Historically there have been points of conflict in understanding what a carbocation is. This is because there are endless reactive molecules that, for some reason or another, acquire a positive charge on one of their carbons. The classical carbocation, so to speak, is one that can be represented by the general formula in the image below.
Note how the positive charge is located exactly on the carbon atom, and that it is also missing a bond. Therefore, the carbocation is electron deficient, this being the cause of its high reactivity.
How are carbocations formed? The answer lies in the mechanisms of the reactions. However, the vast majority share one of the following two aspects in common:
-Adding π electrons to an electrophile
In the heterolytic cleavage, the CG bond, G being any atom or group, breaks unevenly: the electrons of the bond are kept by G, while the carbon atom acquires a positive charge. For instance:
Now, this break can occur by various methods, which in turn will change the mechanism and reaction considered.
Adding π electrons to an electrophile
The other process by which a carbocation is formed, being very common in alkenes and aromatic compounds, is through the attack of electrons from a double bond to an electrophile. The most common cases are the hydrogenations of alkenes by the action of an acid:
This equation corresponds to the formation of the carbocation, which is an intermediate , not a product . The π electrons in the double bond attack the hydrogen in HCl, the electrophile. Hydrogen is added to the carbon to the right of the double bond, forming the -CH 3 group , while the carbon to the left acquires a positive charge.
The general formula presented at the beginning reveals a trigonal plane geometry around the carbon atom. This is due to the hybridization that carbon atomic orbitals undergo to become sp 2 hybrid orbitals , which separate at an angle of 120º. Although it is not represented, the carbocation has a pure and empty p orbital , capable of accepting electrons.
The carbocation is an extremely acidic species, either as defined by Brönsted or Lewis. Its positive charge demands electrons or hydrogen atoms to give them up. It is for this reason that carbocations cannot be formed in very basic media, since they would react immediately to originate other products.
The sp 2 hybridization of the carbocation makes it vulnerable to attack by electron-rich species. This characteristic is further accentuated by its great acidity. Consequently, the carbocation is a very reactive species, which hardly forms and after a short time (in a matter of nanoseconds) they react to give rise to the true product of the chemical reaction.
Types of carbocations
There are several types of carbocations. However, these can be classified according to the following:
-Aromatics or arils
-Vinyl and allylic
Examples of primary carbocations are shown above. They are so called because the positive charge resides on a primary carbon, 1st, so it is only bonded to one carbon atom. Ethane, CH 3 CH 3 , when it loses an H from either of its ends, the carbocation CH 3 CH 2 + or + CH 2 CH 3 originates .
In secondary carbocations the positive charge is located on a secondary carbon, 2nd, which is linked to two carbon atoms. For example, if we remove one H from the central carbon from propane, CH 3 CH 2 CH 3 , we will have the carbocation CH 3 CH 2 + CH 3 .
In tertiary carbocations, the positive charge is located on a tertiary, 3rd carbon, linked to three carbon atoms. Note that unlike the first two types of carbocations, they lack hydrogen atoms.
Thus, we have methylpropane or isobutane, CH (CH 3 ) 3 , which by losing a hydrogen from the central carbon forms the carbocation + C (CH 3 ) 3 .
Aromatic or aryl carbocations are perhaps the most special of all. Its formation is very similar to that described for alkenes in the first section.
In them, the positive charge is located, in principle, on one of the carbons of an aromatic ring, such as that of benzene. However, the positive charge is not fixed, but is dispersed in other positions of the ring by resonance.
This is how the positive charge, as seen above, passes from one carbon to another within the benzene ring. This characteristic gives this carbocation great stability compared to other types of carbocations.
Allyl and vinyl
Other special types of carbocations are allylics and vinyls. The difference between them (above) is the position of the positive charge relative to the double bond.
In the vinyl carbocation the positive charge is on one of the carbons of the double bond; while in the allylic carbocation, the positive charge is located on the carbon following the double bond. It is enough to replace the hydrogens with other groups and we will have a huge family of allylic and vinyl carbocations.
Knowing what the main types of carbocations are, they can be ordered based on their relative stabilities:
Vinyl <Primary <Secondary <Allylic <Tertiary <Aromatic
Now, there may be allylic carbocations that are more stable than a specific tertiary one. Everything will depend on its substituents.
Why is this stability due? The ability of the molecule to disperse or decrease the positive charge of the carbocation. To do this, it needs nearby atoms that give part of their electron density to the carbocation through hyperconjugation. Meanwhile, in allylic and aromatic carbocations this is achieved by resonance.
In the case of the vinyl carbocation, the positive charge is located on a carbon that was already sp 2 , which makes it very unstable.