Carbonium ions Nonclassical carbocations

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

In this section we shall discuss carbocations in which at least one carbon-through a three-center bond (see Section 5.3) is coordinated to four orJive-atoxns. By Olah s terminology these are carbonium ions as opposed to tricoordinated carbenium ions. 61 By older terminology the more highly coordinated carbocations are called nonclassical carbonium ions to differentiate them from the tricoordinated classical carbonium ions. ... 

For many decades chemists had been interested in whether the positively charged intermediate of the SN reaction in Figure 2.28 was a carbenium or a carbonium ion. Also the existence of a rapidly equilibrating mixture of two carbenium ions was considered. It is now known with certainty that this intermediate is a carbonium ion it is known as a nonclassical carboca-tion. In this carbonium ion, there is a bond between the centers Cl, C2, and C6, that consists of two sp2 AOs and one sp3 AO (see MO diagram, lower right, Figure 2.28). It accommodates two electrons. There are many examples of nonclassical carbocations. 

Consideration of the cyclopentyl cation is leading us to the discussion of one of the most controversial of all carbocations, the norbornyl cation, 163 around which the structure the much publicized classical-nonclassical controversy of carbonium ions mainly centered in recent years9). 

Olah emphasizes that the division of cations into classical and nonclassical is frequently arbitrary, since in many cations there is an intenn liate range of delocalization ( partial carbonium-ion character ) as in the 2-methylnorbomyl ion. The author does not want to name classical ions carbonium because it is restricted to highest valeiK state carbocations this requirement is nwt by penta-and tetracoordinate carbocations but not trivalent ones. On the other hand, while in the formation of other onium ions the atom of the donor (nitrogen, oxygen etc.) increases its covalence by one unit upon addition of the acceptor (electrophile), in the formation of a classical ion the covalence of the carbon atom decreases from 4 to 3. As for the name carbenium ion, in the author s opinion it reflects the logical relationships between the carbene and the carbeiunm ion, between the alkene and the carbenium ion ... 

Contents 1 Nonclassical carbocations/by V. A Barkhash — 2 Rearrangements of carbocationsby 1,2-shifts/by V G. Shubin —3 Arenium lons/by V A Koptyug 1. Carbonium ions. 1 Barkhash, V A (Vladimir Alexandrovich). 1933- 11 Shubin, V G (Vyacheslav Gennadievich), 1936- 111. Koptyug, V A IV. Series Topics m current chemistry, 116 etc... 

Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions. Olah, G.A. 

All electrophilic carbon atoms may be described as carbocations. These can be classified further as carbenium ions or carbonium ions. Carbenium ions are trivalent carbocations with electron-deficient centers, and have planar or almost planar 5/ hybridization. In contrast, carbonium ions are four- or fivefold coordinated nonclassical carbocations. In this case, the carbocation consists of three single bonds and a two-electron-two-center bond. Onium ions are formally formed by corresponding addition of heterocompounds to the carbocation, for example. 

Perhaps the "classic" example of a nonclassical carbocation is the 2-norbornyl cation, which was at the center of what has been called "the most heated chemical controversy in our time." In Chapter 8 we will review the experimental evidence, largely based on solvolysis reactions, that led to the proposal of the nonclassical carbonium ion structure shown in Figure 5.48. However, this description was not accepted by all researchers, and an alternative model for the 2-norbomyl cation was a pair of rapidly equilibrating classical (carbenium) ions, as shown in Figure 5.49. Many papers relating to the development of contrasting ideas in this area were published in a reprint and commentary volume by Bartlett. ... 

Further support for the nonclassical structure of the 2-norbomyl cation came from an application of NMR spectroscopy that is based on the difference between the total chemical shift of a carbocation and that of the corresponding alkane. Differences in total chemical shift of 350 ppm or more are associated with classical carbocations, while differences of less than 200 ppm are thought to indicate nonclassical, bridged carbonium ions. For example, the sum of the total NMR chemical shift of propane is 47 ppm, while the sum for the 2-propyl cation is 423 ppm. The difference, 376 ppm, indicates that the 2-propyl cation is a classical ion. For the 2-norbomyl system the total of the shifts is 408 ppm, while the total for norbomane is 233 ppm. The difference, 175 ppm, was taken as evidence for a nonclassical structure. ... 

Cations are kinetic chain carriers in cationic polymerizations. Such cations may be, for example, carbocations or oxonium ions. All electrophilic carbon atoms may be described as carbocations. These can be classified as carbenium ions (trivalent carbocations) and as carbonium ions (carbocations with coordination numbers of four or five). Carbenium ions such as, for example, R3C are classical carbocations. Carbonium ions such as, for example, R5C or R5C2 are nonclassical ions. 

---