The alkyl radicals are a set of unstable molecules that result from the loss of hydrogen from an alkane. They are characterized by having an unpaired electron, so they react quickly to complete the valence octet of the atom where it is located.
These radicals are symbolized with the letter R, like the alkyl groups, but with the difference that a point, R · is added to it. This point indicates the presence of an unpaired electron. For example, consider methane gas, CH 4 . This alkane, upon losing one of its hydrogens, will transform into the methyl radical, CH 3 · (image below).
If you want to go a little deeper into the structure of the CH 3 · radical , you will see that its unpaired electron is at an angle perpendicular to the CH bonds; that is, it is located in a pure p orbital . Therefore, the CH 3 · adopts an sp 2 hybridization , as it generally happens with the other alkyl radicals.
These radicals are the cornerstone of chain reactions where alkanes participate, as is the case with their halogenations: chlorination, bromination, etc. There are 1st, 2nd and 3rd radicals, as well as vinyl and allylic, each with their respective stability and ease of formation.
How are alkyl radicals formed?
The “simplest” reaction by which alkyl radicals are formed is the halogenation of alkanes. In order for them to form, a chain reaction must occur, which only occurs at high temperatures (above 250 ºC), or under the incidence of ultraviolet light at room temperature.
Consider the chlorination of methane under one of the above conditions:
Cl 2 → 2Cl
The energy provided is sufficient to break the Cl-Cl bond of the chlorine molecule, causing a homolytic break; that is, each chlorine atom is left with one of the electrons in the bond. Therefore, two Cl · radicals are formed.
Then a Cl radical attacks a methane molecule:
Cl · + CH 4 → HCl + CH 3 ·
And the methyl radical appears. This CH 3 · is quite unstable, so it will react immediately to gain an extra electron with which its lone electron will pair:
CH 3 + Cl 2 → CH 3 Cl + Cl
The Cl · formed will react with another methane molecule and the cycle will repeat itself over and over again. As the methane is depleted, the following reactions will occur, ending the chain reaction:
Cl + Cl → Cl 2
CH 3 + CH 3 → CH 3 CH 3
CH 3 + Cl → CH 3 Cl
Methane is not the only alkane that is “radicalized” by chlorination. The same is true of ethane, propane, butane, and other isomers. However, what does vary is the necessary energy that must be provided for the Cl · radicals to dehydrogenerate an alkane molecule. In the case of the methyl radical, it is very unstable and therefore difficult to form.
Thus, the radical CH 3 CH 2 · is more stable and easier to form than CH 3 ·. Why? Because the radical CH 3 CH 2 · is primary, 1st, which means that the unpaired electron is on a carbon atom that is linked to another carbon. Meanwhile, CH 3 · nothing else is bound to hydrogen atoms.
What is this about? Recalling the previous representation, the unpaired electron is located in a p orbital , perpendicular to the other bonds. In CH 3 ·, the hydrogen atoms hardly donate electron density to the carbon atom, in an attempt to compensate for the lack of another electron.
In CH 3 CH 2 ·, on the other hand, the carbon with the unpaired electron is linked to two hydrogens and to a methyl group. Consequently, this carbon receives a higher electron density, which helps stabilize the unpaired electron a bit. The same explanation is valid for other 1st alkyl radicals.
In a secondary or 2nd alkyl radical, such as the isopropyl radical, (CH 3 ) 2 C ·, the unpaired electron is even more stabilized because it now receives electronic density from two carbon groups. Therefore, it is more stable than the 1st and methyl radicals.
We also have tertiary or 3rd alkyl radicals, such as the tert-butyl radical, (CH 3 ) 3 C ·. These are more stable than the 1st and 2nd. Now, it is three carbon groups different from the hydrogen atom that stabilize the unpaired electron.
Allyl and vinyl
It is also worth mentioning the allyl radicals, CH 2 = CH-CH 2 ·, and vinyl, CH 2 = CH ·.
Allylic is the most stable of all. This is because the unpaired electron is even capable of moving to the carbon atom at the other end. Vinylic, on the other hand, is the most unstable of all, since the CH 2 = group, more acidic, attracts electronic density towards itself instead of donating it towards carbon with the unpaired electron.
In summary, the relative stabilities of the alkyl radicals, from highest to lowest, are:
Allyl> 3rd> 2nd> 1st> vinyl
The nomenclature of the alkyl radicals is the same as for the alkyl groups: the ending -ano, in the name of the alkane, is changed to the ending -yl. Thus, CH 3 CH 2 · is called the ethyl radical; and the radical CH 3 CH 2 CH 2 CH 3 , n -butyl.
Main alkyl radicals
The main alkyl radicals coincide with the alkyl groups: they are those that have less than six carbon atoms. Therefore, the alkyl radicals derived from methane, ethane, propane, butane, and pentane are the most common. Also, vinyl and allylic radicals are part of this list.
Other less frequently encountered alkyl radicals are those derived from cycloalkanes, such as cyclopropyl, cyclobutane, or cyclopentane.
In the following image several of these radicals are represented with some assigned letters:
Starting with the letter ‘a’, we have:
-a, ethyl radical
-b, Isopropyl radical
-c, sec-butyl radical
-d, propyl radical
-e, n -butyl radical
-f, isobutyl radical
-g, tert-butyl radical
-h, cyclopropyl radical
-i, cyclohexyl radical
Each of these radicals may or may not have their hydrogens substituted by other groups, so they are just as varied as alkanes and their structural isomers.
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