Alkyl: concept, most common radicals, structure

The alkyl, as a group, is symbolized by the letter R. Thus, the molecular structure of many organic compounds can be generalized with the introduction of R. In them, R is just a part of the molecule, linked to its skeleton or to a reactive functional group.

The image above exemplifies the aforementioned. Methane, CH 4 , is an alkane, and when it loses one of its hydrogens, binding to a molecule or group, it becomes methyl, CH 3 -. Methyl is not a compound: it does not exist as a free molecule; unless it is the methyl radical, CH 3 ·, which is highly reactive.


The nomenclature of the rents has nothing to do with the rent of a property. This comes from the same nomenclature of the alkane from which it derives. In the image above, for example, methane transforms into the methyl group. Thus, it is enough to replace the ending -anus , from the name of the alkane, by the ending -ilo .

Another example is ethane, CH 3 CH 3 . By losing one of its hydrogen and bonding to a molecule or group, it becomes the ethyl group, CH 3 CH 2 -.

The same occurs with all other alkanes, even those that consist of cyclic chains such as cyclohexane, which is transformed into cyclohexyl.

Most common alkyl radicals

Alkyl radicals are those “loose and reactive pieces” that are obtained when R is separated from a molecule. Their abundances are proportional to that of the alkyl groups from which they are derived. For example, the methyl group, CH 3 – and the methyl radical, CH 3 ·, are relatively equal in common.

In general, the radicals or alkyl groups that come from alkanes that contain less than five carbon atoms in their skeleton are the most common. That is to say, that above pentane and all its isomers, these radicals become more difficult to find.

And the second is that in nature there are “isomer soups”, which are very difficult to purify by distillation because of the small difference that exists between their boiling points .

Thus, the alkyls and their most abundant radicals are short-chain, with a number of carbon atoms less than six. Examples of these alkyls are: CH 3 -, CH 3 CH 2 -, CH 3 CH 2 CH 2 -, CH 3 CH 2 CH 2 CH 2 -, and their possible isomers.


So far the alkyls discussed have had linear chain structures. Their structures, as expected, are the same as those of the alkanes from which they come.

Linear alkanes will give rise to linear alkyl groups. Branched alkanes, on the other hand, will generate branched alkyls. The same is true for cyclic alkanes.

However, linear alkanes can also give rise to apparently branched alkyls, depending on which of their carbon atoms loses hydrogen. Consider the example of propane:

Structure of propane and its derived alkyl groups. Source: Gabriel Bolívar.

If you lose a hydrogen from any of its primary carbons, that is, from its ends, you get the propyl group, CH 3 CH 2 CH 2 -.

Meanwhile, if hydrogen loses it from its secondary or central carbon, the isopropyl group, (CH 3 ) 2 CH- is obtained. Two RX compounds are shown in the image, with R being propyl or isopropyl.


Alkyl groups do not usually react because their CC or CH bonds are not easy to break. Regardless of their molecular structure, they all share one property in common: hydrophobicity. That is, they show no affinity for water or any polar solvent. But they do it for fats.

When the alkyl group R is very large, or when there are many of them in a molecule, its hydrophobicity increases. This is the same to say that it increases your lipophilicity (love of fats). So, the more “alkylated” a molecule is, the more affinity it will have for fat, and the more difficult it will be to remove it with water.

Examples of alkyl compounds

The term ‘alkyl compounds’ is extremely ambiguous when it comes to organic chemistry. Priority is always given to the group or molecule to which the alkyl group R is attached. And it is these groups or molecules that also define the families of organic compounds.

However, when these groups are more common in inorganic chemistry, such as halogens and sulfates, the alkyl component is given some importance. Some examples will be mentioned to clarify this point.

Alkyl halides

Methyl bromide

Alkyl halides have a general formula RX, where X is a halogen atom (F, Cl, Br and I), and R is any alkyl group or substituent. For example, CH 3 Br is methyl bromide.

Alkyl sulfates

Dimethyl sulfate

Alkyl sulfates have a general formula ROSO 3 R ‘, where R and R’ are two alkyl groups that can be the same or different. Thus, we have dimethyl sulfate, CH 3 OSO 3 CH 3 or Me 2 SO 4 .

Alkyl borates

Alkyl borates have a general formula (RO 3 ) B. For example, (CH 3 CH 2 O) 3 B or (EtO) 3 B is called ethyl borate.


Similarly, we have the triakylborans, whose general formula is R 3 B. For example, (CH 3 ) 3 B or Me 3 B is called trimethylborane (TMB).


Alcohols are also alkyl compounds, and their general formula is ROH. However, the mere presence of the OH group downplays the alkyl groups. Alcohols are not called ‘alkyl hydroxides’, since alkyls as such are not the most fundamental parts of their molecules, but OH.

On the other hand, the previous examples do emphasize the presence of the alkyl groups R; Because, after all, halogens, sulfates, borans and borates are found a lot in inorganic compounds interacting with metal cations, and not with hydrocarbon segments of alkanes.

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