Carbon-hydrogen bond carbocations

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

Other mechanisms with the involvement of an incipient methyl cation were also proposed.418,460,462 An attack of methyl cation released from extensively polarized methoxy groups on the carbon-hydrogen bond forms pentacoordinated carbocation intermediates that, in turn, yields ethyl methyl ether after the loss of a proton ... 

Figure 5.12 Hyperconjugation occurs when a carbon-hydrogen bond lies in the same plane as a carbocation s vacant p orbital.

The abbreviation El stands for Elimination, unimolecular. The mechanism is called unimolecular because the rate-limiting transition state involves a single molecule rather than a collision between two molecules. The slow step of an El reaction is the same as in the SN1 reaction unimolecular ionization to form a carbocation. In a fast second step, a base abstracts a proton from the carbon atom adjacent to the C+. The electrons that once formed the carbon-hydrogen bond now form a pi bond between two carbon atoms. The general mechanism for the El reaction is shown in Key Mechanism 6-8. 

Alcohols do not do uncatalyzed eliminations of water at reasonable temperatures. The carbon-oxygen bond is not a base, and the carbon-hydrogen bond is not an acid. The process drawn on the left is a four-center, four-electron process, which with very few exceptions does not occur thermally. The most common route for water elimination is by acid catalysis, as shown on the right path p.t., protonation of a lone pair, followed by the E2 elimination. If the carbocation is reasonably stable, the reaction may proceed via El. 

One very simple thing for a carbocation to do, is to drop off an adjacent proton to give an olefin as shown in Figure 5.7. The electrons forming the carbon-hydrogen bond, move in to form a double bond between the... 

The positively charged carbon atom in a carbocation is an extremely electron-dehcieni (electrophilic) carbon. As such, its behavior is dominated by a need to obtain an electron pair from any available source. The Sn I reaction illustrates the most obvir>"s fate of a < nr .- -t on c a.bination with an external Lewis base, forming a new bond to carbon. However, the electron deficiency of cationic carbon is so great that even under typical SnI solvolysis conditions, surrounded by nucleophilic solvent molecules, some of the cations won t wait to combine with external electron-pair sources. Instead, they will seek available electron pairs within their own molecular structures. The most available of the.se are electrons in carbon-hydrogen bonds one carbon removed from the cationic center (at llic so-called carbon) ... 

The crucial step in acid-catalyzed conversions of hydrocarbons is the formation of the intermediate trivalent or classical sp hybridized carbocation (car-benium ion). In the case of saturated hydrocarbons, this is interpreted by the interaction of the proton of the superacid and the bonding electron pair of the C—H a bond (a similar interaction between the proton and the C—C a bond would result in the cleavage of the carbon-carbon bond). This is based on the concept of a-basicity developed by Olah (65), which describes the ability of a bonds to share their bonded electrons with electrophiles. A hypervalent, pentacoordinate non-classical carbocation (carbonium ion) is formed, which possesses a three-center, two-electron (3c-2e) bond. This is transformed to the classical trivalent carbocation by the loss of hydrogen, that is, protolysis (protolytic cleavage) of the carbon-hydrogen bond occurs. The process is illustrated by the conversion of hexane to yield the 3-hexyl cation through the pentacoordinate carbonium ion (1) (eq. 45). 

Further research into the reaction mechanism revealed that the reaction rate was correlated with the electron structure of the sulfoxide the more electropositive sulfoxides were the better oxygen donors. Excellent correlation of the reaction rates with the heterolytic benzylic carbon-hydrogen bond dissociation energies indicated a hydride abstraction mechanism in the rate-determining step to yield a carbocation intermediate. The formation of 9-phenylfluorene as by-product in the oxidation of triphenylmethane supports this suggestion. Further kinetic experiments and NMR showed the formation of a polyoxometalate-sulfoxide complex before the oxidation reaction, this complex being the active oxidant in these systems. Subsequently, in a similar reaction system, sulfoxides were used to facilitate the aerobic oxidation of alcohols [29]. In this manner, benzylic, allyUc, and aliphatic alcohols were all oxidized to aldehydes and ketones in a reaction catalyzed by Ke jn-type... 

WORKED PROBLEM 7.18 We have again avoided (p. 138) a detailed discussion of why more substituted carbocations are more stable than less substituted ones. Nevertheless you can now see the outlines of why this is so. Draw a three-dimensional picture of the ethyl cation. Remember. The positively charged carbon and the three surrounding atoms are coplanar—this part of the ethyl cation is flat.) Now consider the overlap of one of the carbon—hydrogen bonds of the methyl group with the empty 2 orbital. Is this interaction stabilizing Why ... 

ANSWER Interactions between filled and empty orbitals are stabiliang. This fundamental tenet of chemistry can he applied in this case. If the filled orbital of the carbon-hydrogen bond overlaps with the empty 2p orbital of the carbocation, the result is the stabilizing interaction called hyperconjugation. 

We have just seen that carbocations are stabilized by delocalization through the overlap of filled and empty orbitals. One example of this effect is fairly straightforward a lone pair of electrons on an adjacent atom interacts with an empty orbital. Less obvious is the last phenomenon, the overlap of filled O orbitals of carbon—hydrogen bonds with empty orbitals, called hyperconjugation. Now we proceed to other factors influencing the stability of carbocations. 

Why should more substituted radicals be more stable than less substituted radicals Let s start with an analogy. In explaining the order of stability for carbocations (tertiary > secondary > primary > methyl), we drew resonance forms in which a filled carbon-hydrogen bond overlapped with the empty 2f orbital in a process called hyperconjugation (Fig. 11.17). 

We can draw similar resonance forms for radicals. As before, the more alkyl groups attached to the carbon bearing the nonbonding electron, the more resonance forms are possible. But there is an important difference Stabilization of the carbocation involves overlap of a filled carbon-hydrogen bond with an empty 2p orbital (Fig. 11.17), whereas in the radical, overlap is between a filled carbon-hydrogen bond and a half-filled 2p orbital (Fig. 11.18). 

Of course, we should analyze the carbon-hydrogen bonds as well. If we do this, we find that the differences in the strengths of the C—H bonds should favor the 1-propyl radical. As with carbocations, it is the C—C bonds that dominate and determine the issue. As a practical matter, it appears that we can get away with an analysis of the C—C bonds alone. 

This reaction is typified by the addition of a hydrogen halide, HX, or water, to an alkene. The process involves two steps, with a carbocation as the intermediate. In the first step (Figure 11.1), an electrophile, for example, a proton, is added to the carbon-carbon bond to form a carbocation. Notice how the curly arrow is drawn—the proton is being added to the upper carbon of the double bond and the electrons are taken away from the lower carbon, leaving it positively charged. In the second step, the counterion, for example, bromide, attacks the carbocation to give a saturated product. Note that we only show one of the new bonds we have made in this example (in red) a carbon-hydrogen bond has also been made. 

A molecule of water removes one of the hydrogens from the P carbon of the carbocation. An electron pair moves in to form a double bond between the a and P carbon atoms. 

Electrophilic addition of HBr to propene gives predominantly the so-called Markovnikov orientation Markovnikov s rule states that addition of HX across a carbon-carbon multiple bond proceeds in such a way that the proton adds to the less-substituted carbon atom, i.e. that already bearing the greater number of hydrogen atoms (see Section 8.1.1). We rationalized this in terms of formation of the more favourable carbocation, which in the case of propene is the secondary carbocation rather than the alternative primary carbocation. 

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

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