Inorganic chemistry, asubstituent is one or a group ofatoms that replaces (one or more) atoms, thereby becoming amoiety in the resultant (new)molecule.[1] (Inorganic chemistry andbiochemistry, the termssubstituent andfunctional group, as well asside chain andpendant group, are used almost interchangeably to describe those branches from the parent structure,[2] though certain distinctions are made inpolymer chemistry.[3] Inpolymers, side chains extend from thebackbone structure. Inproteins, side chains are attached to thealpha carbon atoms of theamino acid backbone.)
The suffix-yl is used when namingorganic compounds that contain asingle bond replacing one hydrogen;-ylidene and-ylidyne are used withdouble bonds andtriple 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 betweenisomers. Substituents can be a combination of theinductive effect and themesomeric effect. Such effects are also described aselectron-rich andelectron withdrawing. Additionalsteric effects result from the volume occupied by a substituent.
The phrasesmost-substituted andleast-substituted are frequently used to describe or compare molecules that areproducts of achemical 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:
Thesuffix-yl is used inorganic chemistry to form names ofradicals, either separate species (calledfree radicals) or chemically bonded parts of molecules (calledmoieties). It can be traced back to the old name ofmethanol, "methylene" (fromAncient Greek:μέθυméthu, 'wine' andὕληhúlē,[4] '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 aparent compound (and also, usually, on the substituent). According to the 1993IUPAC recommendations:[5]
The suffix-ylidine is encountered sporadically, and appears to be a variant spelling of "-ylidene";[6] 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 areconcatenated:-ylylidene (one single and one double),-ylylidyne (one single and one triple),-diylylidene (two single and one double).
The parent compound name can be altered in two ways:[citation needed]
Note that some popular terms such as "vinyl" (when used to mean "polyvinyl") represent only a portion of the full chemical name.[citation needed]
According to the above rules, a carbon atom in a molecule, considered as a substituent, has the following names depending on the number of hydrogens bound to it, and the type of bonds formed with the remainder of the molecule:
| CH 4 | methane | no bonds |
| −CH 3 | methyl group or methanyl | one single bond to a non-hydrogen atom |
| =CH 2 | methylene group or methanylidene or methylidene | one double bond |
| −CH 2− | methylene bridge or methanediyl or methdiyl | two single bonds |
| ≡CH | methanylidyne group or methylidyne | one triple bond |
| =CH− | methine group or methanylylidene or methylylidene | one single bond and one double bond |
| >CH− | methanetriyl group or methtriyl | three single bonds |
| ≡C− | methanylylidyne group or methylylidyne | one triple bond and one single bond |
| =C= | methanediylidene group or methdiylidene | two double bonds |
| >C= | methanediylylidene group or methdiylylidene | two single bonds and one double bond |
| >C< | methanetetrayl group or methtetrayl | four single bonds |
In a chemicalstructural formula, an organic substituent such asmethyl,ethyl, oraryl can be written asR (or R1, R2, etc.) It is a generic placeholder, theR derived fromradical orrest, which may replace any portion of the formula as the author finds convenient. The first to use this symbol wasCharles Frédéric Gerhardt in 1844.[8]
The symbolX is often used to denote electronegative substituents such as thehalides.[9][10]
Onecheminformatics study identified 849,574 unique substituents up to 12 non-hydrogen atoms large and containing onlycarbon,hydrogen,nitrogen,oxygen,sulfur,phosphorus,selenium, and thehalogens 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 themethyl,phenyl,chlorine,methoxy, andhydroxyl substituents. The total number of organic substituents in organic chemistry is estimated at 3.1 million, creating a total of 6.7×1023 molecules.[11] An infinite number of substituents can be obtained simply by increasing carbon chain length. For instance, the substituents methyl (-CH3) and pentyl (-C5H11).
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