In carbocation reactions

Although the geometric relationship suggested by Shiner and by Sunko and their co-workers clearly demonstrates that hyperconjugation is the major contributor to the secondary /3-deuterium KIE in carbocation reactions, Williams (1985) has suggested that there is a significant inductive component to these KIEs. Williams used ab initio MO methods to calculate the geometries of the substrates and the isopropyl carbocation formed in a gas-phase heterolysis (30) of series of isopropyl derivatives at the RHF/4-31G level. 

Hydride shifts and transfers figure prominently in carbocation reactions and these have been thoroughly reviewed (Ahlberg et al., 1983 Brouwer and Hogeveen, 1972 Brouwer, 1980), and are the subject of a recent monograph (Vogel, 1985). 

This kind of ions with a bridged hydrogen can be generated from cycloalkanes with 8-11 carbon atoms, i.e, those for which direct transannular hydrogen shifts in carbocation reactions was postulated Ions with Cg, and -rings as distinct from a cyclodecyl cation, rearrange to contract the cycle and form tertiary ions, e.g. ... 

While S-3 participation in carbocation reactions is common, cyclo-propylcarbinyl cations such as (31) preferentially lead to ring enlargement, and a synthetic procedure for obtaining cyclobutanones has been devised based on this observation. " Thus the tertiary alcohol (32) is ether, upon treatment with aqueous fluoboric acid, gives the spiro ketone (33) quanti-... 

Reactions of the 2-amino-4,5-substituted thiazole (52) in acetic acid with ethylene oxide has been reported to give the N-exocyclic disubstitution product (S3) (201) in a 40% yield (Scheme 38). The reactive species in this reaction is probably the carbocation generated in acetic acid by ethvlene oxide. 

In Chapter 4 you learned that carbocations could be captured by halide anions to give alkyl halides In the present chapter a second type of carbocation reaction has been introduced—a carbocation can lose a proton to form an alkene In the next section a third aspect of carbocation behavior will be described the rearrangement of one carbo cation to another... 

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... 

Section 11 14 Benzylic carbocations are intermediates in SnI reactions of benzylic halides and are stabilized by electron delocalization... 

Carbinolamines are formed by nucleophilic addition of an amine to a carbonyl group and are intermediates in the for mation of imines and enamines Carbocation (Section 4 8) Positive ion in which the charge re sides on carbon An example is tert butyl cation (CH3)3C Carbocations are unstable species that though they cannot normally be isolated are believed to be intermediates in certain reactions... 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

Carbocation rearrangements occur in the reactions of some secondary alco hols with DAST, thus isobutyl alcohol gives a mixture of isobutyl fluoride and tert-hxAy] fluonde [95] (Table 6), and both bomeol and isoborneol rearrange to the same 3-fluoro-2 2,3-tnraethylbicyclo[2 2 IJheptane (72-74%) accompanied by camphene [95]... 

As we have just seen, the rate-determining intermediate in the reaction of tert-butyl alcohol with hydrogen chloride is the carbocation (CH3)3C. Convincing evidence from a variety of sources tells us that carbocations can exist, but are relatively unstable. When carbocations are involved in chemical reactions, it is as reactive intermediates, formed slowly in one step and consumed rapidly in the next one. 

As carbocations go, CH3 is par ticularly unstable, and its existence as an intermediate in chemical reactions has never been demonstrated. Primary carbocations, although more stable than CH3 , are still too unstable to be involved as intermediates in chemical reactions. The threshold of stability is reached with secondary carbocations. Many reactions, including the reaction of secondary alcohols with hydrogen halides, are believed to involve secondary carbocations. The evidence in support of tertiar-y carbocation intermediates is stronger yet. 

Unlike tertiary and secondary carbocations, methyl and primary carbocations are too high in energy to be intermediates in chemical reactions. However, methyl and primary... 

Like carbocations, most free radicals are exceedingly reactive species—too reactive to be isolated but capable of being formed as transient intermediates in chemical reactions. Methyl radical, as we shall see in the following section, is an intermediate in the chlorination of methane. 

These common features suggest that carbocations are key intermediates in alcohol dehydrations, just as they are in the reaction of alcohols with hydrogen halides. Figure 5.6 portrays a three-step mechanism for the acid-catalyzed dehydration of tert-butyl alcohol. Steps 1 and 2 desaibe the generation of tert-butyl cation by a process similar- to that which led to its formation as an intermediate in the reaction of tert-butyl alcohol with hydrogen chloride. 

We have seen this situation before in the reaction of alcohols with hydrogen halides (Section 4.11), in the acid-catalyzed dehydration of alcohols (Section 5.12), and in the conversion of alkyl halides to alkenes by the El mechanism (Section 5.17). As in these other reactions, an electronic effect, specifically, the stabilization of the carbocation intennediate by alkyl substituents, is the decisive factor. The more stable the caibocation, the faster it is fonned. 

Partial but not complete loss of optical activity in SnI reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile. Ionization of the alkyl halide gives a carbocation-halide ion pair-, as depicted in Figure 8.8. The halide ion shields one side of the carbocation, and the nucleophile captures the carbocation faster from the opposite side. More product of inverted configuration is formed than product of retained configuration. In spite of the observation that the products of SnI reactions are only partially racemic, the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular- mechanism. 

Reanangements, when they do occur, are taken as evidence for carbocation intermediates and point to the SnI mechanism as the reaction pathway. Reanangements are never observed in Sn2 reactions. 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intennediates in chemical reactions. The third type of conjugated system that we will examine, conjugated dienes, consists of stable molecules. 

The first step in the reaction of electrophilic reagents with benzene is similar-. An electrophile accepts an electron pair- from the tt system of benzene to form a carbocation ... 

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

Figure 5.4 An energy diagram for the first step in the reaction of ethylene with HBr. The energy difference between reactants and transition state, AG, defines the reaction rate. The energy difference between reactants and carbocation product, AG°, defines the position of the equilibrium.

Since carbocations are involved as intermediates in these reactions, Markovnikov s atle can be restated. 

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