Chapter 2

Carbocations

2.1 Introduction

In many cases, reactions proceed via so-called intermediates, which have in general very short lifetimes. One type of reactive intermediates is the so-called “carbocations.” Charged atoms and groups of atoms are common in inorganic chemistry. All of us know about table salt, which consists of positively charged sodium ions (cations) and negatively charged chloride ions (anions). The opposite is true for the large number of organic compounds, especially hydrocarbons, which are composed of only two elements, carbon and hydrogen. Carbocations have been well established as intermediates in numerous synthetic transformations. In such cases these intermediates had to have an extremely short lifetime, a billionth of a second or less, and due to their high reactivity their concentrations had to be very low. Their existence has been indicated by measurements of reaction rates and observations of the spatial arrangement of the atoms in space. For such purposes, a variety of ingenious experiments have been carried out. However, nobody was able to see these carbocations, not even with the most powerful microscopes or by spectroscopic methods. These techniques can be regarded as extensions of human vision. Consequently, there was no evidence for the existence of carbocations, in other words whether they were a reality independent of human consciousness or were only created by human imagination to describe the experimental results. Because it was not possible to detect carbocations with spectroscopic methods, different scientists interpreted their experiments differently, and a scientific feud took place in organic chemistry during the 1960s and 1970s.

Through a series of brilliant experiments Professor George Olah solved the problem. He created methods to prepare long-lived carbocations in high concentrations, which made it possible to study their structure, stability, and reactions with spectroscopic methods. He achieved this by using special solvents, which did not react with the cations. He observed that in these solvents, at low temperatures, carbocations could be prepared with the aid of superacids (acids 1810 times stronger than concentrated sulfuric acid). Through Olah's pioneering work he and the scientists who followed in his footsteps could obtain detailed knowledge about the structure and reactivity of carbocations. Olah's discovery resulted in a complete revolution for scientific studies of carbocations, and his contributions occupy a prominent place in all modern textbooks of organic chemistry.

Olah found that there are two groups of carbocations, namely, trivalent ones called carbenium ions, in which the positive carbon atom is surrounded by three atoms, and those in which the positive carbon is surrounded by five atoms, called carbonium ions (Figure 2.1). The disputed existence of these pentacoordinated carbocations was the reason for the scientific feud. By providing convincing proof that pentacoordinated carbocations exist, Olah demolished the dogma that carbon in organic compounds could at most be tetracoordinated, or bind a maximum of four atoms. This had been one of the cornerstones of structural organic chemistry since the days of Kekulé in the 1860s.

Figure 2.1 Carbenium and carbonium ions.

Olah found that the superacids were so strong that they could donate a proton to simple saturated hydrocarbons, and that these pentacoordinated carbonium ions could undergo further reactions. This fact has contributed to a better understanding of the most important reactions in petrochemistry. His discoveries have led to the development of methods for the isomerization of straight chain alkanes, which have low octane numbers when used in combustion engines, to produce branched alkanes with high octane numbers. Furthermore, these branched alkanes are important as starting materials in industrial syntheses. Olah has also shown that with the aid of superacids it is possible to prepare larger hydrocarbons with methane as the building block. With superacid catalysis it is also possible to crack heavy oils and liquefy coal under surprisingly mild conditions.

2.2 History

The history of carbocations dates back to 1891 when G. Merling reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water-soluble material, C7H7Br. He did not suggest a structure for it; however, Doering and Knox convincingly showed that it was tropylium (cycloheptatrienylium) bromide (Figure 2.2). This ion is predicted to be aromatic by the Hückel rule.

Figure 2.2 Tropylium bromide.

In 1902 Norris and Kehrman independently discovered that colorless triphenylmethanol gave deep yellow solutions in concentrated sulfuric acid (Scheme 2.1). Triphenylmethyl chloride similarly formed orange complexes with aluminum and tin chlorides. Adolf von Baeyer recognized in 1902 the salt-like character of the compounds formed. He dubbed the relationship between color and salt formation halochromy, of which malachite green is a prime example.

Scheme 2.1 Reaction of triphenylmethanol with conc. H2SO4.

Carbocations are reactive intermediates in many organic reactions. This idea, first proposed by Julius Stieglitz in 1899 (on the constitution of the salts of imido-ethers and other carbimide derivatives), was further developed by Hans Meerwein in his 1922 study of the Wagner–Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction and E1 reaction and in rearrangement reactions such as the Whitmore 1,2 shift. The chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them.

The first NMR spectrum of a stable carbocation in solution was published by Doering et al. It was the heptamethylbenzenonium ion, made by treating hexamethylbenzene with methyl chloride and aluminum chloride. The stable 7-norbornadienyl cation was prepared by Story et al. by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C. The NMR spectrum established that it was nonclassically bridged (the first stable nonclassical ion observed). In 1962 Olah directly observed the tert-butyl carbocation, by nuclear magnetic resonance, as a stable species on dissolving tert-butyl fluoride in magic acid. The NMR of the norbornyl cation was first reported by Schleyer et al. and it was shown to undergo proton scrambling over a barrier by Saunders et al.

2.2.1 Carbonium Ions and Carbenium Ions

A carbocation was previously often called a carbonium ion but questions arose concerning the exact meaning. In present day chemistry, a carbocation is any positively charged carbon atom. Two special types have been suggested: carbenium ions, which are trivalent, and carbonium ions, which are pentavalent or hexavalent. University level textbooks only discuss carbocations as if they are carbenium ions, or discuss carbocations with a fleeting reference to the older phrase of carbonium ion or carbenium and carbonium ions.

Carbocations play a key role in many chemical processes and the study of carbocations as transient or long-lived species has directly influenced the understanding of bonding and solvation, which are fundamental aspects of chemistry. The strengths of acids, as measured by pKa values, range from very weak ones such as hydrocarbons to acids that are much stronger than sulfuric acid. Acids with acidities greater than sulfuric acid are called superacids. Carbocations are most important reactive intermediates, having a formal positive (+) charge on carbon. During much of the recent history of organic chemistry, a structure with a positively charged carbon atom was called a carbonium ion, a term reminiscent of other positively charged species, such as ammonium, phosphonium, sulfonium, and so on (hypervalent cations: having a higher than usual valency, Scheme 2.2). However, these latter terms all refer to species formed by adding a positively charged atom such as a proton to an atom with a nonbonded pair of electrons to form the positively charged ion. Most carbocations, in contrast, are formed by removing a substituent and its electron pair from the carbon, leading to a hypovalent (having less than its usual valency) cation. To keep the nomenclature of organic chemistry consistent, it was proposed that a species such as CH3+ should be thought of as being the addition product of methylene and a proton, so it should more properly be termed a carbenium ion, and that is the term now in general use for species in which a trivalent carbon atom bears a positive charge. However, the more general term carbocation is used instead.

Scheme 2.2 Hypervalent and hypovalent cations.

To continue the analogy of adding the suffix -ium to the term for a neutral species, George Olah (born in Hungry in 1927 but emigrated to the USA and awarded the Nobel Prize for his work on cations in 1994) proposed that the term carbonium ion refer to a species that could be considered to be formed by adding a positive charge to a neutral, tetravalent carbon atom. Such a species in which a carbon atom appears to be bonded to more than four atoms at once is known as a hypercoordinate carbon compound. Figure 2.3 shows the...