All About History Of Carbocations

From the early 1970s to the present, all carbocations were referred to as carbonium ions.

The term “carbocation” refers to any even-electron cation that has a large partial positive charge on the carbon atom in the modern era. 

They are further subdivided into two primary categories based on the coordination number of the charged carbon: 

three in the case of carbenium ions and five in the case of carbonium ions, respectively. G. A. Olah was the one who proposed this nomenclature. 

In their original definition by Olah, carbonium ions were defined as being distinguished by a three-centre, two-electron delocalized bonding scheme. 

They were considered to be synonymous with so-called ‘non-classical carbocations,’ which were defined as carbocations that contained bridging carbonyl–carbon of carbonyl–hydrogen bonds.

The term ‘carbonium ion’ has been carefully defined by some to include only officially protonated or alkylated alkanes (CR₅⁺, where R is H or alkyl), with non-classical carbocations such as the 2-norbornyl cation being excluded.

History of Carbocations

In 1891, G. Merling reported that he had added bromine to propylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water-soluble substance, C₇H₇Br. 

Since then, carbocations have been studied extensively. Doering and Knox were able to demonstrate conclusively that it was tropylium (cycloheptatrienyl ium) bromide, despite the fact that he did not provide a structure.

According to Hückel’s rule, this ion is predicted to be aromatic.

Norris and Kehrman independently discovered in 1902 that colourless triphenylmethanol produces deep-yellow solutions in concentrated sulfuric acid, which they named “triphenylmethanol solution.”

Triphenylmethyl chloride generated orange complexes with aluminium and tin chlorides in a manner similar to that described above. 

Adolf von Baeyer discovered the salt-like nature of the compounds that were created in 1902 and published his findings. 

Trityl carbocation (shown below) has been employed as a homogeneous organocatalyst in organic synthesis because it is a stable carbocationic system with a long half-life.

He coined the term “halochromy” to describe the link between colour and salt production, with malachite green serving as a prime example.

A wide variety of organic reactions involve the use of carbocations as intermediates. 

Originally proposed by Julius Stieglitz in 1899, Hans Meerwein further expanded the concept in his 1922 research of the Wagner–Meerwein rearrangement, which was published in the journal Music Theory. 

Carbocations have also been discovered to be involved in the SN¹ reaction, the E₁ reaction, and rearrangement processes such as the Whitmore 1,2 shift, among other reactions. 

The chemical establishment was hesitant to accept the concept of a carbocation, and for a long time, writings mentioning them were rejected by the Journal of the American Chemical Society.

A stable carbocation in solution was discovered by Doering and colleagues in 1958, and their NMR spectra was reported the following year.

Ion heptamethyl benzenium was created by reacting hexamethyl benzene with methyl chloride and aluminium chloride, and it was found to be toxic. 

It was discovered by Story et al. in 1960 that the stable 7-norbornadiene cation could be made by reacting norbornadiene chloride with silver tetrafluoroborate in sulphur dioxide at temperatures as low as 80 degrees Celsius. 

The NMR spectra demonstrated that it was not a conventionally bridged structure (the first stable non-classical ion observed).

After dissolving tert-butyl fluoride in magic acid and observing the tert-butyl carbocation using nuclear magnetic resonance, Olah was the first to observe it as a stable species in 1962. 

Schleyer et al. were the first to report the NMR of the norbornyl cation, while Saunders et al. were the first to demonstrate that the cation undergoes proton-scrambling when it crosses a barrier.

Huckel’s Rule

Erich Hückel, a physical chemist who worked in the 1930s, was the first to discover the quantum mechanical foundation for its formulation.

As stated in the name, Hückel’s rule predicts that an aromatic planar ring molecule possesses 4n + 2 electrons, where n is an even non-negative integer, and that this molecule will exhibit aromatic properties. 

Triphenylmethanol

A triphenylmethanol compound (also known as triphenylcarbinol, or TrOH) is an organic compound with the chemical formula. 

Benzene is an insoluble white crystalline solid that is insoluble in water and petroleum ether, but is highly soluble in ethanol, diethyl ether, and benzene, among other things. 

It generates a bright yellow colour in strongly acidic solutions, which is owing to the development of a stable “trityl” carbocation, which is stable in acidic solutions. 

Many triphenylmethanol compounds are used in the production of colours.

The Structure and Properties Of The Object

Triphenylmethanol is a compound composed of three phenyl rings and an alcohol group that are all linked together by a central tetrahedral carbon atom. 

Every one of the three C–Ph bonds is typical of sp3–sp2 carbon–carbon bonds, with lengths of roughly 1.47, although the C–O bond has a length of approximately 1.42. The C–O bond is the longest of the three bonds, with a length of approximately 1.47.

A particular property of alcohol is conferred by the presence of three contiguous phenyl groups, which is evident in the reactivity of the alcohol.

For example, when it interacts with acetyl chloride, it does not produce the ester but rather triphenylmethyl chloride:

Ph₃COH + MeCOCl  →  Ph₃CCl + MeCO₂H is a chemical reaction.

Steric protection is also provided by the three phenyl groups. The reaction with hydrogen peroxide results in the formation of Ph₃COH, a hydroperoxide that is unusually stable.

Conclusion

Carbocations are the most significant reactive intermediates in all of organic chemistry, and they have a wide range of applications. 

A century of carbocation chemistry can be split into about three equal periods, such as the beginning, which spans from 1900 to the late 1930s, the golden age, which spans from the 1940s to the mid-1970s, and the modern period, which spans from the 1980s to the present. 

This chapter presents a high-level overview of each historical epoch. It is generally agreed that the independent observations and reports of J. F. Norris in the American Chemical Journal and F. 

Kehrmann in Chemische Berichte in 1901, both published shortly after Moses Gomberg’s discovery of the triphenylmethyl radical in 1900, are primarily responsible for the discovery of carbocations, which is celebrated this year on the occasion of our 100th anniversary. 

J. Corey, the paragon of synthetic organic chemistry, carried out mechanistic research and explorations of carbocations, which is something that one would not normally identify with physical organic chemistry.