Tertiary carbenium ion

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. 

As a catalyst sulfuric acid is most often used phosphoric acid, boron trifluoride or an acidic ion exchange resin have also found application. 1,1-disubstituted alkenes are especially suitable substrates, since these are converted to relatively stable tertiary carbenium ion species upon protonation. o ,/3-unsaturated carbonyl compounds do not react as olefinic component. 

Alkylation of tetrapropylene follows over a tertiary carbenium ion, so that the benzene ring is always connected to a quaternary carbon atom (2) ... 

The chemistry of acid catalyzed reactions is deeply documented and will not be considered here. It is sufficient to say that they are based in carbenium ion reactions, which yield lower molecular weight compounds and branched paraffins (since tertiary carbenium ions are the most stable). The result of the cracking of a paraffin of n-carbon atoms is another paraffin with (n-x) carbon atoms and an olefin of x-carbon atoms. 

Ionic hydrogenations of C=C bonds generally work well only in cases where a tertiary or aryl-substituted carbenium ion can be formed through protonation of the C=C bond. Alkenes that give a tertiary carbenium ion upon protonation include 1,1-disubstituted, tri-substituted and tetra-substituted alkenes, and each of these are usually hydrogenated by ionic hydrogenation methods in high yields. 

There are substantial differences between gas-phase and liquid-phase hydride transfer reactions. In the latter, the hydride transfer occurs with a low activation energy of 13-17 kJ/mol, and no carbonium ions have been detected as intermediates when secondary or tertiary carbenium ions were present (25). 

The cations formed from simple olefins are secondary or tertiary carbenium ions ... 

At about the same time, Mayr measured the rate of attack of various types of carbenium ions on alkenes by new and ingenious methods. His rate constants k2 for tertiary carbenium ions are ca. 4 orders of magnitude greater than the kp+ values recommended by this author [1], Despite detailed discussions between Mayr and the present writer, involving also a detailed examination of Mayr s methods, a reason for the discrepancy could not be discovered in the mid-1990s [2]. The purpose of the present paper is to suggest one, and thus to explain the discrepancy. 

The author s theory which has been used here was developed in detail to explain the polymerisations by ionising radiations of some alkyl vinyl ethers, the polymerisations of which proceed by secondary ions. Although it was shown that the theory is also perfectly serviceable for the tertiary carbenium ions considered here, it must be realised that there is a fundamental difference between these two types of carbenium ions. When one of the bonds of the carbenium ion is a C—H bond, the solvators, especially of course an ion, can get much closer to the positive centre, and they are therefore correspondingly more firmly held to which effect is added that of a smaller steric hindrance. The most researched monomer propagating by secondary cations, apart from the alkyl vinyl ethers, is, of course, styrene. Thus, Mayr s many studies with diaryl methylium cations are directly relevant to the polymerisation of styrene. 

Example The El mass spectmm of -decane is typical for this class of hydrocarbons (Fig. 6.18a). Branching of the aliphatic chain supports cleavage of the bonds adjacent to the branching point, because then secondary or tertiary carbenium ions and/or alkyl radicals are obtained (Fig. 6.18b,c). This allows for the identification of isomers to a certain degree. Unfortunately, hydrocarbon molecular ions may undergo skeletal rearrangements prior to dissociations, thereby obscuring structural information. 

Figure 13.18 Protonation mechanism for i-butyiene over a Bronsted acid site-tertiary carbenium ion or an alkoxide.

It is important to note that, while the primary product Cg carbenium ions that are formed (after reaction with 2-butene or 1-butene) are secondary, they can undergo hydride shift or methyl shift and form a tertiary carbenium ion in each case. In that case the driving force is diminished for either of the two tertiary Cg carbenium ions to abstract a hydride ion from i-butane since this now becomes a transition from a large tertiary carbenium ion to a smaller tertiary carbenium ion. Nevertheless, this hydride transfer can still occur due to the high ratio of i-butane to tertiary Cg carbenium ion that exists in the reaction medium. At the same time the tertiary Cg carbenium ion may get alkylated with another butylene molecule to make the more stable C12 carbenium ion, which would then lead to heavies. 

In the rearrangement of l-methylbicyclo[3.1.1]heptan-6-one (8) the tertiary carbenium ion 9 in which the configuration at C6 is still retained is apparently more stable than the secondary ion, thus leading to ra-5-methyIbicyclo[3.2.0]heptan-6-one (10) with retention of its chiral information.44... 

CH2—CH(iPr)ion. According to these workers, surprisingly, the content of isomerized units is independent of monomer concentration, conversion and molecular weight. To account for these observations Kenndy et al. (4) proposed a model according to which the initially formed secondary ion-counteranion pair CSG or the more stable tertiary carbenium ion counteranion pair C,G (formed from the former by rate constant kH) can add monomer M to form ternary complexes CSGM or C,GM (with rate constants kc and k c) respectively. Within the former complex the secondary ion can isomerize to the tertiary ion with rate constant k H. Propagation of either ion can occur by incorporation of the complexed monomer into the chain end (with rate constants k or k[, respectively)... 

As shown, the ratio was very high on zeolite catalysts, while that on mesoporous silica was as low as those on AMS and quartz chip. The high ratio on zeolites can not be explained by classical mechanism of acid-catalyzed cracking supposing higher stability of tertiary carbenium ion and its cracking by P-scission, because this supposition predicts that the reaction (2) proceeds in preference to the reaction (1). Rather, a-scission of carbocation [12] may rationalize the higher C3/C4 ratio on zeolite catalysts. 

Higher protonated alcohols cleave to stable tertiary alkyl cations. For protonated primary and secondary alcohols, the initially formed primary and secondary carboca-tions rapidly rearrange to the more stable tertiary carbenium ions under the conditions of the reaction. For example, protonated ra-butyl alcohol 7 cleaves to ra-butyl cation which rapidly rearranges to tot-butyl cation (k2 ki) [Eq. (4.4)]. 

Peterson and Bonazza417 have reported that ionization of cis-1,2-bis(chloro methyl) cyclohexane in SbF5-S02 solution at —78°C gives the bicyclic five-member-rig chloronium ion 186 along with smaller amounts of other species [Eq. (4.128)]. Warming the solution containing the ion to 10°C leads to the formation of the open-chain tertiary carbenium ion 187. 

The formation of C6 and C7 acids along with some ketones was reported in the reaction of isopentane, along with methylcyclopentane and cyclohexane with CO in HF-SbF5 at ambient temperatures and atmospheric pressure.406 Yoneda et al.407 have also found that other alkanes can be carboxylated as well with CO in HF-SbF5. Tertiary carbenium ions, which are produced by protolysis of C—H bonds of branched alkanes in HF-SbF5, undergo skeletal isomerization and disproportionation before reacting with CO. Formation of the tertiary carboxylic acids in the... 

The conversion is thought to involve formation of the carboxonium ion (77) by protonation of the carbonyl oxygen, and subsequent protonation then occurs at the C-H bond. The resulting carboxonium-carbonium dication (78) possesses the maximum possible charge-charge separation for this bicyclic framework. Subsequently, an intermediate carboxonium-carbenium dication (79) is produced, which isomerizes to the tertiary -carbenium ion, and deprotonation provides the product enone (80). Similar distonic superelectrophiles are proposed in other rearrangements of terpenes in superacid.28... 

The formation of the ring-opened products by the electrooxidation may be explained by one-electron oxidation to the cation radical followed by C—C bond cleavage forming a tertiary carbenium ion and an allyl radical. Deprotonation, further one-electron oxidation, and attack of the nucleophile yield 18 and 19. 

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