Stability of Carbocations and Free Radicals

The creation of two oppositely charged ions arises from the cleavage of a covalent chemical bond, with the negative charge emerging on the more electronegative atom of the linked pair and the positive charge developing on the less electronegative atom. As the difference in electronegativity between two linked atoms decreases, the bond between them has a more significant potential to break homolytically, with each atom retaining one of the bond’s electrons. Free radicals are species in which an atom contains an unpaired electron. A carbocation molecule has three bonds and a positively charged carbon atom.

Stability of free radicals and carbocations

Free radicals are atoms, molecules, or ions with unpaired valence electrons or an open electron shell, and hence one or more “dangling” covalent bonds, as defined in chemistry. With a few exceptions, these “dangling” connections make free radicals very chemically reactive towards other substances, including themselves: their molecules will frequently dimerise or polymerise spontaneously if they come into contact. Only at low concentrations in an inert medium or a vacuum are most radicals relatively stable.

The hydroxyl radical (HO•), one hydrogen atom short of a water molecule and hence has one link “dangling” from the oxygen, is a well-known example of a free radical. The carbene molecule (CH2) contains two dangling bonds. The superoxide anion (O2), the oxygen molecule O2 with one additional electron and has one dangling bond, are two further examples of free radicals.

You could already have a sense of things that might stabilise free radicals if you are electron deficient. If charges are spread out across a larger volume, their stability improves. Neighbouring atoms that can give electron density stabilise electron-poor species. The most popular approach to interpreting “rich neighbours” here is discovering that increasing the number of alkyl groups on the carbon harbouring the free radical enhances its stability is the most popular approach to interpreting “rich neighbours” here. The order of methyl primary, secondary, and tertiary boosts radical stability.

Delocalisation helps stabilise free radicals

Any mechanism that causes the electron-deficient site to be distributed across a greater region stabilises electron-poor species. When a carbocation is next to a bond, for example, its positive charge is significantly stabilised. That’s because the carbocation is sp2 hybridised and has an empty p orbital, which allows for overlap with neighbouring p orbitals and so causes the positive charge to be delocalised over many carbon atoms, as illustrated by resonance structures. Because carbocations are flat, it’s simple to imagine the p orbital of a double bond aligned with nearby p orbitals.

Free Radicals have a “Shallow Pyramid” geometry, which allows the half-filled p-orbital to overlap with adjacent pi bonds. When the electrons in a typical alkyl free radical are drawn out, you can see that there are three bonding pairs and one unpaired electron, totalling four occupied orbitals. By comparison with amines, you would anticipate the molecule’s hybridisation to be sp3 and the geometry of a free radical to be trigonal pyramidal. Except that the “pyramid” is a bit steeper than for molecules with a complete lone pair, this is a fair approximation.

If the p orbitals are all in line and can overlap when the free radical is next to a bond, there is considerable stabilisation. When the “shallow pyramid” is smoothed down, the overlap increases. To conceive a free radical near a bond as being “sp2 hybridised” is a decent approximation. So, the bottom line hereDelocalisationon of the electron-deficient free radical over many carbons is possible. Free radicals are, therefore, stabilised by resonance.

Carbon atoms stabilise carbocations

As you get from primary to secondary to tertiary carbons, the stability of carbocations improves. There are two possible explanations for this. According to the traditional response, carbons (alkyl groups in particular) are thought to be “electron-releasing” groups due to inductive effects. A carbon coupled to hydrogen will have a lot of electrons and can give part of them to a nearby carbocation. To put it another way, the surrounding carbon compensates for the carbocation by stealing electrons from the hydrogens. The second is hyperconjugation, which causes stability by donating electrons from C-H sigma bonds to the carbocation’s empty p orbital.

Neighbouring Carbon-Carbon multiple bonds stabilise carbocations

Because the vacant p orbital of the carbocation overlaps with the p orbitals of the bond, the charge can be exchanged between numerous atoms. Carbocations close to another carbon-carbon double or triple bond exhibit particular stability. The charge “moves” from atom to atom in this action known as “delocalisation.” Even primary carbocations, which are ordinarily exceedingly unstable, are amazingly simple to generate when close to a double bond, to the point that they will participate in SN1 reactions.

Adjacent atoms with lone pairs help to stabilise Carbocations

An adjacent atom gives a pair of electrons to the electron-poor carbocation, which acts as a vital stabilising factor. It’s worth noting that this always results in forming a double bond (bond), with the charge moving to the atom contributing to the electron pair. As a result, the term “contribution” is frequently used.

Because the potency of this impact varies with basicity, the strongest donors are nitrogen and oxygen. Surprisingly, halogens can aid in the stabilisation of carbocations by donating a single pair. This effect is crucial in aromatic ring reactions and enolate chemistry, where double bonds connected to donating groups can be millions of times more nucleophilic than alkenes without these groups.

Conclusion 

Because radicals and carbocations contain varying quantities of valence electrons, their relative stability differs. The valence shell of a free radical has just 7 electrons. They have more energy than atoms that have eight valence electrons. Carbocations are electron-poor species as well. Carbocations contain more energy than free radicals since they only have 6 valence electrons. The gap in the stability between carbocations and free radicals is substantially more significant. The takeaway from this post is that knowing the elements that influence the stability of carbocations and free radicals may provide you with a lot of insight into various reactions, even if they appear to be quite different.