Alkyl

Alkyl

[al-kuhl]
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 CnH2n+1. They include methyl, CH3· (named after methane), ethyl (C2H5·), propyl (C3H7·), butyl (C4H9·), pentyl (C5H11·), and so on. They are usually constituents of larger molecules, but, when occurring independently, they are 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 are 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 in which 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.

In addition, an alcohol is formed when a hydroxyl group is bound to the carbon atom of an alkyl.

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):

Number of carbons 1 2 3 4 5 6 7 8 9 10 11 12
Prefix meth- eth- prop- but- pent- hex- hept- oct- non- dec- undec- dodec-
Molecule name Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl Nonyl Decyl Undecyl Dodecyl

Branched chained structures

These names are used to name branched chained structures, for example 3-methylpentane:

The structure of 3-methylpentane consists of two parts. First, the longest straight chain of carbon atoms, 5 carbons long with all single bonds, which is named pentane (highlighted blue). Second, there is 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 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.

See also

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