Alkylation carbocation

A wide range of caibocation stability data has been obtained by measuring the heat of ionization of a series of chlorides and cafbinols in nonnucleophilic solvents in the presence of Lewis acids. Some representative data are given in Table 5.4 These data include the diarylmediyl and triarylmethyl systems for which pX R+ data are available (Table 5.1) and give some basis for comparison of the stabilities of secondary and tertiary alkyl carbocations with those of the more stable aryl-substituted ions. 

The mechanism of alkyne additions is similar but not identical to that of alkene additions. When an electrophile such as HBr adds to an alkene (Sections 6.7 and 6.8), the reaction takes place in two steps and involves an alkyl carbocation intermediate. If HBr were to add by the same mechanism to an alkyne, an analogous vinylic carbocation would be formed as the intermediate. 

A vinylic carbocation has an sp-hybridized carbon and generally forms less readily than an alkyl carbocation (Figure 8.2). As a rule, a secondary vinylic carbocation forms about as readily as a primary alkyl carbocation, but a primary vinylic carbocation is so difficult to form that there is no clear evidence it even exists. Thus, many alkyne additions occur through more complex mechanistic pathways. 

Figure 8.2 The structure of a secondary vinylic carbocation. The cationic carbon atom is sp-hybridized and has a vacant p orbital perpendicular to the plane of the tt bond orbitals. Only one R group is attached to the positively charged carbon rather than two, as in a secondary alkyl carbocation. The electrostatic potential map shows that the most positive (blue) regions coincide with lobes of the vacant p orbital and are perpendicular to the most negative (red) regions associated with the ir bond.

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the... 

Because of resonance stabilization, a primary allylic or benzylic carbocation is about as stable as a secondary alkyl carbocation and a secondary allylic or benzylic carbocation is about as stable as a tertiary alkyl carbocation. This stability order of carbocations is the same as the order of S l reactivity for alkyl halides and tosylates. 

Yet a final limitation to the Friedel-Crafts reaction is that a skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used. Treatment of benzene with 1-chlorobutane at 0 °C, for instance, gives an approximately 2 1 ratio of rearranged (sec-butyl) to unrearranged (butyl) products. 

The simple alkyl carbocations have already been seen (p. 83) to follow the stability sequence,... 

Table 3 Relative stabilities" (in kcalmol of substituted bromonium ions and alkyl carbocations in the gas phase.11...

Estimating stability it is possible to apply criteria commonly used in organic chemistry. Tertiary alkyl carbocation is more stable than the secondary one which is in its turn more stable than the primary one. For the carbon ions of this type the row of the stability is reversed. Allyl and benzyl cations are stable due to the resonance stabilization. The latter having four resonance structures may rearrange to be energetically favorable in the gas phase tropilium cation possessing seven resonance forms (Scheme 5.3). 

On the basis of hyperc onjugation, among alkyl carbocations, tertiary butyl carbocation has been shown to be most stable, because in the following reaction,... 

These calculations act as a backdrop for the next part of this Chapter, in which we describe calculations of ground-states, transition-states and products, for the loss of H2 from simple secondary alkyl carbocations, using the cyclodecyl cations as our model. 

The racemization of an optically active secondary halide with the chiral carbon carrying the halogen (e.g., 2-chlorobutane) may occur ift solution and, usually, the more polar and better ionizing the solvent is, the more readily the substance is racemized. Ionization of the halide by an SK1 process probably is responsible, and this certainly would be promoted by polar solvents (see Section 8-6). All indications are that an alkyl carbocation once dissociated from its accompanying anion is planar and, when such an ion recombines with the anion, it has equal probability of forming the d and L enantiomers ... 

X-ray diffractometry is the most powerful method to determine atomic coordinates of molecules in the solid state. X-ray crystal structure analysis was, however, rarely applied in the early years of development of persistent, long-lived alkyl carbocations and studies were only performed to investigate structures of carbocations of aryl derivatives and aromatic systems.65 This is due to the low thermal stability of alkyl carbocations and to the difficulties in obtaining single crystals of carbocations suitable for analysis. Since then, however, methods and instrumentation have improved significantly and X-ray crystal structure analysis has become a powerful tool to solve structural problems of carbocations.65,66... 

Antimony pentafluoride itself (neat or in S02 or S02C1F solution) also ionizes alcohols to form alkyl carbocations [Eq. (3.15)]. 

The second major type of nucleophilic substitution mechanism is the S 1 mechanism. This mechanism proceeds via two steps. The first step (the slow step) involves the breakdown of the alkyl halide into an alkyl carbocation and a leaving group anion. The second step (the fast step) involves the formation of a bond between the nucleophile and the alkyl carbocation. 

Because the activated complex contains only one species—the alkyl carbocation—the substitution is considered unimolecular. 

Vinyl and aryl halides generally do not undergo SN1 or Sn2 reactions. An SN1 reaction would require ionization to form a vinyl or aryl cation, either of which is less stable than most alkyl carbocations. An Sn2 reaction would require back-side attack by the nucleophile, which is made impossible by the repulsion of the electrons in the double bond or aromatic ring. 

Formation of an alkyl-substituted benzene derivative by substitution of an alkyl carbocation or carbocation-like species on an aromatic ring. (p. 777)... 

The intermediate in addition reactions is a vinylic carbocation, which forms less readily than an alkyl carbocation. 

Olah and coworkers have carried out reactions of alkanes in superacid solution (SbFs-HF or SbFs-FSOsH), leading to observable (by NMR) tertiary alkyl carbocations (17, 18, 19). This reaction is pictured as a protonation of the alkane to give a pentacoordinated intermediate which then loses H2. Thus, a plausible mechanism for the cyclodecyl -> 9-decalyl cation reaction could involve loss of the bridging hydrogen (as H ) to form decalin, and then reprotonation of the decalin to eventually form H2 + 9-decalyl cation. However, when the cyclodecyl cation 1 was prepared in SbFs-FSOsD solution, there was no evidence of H-D formation in the hydrogen gas that was produced. It was concluded therefore that the H2 gas must come directly from the carbocation hydrogens without any involvement by the superacid system. 

The aryl group participation process is mechanistically a Friedel-Crafts alkylation. The r value of the Friedel-Crafts alkylation by alkyl carbocations was found to be significantly lower than that of protonation or of halogenation of aromatic substrates (Yukawa et al., 1966). Olah interpreted the low r value in terms of an earlier transition state, i.e. less advanced aryl C bond formation with r = 0.6 at the transition state prior to formation of the... 

Kelly and Brown (1976) describe a method for the preparation of relatively concentrated ( 1 M) solutions of alkyl carbocations in SbEj-SOjClF suitable for C-nmr investigations. This method, which uses syringe techniques, allows the quantitative conversion of precursors soluble in SOjClF at —78°C into the corresponding cations. 

In another approach Borodkin et al. (1976c) established linear relations between the rates (log k) of two different migrating groups (CHj and H) in several carbocations with different core structure. Interestingly, the rates of such different reactions as 1,2-shifts in simple alkyl carbocations and sigma-tropic rearrangements of cyclopentadienes could be satisfactorily predicted by these relations. 

The alkyl carbocations formed from alkyl halides, aikenes and alcohols act as an electrophile in Friedel-Crafts alkylation reactions. ... 

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