Carbocation trivalent

Reactions in acid often involve the formation of carbocations—trivalent, positively charged carbon atoms—as intermediates. The order of stability of carbocations containing only alkyl substituents is 3° > 2° > 1° > CHj. Cation stability is influenced by several factors ... 

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. 

Concerning carbocations, previous usage named the trivalent, planar ions of the type carbonium ions. If the name is considered anal-... 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

TIricooRlinate caibocations are fiequendy called carbonium ions. The terms methyl cation, butyl cation, etc., are used to describe the c >rTesixiiulir.ji tricoordinate cations. Chemical Abstracts uses as specific names methylium, ethyUum, propylium. We will use carbocation as a generic term for trivalent carbon cations. 

The C-NMR chemical shift of the trivalent carbon is a sensitive indicator of carbocation structure. Given below are the data for three carbocations with varying aryl substituents. Generally, the larger the chemical shift, the lower is the electron density at the carbon atom. 

Carbocations, ions that contain a trivalent, positively charged carbon atom, react with water to give alcohols ... 

Figure 6.9 The structure of a carbocation. The trivalent carbon is sp -hybridized and has a vacant p orbital perpendicular to the plane of the carbon and three attached groups.

To understand why a racemic product results from the reaction of T120 wjtl 1-butene, think about the reaction mechanism. 1-Butene is first protonaled tc yield an intermediate secondary (2°) carbocation. Since the trivalent carbon i sp2-hybridized and planar, the cation has no chirality centers, has a plane o symmetry, and is achiral. As a result, it can react with H20 equally well fron either the top or the bottom. Reaction from the top leads to (S)-2-butano through transition state 1 (TS 1) in Figure 9.15, and reaction from the bottorr leads to R product through TS 2. The two transition states are mirror images. The] therefore have identical energies, form at identical rates, and are equally likeb to occur. 

Carbocation (Sections 5.5, 6.9) A carbon cation, or substance that contains a trivalent, positively charged carbon atom having six electrons in its outer shell (R3C+). 

The existence of trivalent silicenium cations as reactive species in solution is more controversial. Many early attempts to demonstrate the solution-phase existence of stable silicenium ions by using techniques analogous to those successfully applied to carbocation formation failed.34-36 Other reports of attempts... 

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

The RC60+ cations are carbenium ions (trivalent carbocations), which are formally conjugated with the entire 7i-system of the fullerene cage. This chapter focuses on recent developments in the author s laboratory regarding the prepara-... 

The most significant conclusion that can be drawn from the data summarized in Table III is that substituent effects do not exert the same overwhelming importance for the thermodynamic stability of the higher homologues of carbenium ions, thus they do not play the dominant role as in carbocation chemistry. This can be traced back on (i) the inherent higher stability of the trivalent cations of the elements Si Pb and (ii) the weakness of the stabilizing interaction (in many cases of ii-type) of the most common substituents with the central element atom. 

A term for a positively-charged trivalent carbon atom (a carbocation). RsC is a carbenium ion. The term car-... 

Some of the most important reaction intermediates in organic chemistry are the carbocations. Neglecting some heteroatom-stabilized cations, most carbocations are divided into two groups trivalent carbenium ions and five-coordinate or higher coordinate carbonium ions. The parent carbenium ion is CHJ, and the parent carbonium ion is CHJ. Carbonium ions have been proposed as reactive intermediates in superacid-catalyzed reactions however, they have never been directly observed in condensed media. In contrast, a variety of carbenium ions have already been prepared in superacidic media and been characterized by various physical methods, mainly 13C NMR spectroscopy (5). 

The carbocations involved in these reactions are trivalent carbenium ions, of which CH3+ is parent. It was Whitmore in the 1930s who first generalized their importance in hydrocarbon transformations based on fundamental studies by Meerwein, Ingold, Pines, Schmerling, Nenitzescu, Bartlett, and others. 

Protolytic reactions of saturated hydrocarbons in superacid media21 were interpreted by Olah as proceeding through the protonation (protolysis) of the covalent C—H and C—C single bonds. The reactivity is due to the electron donor ability of the <7 bonds via two-electron, three-center bond formation. Protolysis of C—H bonds leads via five-coordinate carbocations with subsequent cleavage of H2 to trivalent ions, which then themselves can further react in a similar fashion ... 

In contrast, cracking of longer-chain alkanes with conventional acid catalysts is considered to proceed via p scission involving initial formation of trivalent carbocations ... 

The acidic centers, both the Br0nsted and Lewis types, are generated by the isomorphous substitution of trivalent aluminum for a tetravalent silicon in the silica lattice.61-63 Additionally, carbocations may be formed by protonation of alkenes,43,52,56,61 which explains their higher reactivity in catalytic cracking. 

In the ethane-ethylene reaction in a flow system with short contact time, exclusive formation of n-butane takes place (longer exposure to the acid could result in isomerization). This indicates that a mechanism involving a trivalent butyl cation depicted in Eqs. (5.1)—(5.5) for conventional acid-catalyzed alkylations cannot be operative here. If a trivalent butyl cation were involved, the product would have included, if not exclusively, isobutane, since the 1- and 2-butyl cations would preferentially isomerize to the rm-butyl cation and thus yield isobutane [Eq. (5.9)]. It also follows that the mechanism cannot involve addition of ethyl cation to ethylene. Ethylene gives the ethyl cation on protonation, but because it is depleted in the excess superacid, no excess ethylene is available and the ethyl cation will consequently attack ethane via a pentacoordinated (three-center, two-electron) carbocation [Eq. (5.10)] ... 

There is clear differentiation of the alkylation of alkenes (jt-donor nucleophiles) and alkanes (a donors). The former follows Markovnikov addition, giving a trivalent carbocation and derived branched products. The latter proceeds through a five-coordinate carbocation without involvement of trivalent carbenium ions and thus without necessary branching. 

The stereochemical outcome of Friedel-Crafts alkylation may be either inversion (nucleophilic displacement) or racemization (involvement of a trivalent flat carbocation). Most transformations were shown to occur with complete racemization. In a few instances, inversion or retention was observed. For example, the formation of (—)-l,2-diphenylpropane [(—)-28] from (—)-26 was interpreted to take place through the 27 nonsymmetrically Jt-bridged carbocation, ensuring 50-100% retention of configuration 136... 

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. 

Carbocations with trivalent carbon may have carbon with coordination number 2. Acyl ions have already been mentioned the vinyl cations, or car-bynium ions (26), have been detected as intermediates in addition of electrophiles to acetylenes and allenes and in solvolysis reactions with the highly reactive trifluoromethanesulfonate (triflate) leaving group.84 Vinyl cations are expected... 

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. 

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