In chemistry, an alkyl group is a hydrocarbon; typically an alkyl is a part of a larger molecule. The term is usually used loosely; there is no general formula for an alkyl group. In structural formulae, an alkyl group is represented with an R. Usually, alkyl groups resemble hydrocarbons, but with one less hydrogen atom. The smallest alkyl group is methyl, with the following structure:
The species CH3 is rarely encountered, but there are many methyl compounds.
Lower Alkyl refers to an alkyl with one to six carbon atoms.
The word root alkyl is encountered in several contexts in everyday life. Alkylation is an important operation in refineries, for example in the production of high-octane gasoline. Alkylating antineoplastic agents refer to a class of compounds that cause cancer. In such case, the term alkyl is used loosely. For example, nitrogen mustards are well-known alkylating agents, but they are more complex than a mere hydrocarbon. In chemistry, alkyl refers to a group, a substituent, that is attached to other molecular fragments. For example, alkyl lithium reagents have the empirical formula Li(alkyl), where alkyl = methyl, ethyl, etc. A dialkyl ether is an ether with two alkyl groups, e.g., diethyl ether (O(C2H5)2).
Usually alkyl groups are attached to other atoms or groups of atoms. Free alkyls occur as neutral compounds, as anions, or as cations. The neutral alkyls are called free radicals. The cations are called carbocations. The anions are called carbanions. Such species are usually encountered only as transient intermediates, but some are quite stable and can be "put into a bottle." Typically alkyl cations are generated using super acids, alkyl anions are observed in the presence of strong bases, and alkyl radicals are generated by a photochemical reaction. Alkyls are commonly observed in mass spectrometry of organic compounds.
Alkyls form homologous series. The simplest series have the general formula CnH2n+1. Alkyls include methyl, CH3· (named after methane), ethyl (C2H5·), propyl (C3H7·), butyl (C4H9·), pentyl (C5H11·), and so on. Alkyl groups that contain one ring have the formula CnH2n-1, e.g. cyclopropyl and cyclohexyl. The naming convention is taken from IUPAC nomenclature:
|Number of carbons||1||2||3||4||5||6||7||8||9||10||11||12|
These names are used to name branched chained structures, for example 3-methylpentane:
The structure of 3-methylpentane is viewed as consisting of two parts. First, five atoms comprise the longest straight chain of carbon centers. The parent five-carbon compound is named pentane (highlighted blue). The methyl "substituent" or "group" is highlighted red. According the usual rules of nomenclature, alkyl groups are included in the name of the molecule before the root, as in methylpentane. This name is, however, ambiguous, as the methyl branch could be on various carbon atoms. Thus, the name is 3-methylpentane to avoid ambiguity: The 3- is because the methyl is attached to the third of the 5-carbon atoms.
If there is more than one of the same alkyl group attached to a chain, then the prefixes are used on the alkyl groups to indicate multiples (i.e., di, tri, tetra, etc.)
This compound is known as 2,3,3-trimethylpentane. Here, there are three identical alkyl groups attached to carbon atoms 2, 3, and 3. These numbers are included in the name to avoid ambiguity on the position of the groups, and 'tri' indicates that there are three identical methyl groups. If one of the methyl attached to the third carbon atom were an ethyl group, then the name would become 3-ethyl-2,3-dimethylpentane. Note that, when there are different alkyl groups, they are listed in alphabetical order.