Carbonium Ions

Carbonium ions belong to a category of organic chemicals that are characterized by positive charges present on the carbon atom. Typically, ions are short-lived and exist only for short periods during a chemical reaction. Some carbonium ions, however, can be stabilized enough to research their chemical structure. It is necessary to study the carbonium ion structure and its properties to understand their roles in biochemical or synthetic reactions. 

The carbonium ion was first discovered in 1901. Almost twenty years later, Hans Meerwein proposed and proved the idea of a carbonium ion as a crucial intermediate in chemical reactions. This idea was further popularized and strengthened by other scientists. It was only in the 1960s however, that a stable carbonium ion structure was made available. These ions were stabilized and made long-lasting enough to conduct proper studies on them.

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Types of Carbonium Ions

There are two major types of carbonium ions. 

  1. Classical Carbonium Ions: The centre of the ion which has the carbon atom has a valency of 3 in a hybridized state of sp2. This means that three electrons of the carbon atoms occupy three orbitals that lie on the same plane. The positive centre of the molecule is planar. The methyl cation with a chemical formula of CH3+ is the parent ion. 

  2. Non- Classical or Pentacoordinated Carbonium Ions: These ions have three single bonds that connect the carbon atom to another atom and a double bond that joins three atoms instead of 2. These atoms have a single pair of electrons. The methonium ion with a chemical formula of CH5+ is the parent ion. 

Structure of Carbonium Ions and their Chemical Formula

Traditionally within tertiary carbonium ions, three alkyl groups bond with the sp2 carbon. On the other hand, in secondary carbonium ions, the bond between sp2 carbon atoms is formed in one hydrogen atom and two alkyl groups. Electron emission spectroscopy using X-rays is yet another method to obtain information regarding the bond energies.

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How to Prepare the Most Stable Carbonium Ion?

The following are the major methods of preparing carbonium ions:

  1. Heterolytic cleavage of the carbon atom and an attached functional group

  2. Removal of the electron from a neutral organic compound

  3. Adding a positively charged ion or positive charge to an unsaturated group

  4. Protonation and alkylation

Carbonium ions being positively charged can be prepared in solvents that have a polar nature i.e. solvents with an asymmetrical number of electrons. These solvents help to disperse charges throughout it. Some examples of these solvents are acetone in a liquid form, acetic acid methanol, and trifluoroacetic acid

Methods to Observe Carbonium Ions

What kind of carbonium ion structure will be formed depends upon the nature of the medium and the stability of the ion. Carbonium ions exist temporarily because they can easily react with the solvent that acts as a medium or with any other negatively charged particle. This property creates a situation where only indirect methods can be applied for research. 

The kinetic method involves measuring the rate of reaction, another method involves studying the end product of the chemical reaction of the carbonium ion, yet another method involves looking at the arrangement of atoms in the molecules. Finally, radioactive isotopes can be used for the identification of particular atoms. 

Stability With Solvents

Stable carbonium ions can exist with solvents that do not react much with carbocations. For instance, hydrogen fluoride and fluorosulfuric acid along with antimony pentafluoride and sulfur dioxide are solvents in which the life of carbonium ions is prolonged enough to allow direct study. 

Relative Stability of Carbonium Ions

Primary carbonium ions are the least stable, secondary carbonium ions are more stable than primary, and tertiary carbonium ions are the most stable carbonium ion. This is because, in tertiary ions, the carbon with sp2 hybridization is bonded to three alkyl groups. In secondary carbonium ions, the carbon atom with sp2 hybridization is bonded to one hydrogen atom and two alkyl groups. 

In primary carbonium ions, the carbon atom with sp2 hybridization is bonded to either 3 atoms of hydrogen or 2 hydrogen groups and one alkyl group. Since an alkyl group displaces the positive charge on the sp2 hybridized carbon, the more alkyl groups, the lesser will be the reactivity, and more will be the stability. 

Fun Fact About the Most Stable Carbonium Ion

Did you know that benzyl cations have more stability than primary cations? This is so because the aromatic ring of the benzyl group can displace the positive charge on the carbon atoms. The positive charge is distributed and contributes to building the entire structure. This action can exist in many forms. 

FAQs (Frequently Asked Questions)

1. How Do Carbonium Ions React?

Ans: Carbonium ions are positively charged. This means that they have an electron deficiency.  They react with molecules that can donate their electrons. These molecules are also called nucleophiles. Nucleophiles can exist as nonbonded pairs (n- base), pi- bonded pairs (pi bonds are found in aromatic and unsaturated solutions), and sigma- bonded pairs (sigma bonds are covalent bonds that generally exist between atoms). The nucleophile can exist within the cation itself (internal nucleophile) or may be introduced through external means. In the case of internal nucleophiles, there is a rearrangement of atoms.  

2. What are the Examples of Major Reactions of Carbonium Ions?

Ans: The following are the major types of reactions that involve nucleophiles and rearrangement:

  • Reaction with an External Nucleophile: if isobutylene is hydrated in the presence of acid as a catalyst then the free electron pair on the oxygen atom can react with the carbonium ion. 

  • Reaction with an External Base: If an alkyl group is added to benzene using isopropyl chloride as a catalyst through a process known as Friedel–Crafts reaction, then benzene donates its electrons from its pi bonds. 

  • Reaction with Internal Nucleophiles: It is also called cyclization reaction. In this reaction, the electron pair which is not bonded on the oxygen atom acts as the donor.

  • Reaction with an External Sigma Base: In this reaction, hydrides are transmitted. The sigma bond between carbon and hydrogen in isobutane acts as the donor.