Which carbonium ion is the most stable

Carbenium ions

Carbenium ions are organic compounds with a trivalent, positively charged carbon atom. According to the spherical cloud model, this carbon atom has an empty spherical cloud. According to the orbital model, the C atom is a sp2-hybridized carbon atom with an empty pz-Orbital.

Carbenium ions are a subclass of carbocations. In addition to carbenium ions, carbonium ions also belong to the carbocations. Carbonium ions are pentavalent and difficult to describe even according to the orbital model. For the sake of simplicity, imagine that a methane molecule reacts with an extremely strong acid. The acid then releases a proton to the methane molecule. The CH5+-Ion would then be a carbonium ion.

Carbenium ions play an important role in several types of organic reactions, especially electrophilic addition and nucleophilic substitution.


Protonation of an alcohol

When an alcohol is protonated by adding an acid, the -OH that is formed becomes2+-Group split off relatively easily as a water molecule. A carbenium ion then remains. This cleavage of the OH group is particularly easy with secondary or tertiary alcohols because particularly stable secondary or tertiary carbenium ions are then formed.

Protonation of an alkene

An alkene such as propene can also be converted into a carbenium ion relatively easily if it is protonated. Sulfuric acid is often used for this because the acid residue anion HSO4- is not nucleophilic[2]. In the case of protonation with hydrochloric acid HCl, for example, the acid residue Cl- attach to the carbenium ion, causing it to cease to exist. The hydrogen sulfate ion and the sulfate ion, on the other hand, show no tendency to attach to a carbenium ion.

Reaction of an alkyl halide with a Lewis acid

A carbenium ion can be easily produced from an alkyl halide such as 2-chloropropane, for this one has to remove the halogen atom from the compound as a halide ion. This is done with the help of a Lewis acid. Usually aluminum chloride AlCl is used for this purpose3 used, but also antimony fluoride SbF5, Titanium chloride TiCl4, Tin chloride SnCl4 and boron fluoride BF3 are often used as Lewis acids[1].


Addition of a nucleophile

A carbenium ion can be obtained from a nucleophile such as Br- to be attacked. A halogen-alkane or a similar compound is then formed. This step plays a major role in electrophilic addition, for example, but also in nucleophilic substitution after the SN1 mechanism.

Splitting off a proton

A carbenium ion can also stabilize itself by releasing a proton. Then an alkene is formed. This step occurs, for example, with elimination.

Addition of an aromatic

Depending on your perspective, you can look from the "Addition of an aromatic to a carbenium ion" speak, then one would have to use the mechanism as a nucleophilic substitution according to the SN1 mechanism. Alternatively, you can also use the "Addition of a Carbenium Ion to an Aromatic" speak, then this mechanism would be called Friedel-Crafts alkylation, a special case of electrophilic substitution.

Stability of Carbenium Ions

Tertiary carbenium ions are the most stable because the + I effects of the three pendant alkyl groups slightly increase the electron density on the positive carbon atom. Secondary carbenium ions are less stable, and primary ones even less.

Tertiary carbenium ions are so stable because the positive charge is somewhat compensated for by the + I effects of three alkyl groups. Even more stable than tertiary carbenium ions are carbenium ions, in which the sp2-hybridized positive carbon atom is connected with a C = C double bond. The two carbon atoms of the double bond are also sp2-hybridized, and the two pi electrons of the two pz-Orbitals of the double bond can now also be used in the pz- Cross the orbital of the positive carbon atom and thus reduce the positive charge.

Stability of Carbenium Ions

Even more stable than such carbenium ions are carbenium ions, in which the sp2-hybridized positive carbon atom connected to an aromatic ring. Here the pi-electrons of the aromatic can enter the pz-Cross orbital of the positive C-atom and thus reduce the positive charge quite strongly.