CARBOCATIONS (or CARBONIUM IONS)

All carbocations (previously known as carbonium ions) carry a positive charge on a carbon atom. The name tells you that - a cation is a positive ion, and the "carbo" bit refers to a carbon atom. However there are important differences in the structures of various types of carbocations.

Some examples of primary carbocations include:

Notice that it doesn't matter how complicated the attached alkyl group is. All you are doing is counting the number of bonds from the positive carbon to other carbon atoms. In all the above cases there is only one such link.

Using the symbol R for an alkyl group, a primary carbocation would be written as in the box.

Secondary carbocations

In a secondary (2°) carbocation, the carbon with the positive charge is attached to two other alkyl groups, which may be the same or different.

Examples:

A secondary carbocation has the general formula shown in the box. R and R' represent alkyl groups which may be the same or different.

Tertiary carbocations

In a tertiary (3°) carbocation, the positive carbon atom is attached to three alkyl groups, which may be any combination of same or different.

A tertiary carbocation has the general formula shown in the box. R, R' and R" are alkyl groups and may be the same or different.

At the end of 2024, the Royal Society of Chemistry (RSC) emailed UK Exam Boards suggesting that the way this has been taught traditionally by way of an inductive effect of alkyl groups (often described as an "electron pushing effect") can be shown to be wrong.

I'm going to give you the traditional version and the new approved version. Which should you use? The answer to this is always use whatever your examiners want - check your syllabus.

I know that AQA have said that until they publish a new syllabus sometime in the future they will accept either version. It may well make sense to continue to use the traditional explanation until your syllabus changes because there will be enough evidence in past papers for exactly what they want you to say. Take the advice of your teachers.

Alkyl groups do precisely the opposite and, rather than draw electrons towards themselves, tend to "push" electrons away.

This means that the alkyl group becomes slightly positive (+) and the carbon they are attached to becomes slightly negative (-). The alkyl group has a positive inductive effect.

This is sometimes shown as, for example:

The arrow shows the electrons being "pushed" away from the CH₃ group. The plus sign on the left-hand end of it shows that the CH₃ group is becoming positive. The symbols + and - simply reinforce that idea.

If this is what your syllabus wants, don't bother with the next bit - you will just get confused. Instead, jump over it to the rest of the page by clicking here.

The more accurate approach: hyperconjugation

We'll look at a simple carbonium ion from the ones in the diagram above.

Hyperconjugation looks at what is happening at the orbital level. I want to concentrate on the positive carbon atom and one of the carbon-hydrogen bonds, looking at the ion CH₃CH₂⁺.

The sigma bond shown between the hydrogen and carbon is simply a pair of shared electrons. The empty orbital on the positive carbon is because the two electrons originally in that orbital were removed by something else. These ions only form as a short-lived stage of a reaction mechanism.

Notice that the two orbitals shown aren't far off being parallel. And remember that these diagrams are shown in an "exploded" form. The orbitals actually touch to an extent - not enough to form a bond by sideways overlap, but enough to have some temporary interaction.

Why "temporary"?

Because there is free rotation around a single bond - the CH₃ is constantly spinning around putting one after another C-H bond in the right position to interact with the empty p orbital.

The hydrogens at the ends of the sigma bonds have a slight positive charge because on average the electrons in the bond have moved slightly towards the positively charged carbon. The positive nuclei are still there, but their charge is no longer quite covered by the bond electrons.

At the same time, addition of some negative charge around the right-hand carbon is slightly cutting down its positive charge.

If there is more than one CH₃ group attached to the positive carbon, this same thing is happening to all of them, leading to even more δ- charge accumulating around the positive carbon.

Be careful if you have a more complicated alkyl group like ethyl, CH₃CH₂, bonded to the positive C. Only the two hydrogens on the carbon directly attached to the positive carbon can get involved in this. The others on the CH₃ group are too far away.

The importance of spreading charge around in making ions stable

The general rule-of-thumb is that if a charge is very localised (all concentrated on one atom) the ion is much less stable than if the charge is spread out over several atoms.

Applying that to carbocations of various sorts . . .

You will see that the electron transfer from the CH₃ group is placing more and more negative charge on the positive carbon as you go from primary to secondary to tertiary carbocations. The effect of this, of course, is to cut down that positive charge.

At the same time, the region around the various CH₃ groups is becoming somewhat positive. The net effect, then, is that the positive charge is being spread out over more and more atoms as you go from primary to secondary to tertiary ions.

The more you can spread the charge around, the more stable the ion becomes.

The stability of the carbocations in terms of energetics

When we talk about secondary carbocations being more stable than primary ones, what exactly do we mean? We are actually talking about energetic stability - secondary carbocations are lower down an energy "ladder" than primary ones.

This means that it is going to take more energy to make a primary carbocation than a secondary one.

If there is a choice between making a secondary ion or a primary one, it will be much easier to make the secondary one.

Similarly, if there is a choice between making a tertiary ion or a secondary one, it will be easier to make the tertiary one.

This has important implications in the reactions of unsymmetrical alkenes. If you are interested in these, follow the link below to the electrophilic addition reactions menu.