Everything about Alkyl totally explained
An
alkyl is a
univalent radical consisting of
carbon and
hydrogen atoms, arranged in a chain. The alkyls form a
homologous series with the general formula C
nH
2n+1. They include
methyl, CH
3· (named after
methane),
ethyl (C
2H
5·),
propyl (C
3H
7·),
butyl (C
4H
9·),
pentyl (C
5H
11·), and so on. They are usually constituents of larger molecules, but when occurring independently, they're known as "
free radicals" and are very reactive.
The structure of an alkyl is like that of its
alkane counterpart, but with one less hydrogen atom. For example, this is the structure of methyl, the smallest alkyl:
A
secondary alkyl group is branched with the central carbon atom linked to two carbon residues. Likewise, in
tertiary alkyl groups, there are three carbon-linked substituents. In
neopentane, the central carbon atom is quaternary.
Reactions
Unbonded alkyls are
free radicals, therefore they're formed only as instantaneous intermediates in reactions usually involving other free radicals. When a free radical is formed it very quickly reacts with something else to lose the unpaired electron.
An example of a reaction where alkyls are formed instantly is chlorination. This reaction involves chlorine free radicals, formed by
photodissociation, reacting with an organic molecule, usually an alkane. The chlorine free radical will bond with a hydrogen from an alkane forming hydrochloric acid and leaving an alkyl. This alkyl will then bond to a chlorine to form the product; a chloroalkane. The reaction is very energetic and can be explosive if proper precautions are not taken. This is because of the reactivity of free radicals.
Alkyls are very commonly observed in
mass spectroscopy of
organic compounds.
Naming Alkyls
The naming convention for alkyls is much the same as
alkanes. The suffix is always
-yl. The prefix depends on how many carbon atoms are in the molecule. This uses the same system as for alkanes as shown in this table (taken from
IUPAC nomenclature):
Branched chained structures
These names are used to name branched chained structures, for example 3-methylpentane:
The structure of 3-methylpentane consist of two parts. Firstly, the longest straight chain of
carbon atoms, 5 carbons long with all single bonds, which is named pentane (highlighted blue). Secondly, there's an alkyl group, which is one carbon long, which is methyl (highlighted red). 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's more than one of the same alkyl group attached to a chain, then the prefixes are used on the alkyl groups to indicate multiples (for example 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 was an ethyl group, then the name would become
3-ethyl-2,3-dimethylpentane. Note that when there are different alkyl groups, they're listed in alphabetical order.
Further Information
Get more info on 'Alkyl'.
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