Carbocations fert-butyl cation

Preformed or in situ-prepared carbocation salts (tropylium, trityl, etc.) are also active in transforming alkenes to carbocations.119,138,140 Preformed carbocation salts are the simplest initiators in cationic polymerization and ideal if the cation is identical to the one derived from the momomer (e.g., fert-butyl cation in the polymerization of isobutylene). 

Then, the fert-butyl cation intermediate can attack a molecule of butene to give the corresponding C8 carbocation. Depending on the particular butene isomer that is alkylated, a different C8 carbocation will be formed (2,2,4-TMP+ from isobutene, 2,2,3-TMP+ from 2-butene, and 2,2-DMH+ from 1-butene) ... 

There is direct evidence, from ir and nmr spectra, that the fert-butyl cation is quantitatively formed when ferf-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 1019) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride and 1-chloroadamantane alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole, but not benzene. It was noted on p. 476 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates. Lewis acids, such as BF3 or AIEta, can also be used to alkylation of aromatic rings with alkene units. 

For both tertiary cations and secondary benzylic carbocation, the ratio of substitution to elimination is quite high. For example, for l-(/i-tolyl)ethyl cation in 50 50 TFE-water, the ratio is 1400."° For the fert-butyl cation, the ratio is about 30 in water" and 60 in 50 50 TFE-water. " These ratios are on the order of 10 if account is taken of the need for solvent reorganization in the substitution process." The generalization is that under solvolysis conditions, tert-alkyl and sec-benzyl carbocations prefer substitution to elimination. 

To answer this question, we need to look at the mechanism of the reaction. Recall that the first step of the reaction—the addition of H" " to an sp carbon to form either the fert-butyl cation or the isobutyl cation—is the rate-determining step (Section 3.7). If there is any difference in the rate of formation of these two carbo-cations, the one that is formed faster will be the preferred product of the first step. Moreover, because carbocation formation is rate determining, the particular carbo-cation that is formed in the first step determines the final product of the reaction. That is, if the ferf-butyl cation is formed, it will react rapidly with CF to form tert-butyl chloride. On the other hand, if the isobutyl cation is formed, it will react rapidly with Cr to form isobutyl chloride. It turns out that the only product of the reaction is ferf-butyl chloride, so we know that the ferf-butyl cation is formed faster than the isobutyl cation. 

The question now is, Why is the fert-butyl cation formed faster than the isobutyl cation To answer this, we need to take a look at the factors that affect the stability of carbocations and, therefore, the ease with which they are formed. 

Why does the stability of a carbocation increase as the number of alkyl substituents bonded to the positively charged carbon increases Alkyl groups decrease the concentration of positive charge on the carbon—and decreasing the concentration of positive charge increases the stability of the carbocation. Notice that the blue— recall that blue represents electron-deficient atoms—is most intense for the least stable methyl cation and is least intense for the most stable fert-butyl cation. 

The ferf-butyl group is cleaved as the corresponding carbocation. Loss of a proton from fert-butyl cation converts it to 2-methylpropene. Because of the ease with which a tert-butyl group is cleaved as a carbocation, other acidic reagents, such as trifluoroacetic acid,... 

In Chapter 5, we study a group of organic cations called carbocations. Following is the structure of one such carbocation, the fert-butyl cation ... 

In a carbocation, the carbon bearing the positive charge is bonded to three other atoms and, as predicted by VSEPR, the three bonds about the cationic carbon are copla-nar and form bond angles of approximately 120°. According to the orbital hybridization model, the electron-deficient carbon of a carbocation uses sp hybrid orbitals to form a bonds to the three attached groups. The unhybridized 2p orbital lies perpendicular to the a bond framework and contains no electrons. Figure 6.3 shows a Lewis structure, a cartoon, and a calculated structure indicating the vacant 2p orbital for the fert-butyl cation. 

Similarly, in the reaction of HBr with 2-methylpropene, proton transfer to the carbon-carbon double bond might form either the isobutyl cation (a 1° carbocation) or the fert-butyl cation (a 3° carbocation). 

Carbocations are classified according to the degree of substitution at the positively charged carbon. The positive charge is on a primary carbon in CHjCH2, a secondary carbon in (CH3)2CH, and a tertiary carbon in (CH3)3C+. Ethyl cation is a primary carbocation, isopropyl cation a secondary carbocation, and fert-butyl cation a tertiary carbocation. 

The alkylation of isobutane with C3-C5 olefins involves a series of consecutive and simultaneous reactions with carbocation species as the key intermediates. Scheme 6.10.2 shows the reaction of 2-butene and isobutane as a typical example. In the initial step, proton addition to 2-butene affords a sec-butyl cation. This sec-butyl cation can either isomerize or accept a hydride from a molecule of isobutane, giving n-butane and the thermodynamically more stable fert-butyl cation. These initiation reactions are required to generate a high level of ions in the start-up phase of alkylation but become less important under steady state conditions. 

The preference for fragmentation at a highly snbstituted center is even more pronounced in the mass spectrum of 2,2-dimethylpropane (Figure 11-27). Here, loss of a methyl radical from the molecular ion produces the 1,1-dimethylethyl (fert-butyl) cation as the base peak at m/z = 57. This fragmentation is so easy that the molecular ion is barely visible. The spectrum also reveals peaks at m/z = 41 and 29, the result of complex structural reorganizations, such as the carbocation rearrangements considered in Section 9-3. 

The reaction proceeds via a pentacoordinate hydroxycarbonium ion transition state, which cleaves to either fert-butyl alcohol or the tert-butyl cation. Since 1 mol of isobutane requires 2 mol of hydrogen peroxide to complete the reaction, one can conclude that the intermediate alcohol or carbocation reacts with excess hydrogen peroxide, giving fcrt-butyl hydroperoxide. The superacid-induced rearrangement and cleavage of the hydroperoxide results in very rapid formation of the dimethylmethyl-carboxonium ion, which, upon hydrolysis, gives acetone and methyl alcohol. 

Answer The solvents and leaving groups are the same. The SnI reaction will be faster via the more stable carbocation. The most stable carbocation and fastest SnI reaction is via the diphenylmethyl carbocation next is via the fert-butyl carbocation. The least stable isopropyl cation is the slowest, not expected to form at a significant rate. 

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