Carbocations trivalent classical

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

This chapter begins with a short historic retrospect about the development of the carbocation concepts and covers the techniques used for their generation, observation, and characterization under superacidic long-lived conditions. This is followed by an extensive coverage of the multitude of trivalent (classical) and equilibrating (degenerate) and higher (five or six) coordinate (nonclassical) carbocations. 

The superacidic methods employed to generate trivalent classical carbocations work equally well for the preparation of nonclassical carbocations. The most commonly employed superacid systems are FSO3H, FSOgH-Shl s, HF-SbFs, or SbFs, although other superacids have also been used. The most convenient... 

In 1972 Olah proposed a classification for carbocations dividing them into trivalent ( classical ) carbenium ions and penta- and tetracoordinate ( non-classical ) carbonium ions. 

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. 

Timberlake and coworkers have studied the degenerate rearrangement of pentacyclo-propylethyl cation 56 (involving 1,2-cyclopropyl shifts) under long-lived stable ion conditions81 82 (equation 39a). The rearrangement could not be frozen even at -80 °C. However, additivity of 3C NMR chemical shift analysis7 indicates the classical trivalent nature of the carbocation. 

An empirical criterion based on additivity of 13C NMR chemical shifts for distinguishing classical trivalent and higher coordinate carbocations has been developed by... 

Olah s definition of two main classes of carbocations is most appropriate. A carbenium ion is a classical entity which contains an electron-deficient trivalent carbon atom possessing sp hybridisation and six electrons in the valence shell, e.g., CH3, (CH3)3C, CgH7 (benzenium ion), etc. The three atoms bound to such as carbon atom tend to be... 

Lewis valence bond structures]. The methyl cation or methenium ion (CH3+) may be considered the parent of the trivalent carbocations. The trivalent carbocations are also stiU referred to as classical ions. 

The deuterium isotopic perturbation technique developed by Saunders et al. is capable of providing a convenient and valuable means to differentiate between rapidly equilibrating classical trivalent and nonclassical carbocations containing hypercarbons. 

Applying the additivity of chemical shift analysis to the 2-norbornyl cation also supports the nonclassical bridged nature of the ion. The chemical shift difference of 168 ppm between 2-norbomyl cation (C7H11+) and its parent hydrocarbon norbornane 129 is characteristic of the <200 ppm difference observed between a nonclassical ion and its parent hydrocarbon. In contrast, an ordinary classical trivalent carbocation such as the cyclopentyl cation (75) reveals a chemical shift difference of >360 ppm (between the ion and the parent hydrocarbon, cyclopentane). This is consistent with the 350ppm difference characteristic of classical carbocations and their precursor hydrocarbons. 

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 ... 

Protonated saturated hydrocarbons contain a carbon atom which is formally 5-coordinate. There are insufficient electrons to describe such a carbocation by a classical structure and it is necessary to invoke a 3-centre 2-electron bond. This family of non-classical alkonium ions are generally known as carbonium ions. Removal of H2 from an alkonium ion leaves a carbocation containing a 3-coordinate carbon. These trivalent alkyl cations are known as carbenium ions. The interrelation between the two classes of carbocations is shown for the parent ions in equation 2. 

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. 

The crucial step in acid-catalyzed conversions of hydrocarbons is the formation of the intermediate trivalent or classical sp hybridized carbocation (car-benium ion). In the case of saturated hydrocarbons, this is interpreted by the interaction of the proton of the superacid and the bonding electron pair of the C—H a bond (a similar interaction between the proton and the C—C a bond would result in the cleavage of the carbon-carbon bond). This is based on the concept of a-basicity developed by Olah (65), which describes the ability of a bonds to share their bonded electrons with electrophiles. A hypervalent, pentacoordinate non-classical carbocation (carbonium ion) is formed, which possesses a three-center, two-electron (3c-2e) bond. This is transformed to the classical trivalent carbocation by the loss of hydrogen, that is, protolysis (protolytic cleavage) of the carbon-hydrogen bond occurs. The process is illustrated by the conversion of hexane to yield the 3-hexyl cation through the pentacoordinate carbonium ion (1) (eq. 45). 

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