.g. side chain, pendant group) are used almost interchangeably to describe branches from a parent structure, though certain distinctions are made in the context of polymer chemistry. 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 species (called free radicals) or chemically bonded parts of molecules (called moieties). 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: The suffix -ylidine (with 'ine' instead of 'yne' or 'ene') is encountered sporadically, and appears to be a variant spelling of '-ylidene'. 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)