Carbocations, stability stable

Clearly the steric crowding that influences reaction rates in 8 2 processes plays no role in Stvfl reactions The order of alkyl halide reactivity in 8 1 reactions is the same as the order of carbocation stability the more stable the carbocation the more reactive the alkyl halide... 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

Very stable carbocation (stabilized by both alkoxy function and aromaticity)... 

One way of determining carbocation stabilities is to measure the amount of energy required to form the carbocation by dissociation of the corresponding alkyl halide, R-X - R+ + X . As shown in Figure 6.10, tertiary alkyl halides dissociate to give carbocations more easily than secondary or primary ones. As a result, trisubstituted carbocations are more stable than disubstituted ones, which are more stable than monosubstituted ones. The data in Figure 6.10 are taken from measurements made in the gas phase, but a similar stability order is found for carbocations in solution. The dissociation enthalpies are much lower in solution because polar solvents can stabilize the ions, but the order of carbocation stability remains the same. 

The mechanism involves a simple 1,2 shift. The ion (52, where all four R groups are Me) has been trapped by the addition of tetrahydrothiophene. It may seem odd that a migration takes place when the positive charge is already at a tertiary position, but carbocations stabilized by an oxygen atom are even more stable than tertiary alkyl cations (p. 323). There is also the driving force supplied by the fact that the new carbocation can immediately stabilize itself by losing a proton. 

As we saw in the previous section, Markovnikov s rule tells us to place the H on the less substituted carbon, and to place the X on the more substituted carbon. The rule is named after Vladimir Markovnikov, a Russian chemist, who first showed the regiochemical preference of HBr additions to alkenes. When Markovnikov recognized this pattern in the late 19th century, he stated the rule in terms of the placement of the proton (specifically, that the proton will end up on the less substituted carbon atom). Now that we understand the reason for the regiochemical preference (carbocation stability), we can state Markovnikov s rule in a way that more accurately reflects the underlying principle The regiochemistry will be determined by the preference for the reaction to proceed via the more stable carbocation intermediate. 

These substituent effects are due to the stabilization of the carbocations that result from protonation at the center carbon. Even if allylic conjugation is not important, the aryl and alkyl substituents make the terminal carbocation more stable than the alternative, a secondary vinyl cation. 

Substituent effects Carbocations are formed in the S l reactions. The more stable the carbocation, the faster it is formed. Thus, the rate depends on carbocation stability, since alkyl groups are known to stabilize carbocations through inductive effects and hyperconjugation (see Section 5.2.1). The reactivities of SnI reachons decrease in the order of 3° carbocation > 2° carbocation > 1° carbocation > methyl cation. Primary carbocation and methyl cation are so unstable that primary alkyl halide and methyl halide do not undergo SnI reachons. This is the opposite of Sn2 reactivity. 

The transition state is closer in energy to the carbocation and more closely resembles it than the alkyloxonium ion. Thus, structural features that stabilize carbocations stabilize transition states leading to them. It follows, therefore, that alkyloxonium ions derived from tertiary alcohols have a lower energy of activation for dissociation and are converted to their corresponding carbocations faster than those derived from secondary and primary alcohols. Simply put more stable carbocations are formed faster than less stable ones. Figure 4.17 expresses this principle via a potential energy diagram. 

Oxides 28 and 29 react with nucleophiles like p-toluidine, although the reactivities of epoxides 1,4,28, and 29 differ by a factor of less than three. It is interesting that 253 is most stable toward the attack of p-toluidine, showing that the carbocation-stabilizing group does not necessarily facilitate the reaction with nucleophiles. 

This can be tested by drawing on extensive information on carbocation stabilities in the gas phase. Heats of formation of ethyl, isopropyl, sec-butyl and /-butyl cations2 are shown below. From these values it is evident that the /-butyl cation is more stable than the sec-butyl cation by 13kcalmol 1. This corresponds to the direct comparison of (isomeric) ion stabilities noted above by Arnett and Mayr. 

The order of carbocation stability is tertiary > secondary > primary. There is only one C5Hn+ car-bocation that is tertiary, and so that is the most stable one. 

For the SN1 reaction, formation of the carbocation is the rate-limiting step. We have already seen that the transition state for this step resembles the carbocation. Any change that makes the carbocation more stable will also make the transition state more stable, resulting in a faster reaction. Carbocation stability controls the rate of the SN1 reaction. Many studies have provided the following order of carbocation stabilities ... 

SnI reactivity is related to carbocation stability. Thus, substrates that form the most stable carbocations are the most reactive in SnI reactions. 

When we speak of carbocation stability, we really mean relative stability. Tertiary carbocations are too unstable to isolate, but they are more stable than secondary carbocations. We will examine the reason for this order of stability by invoking two different principles inductive effects and hyperconjugation. 

A second explanation for the observed trend in carbocation stability is based on orbital overlap. A 3° carbocation is more stable than a 2°, 1°, or methyl carbocation because the positive charge is delocalized over more than one atom. 

The preceding reaction steps present another difficulty, namely, attack by the methyl cation. Primary carbocations that lack stabilizing groups are highly unstable, and the methyl cation is the least stable of the carbocations. In fact, even in superacid (FS03H-SbF5), no primary carbocation is stable enough to be detected. Consequently, a mechanism that invokes such a species should be looked upon with suspicion. 

Cations at sp - or 5p-hybridized carbons are especially unstable. In general, the more 5 character in the orbitals, the less stable the cation. An approximate order of carbocation stability is CRjCO" (acetyl cation) (CHJ,C+ ... 

PATr+ is the pK value for the reaction R+ + 2 H2O ROH + HsO" " and is a measure of the stability of the carbocation. The //r parameter is an early obtainable measurement of the stabihty of a solvent (see p. 371) and approaches pH at low concentrations of acid. In order to obtain pA) +, for a cation R" ", one dissolves the alcohol ROH in an acidic solution of known //r. Then the concentration of R" and ROH are obtained, generally from spectra, and pATr+ is easily calculated. A measure of carbocation stability that applies to less-stable ions is the dissociation energy D(R -H ) for the cleavage reaction R — H R" " -f H , which can be obtained from photoelectron spectroscopy and other measurements. Some values of D(R+ H ) are shown in Table 5.2. Within a given class of ion (primary, secondary, aUylic, aryl, etc.), D(R H ) has been shown to be a linear function of the logarithm of the number of atoms in R, with larger ions being more stable. " ... 

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