Halides dissociation

The other halides dissociate at lower temperatures and, if put into water, all are decomposed, the proton transferring to water which is a better electron pair donor ... 

Step (1) Alkyl halide dissociates by heterolytic cleavage of carbon-halogen bond (Ionization step)... 

Step 1 The alkyl halide dissociates to a carbocation and a halide ion... 

Figure 6.10 A plot of dissociation enthalpy versus substitution pattern for the gas-phase dissociation of alkyl chlorides to yield carbocations. More highly substituted alkyl halides dissociate more easily than less highly substituted ones.

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 scope and limitations of the Lewis acid-catalyzed additions of alkyl chlorides to carbon-carbon double bonds were studied.51 Since Lewis acid systems are well-known initiators in carbocationic polymerizations of alkenes, the question arises as to what factors govern the two transformations. The prediction was that alkylation products are expected if the starting halides dissociate more rapidly than the addition products.55 In other words, addition is expected if the initial carbocation is better stabilized than the one formed from the dissociation of the addition product. This has been verified for the alkylation of a range of alkyl-and aryl-substituted alkenes and dienes with alkyl and aralkyl halides. Steric effects, however, must also be taken into account in certain cases, such as in the reactions of trityl chloride.51... 

Reaction (IV) is possible and in fact quite likely. The breaking of a carbon-bromine bond involves the absorption of 58,000 calories and this minimum is close enough to the 55,000 calories to be within the limit of uncertainty of the constants involved. This reaction can account for the experimental results and is in line with spectroscopic evidence that ethyl halides dissociate photo-chemically into the halogen atom and the free radical. 

Despite the fact that a coordination site is not formally required for reductive elimination reactions, ligand dissociation from the metal center apparently facilitates reductive elimination. In the iridium examples described in Table 11, halide dissociation is important in other examples, phosphine ligand dissociation is important. 

The reactions, summarized in Table 1, show that reductive elimination from Ir " results in formation of different types of bonds. These reductive elimination reactions occur under very similar conditions, indicating that the nature of R and R does not significantly affect the rateh However, the coupling of two sp carbon centers does not occur. Theoretical studies have suggested that the directionality of the sp hybrid inhibits bond formation. Although a coordination site is not required for reductive elimination reactions, ligand dissociation from the metal center apparently facilitates reductive elimination. In the iridium examples, described in Table 1, halide dissociation is important, in other examples, phosphine ligand dissociation is important. 

Figure 4.8 ECD data plotted as In ftp versus 1,000/7. These alkyl halides dissociate via activation of the molecule. They are designated DEC(l) for dissociative electron capture via activation of the molecule. The slope multiplied by R is equal to the activation energy in the high-temperature region. The low-temperature data were originally not explained, but could be an indication of a low molecular electron affinity. The curves were fit using both dissociation and molecular ion formation. Data from [16-19].

A substantia] AF difference between the halide and ammine photolabiliz-ations would be expected. The AFS contributions to AFx would be negative owing to charge creation as halide dissociates from the dipositive Rh(NH3)5X2 + ion to form the tripositive Rh(NH3)s + plus the uninegative X-. In contrast, the dissociation of NH3 should afford no appreciable charge creation, hence only minor contributions from AF, would be expected. 

The foregoing mechanism shows that an El reaction has two steps. In the first step, the alkyl halide dissociates heterolytically, producing a carbocation. In the second step, the base forms the elimination product by removing a proton from a carbon that is adjacent to the positively charged carbon (i.e., from the /8-carbon). This mechanism agrees with the observed first-order kinetics. The first step of the reaction is the rate-determining step. Therefore, increasing the concentration of the base—which comes into play only in the second step of the reaction—has no effect on the rate of the reaction. 

Now let s look at what happens when conditions favor Sn1/E1 reactions (a poor nucleophile/weak base). In Sn1/E1 reactions, the alkyl halide dissociates to form a carbocation, which can then either combine with the nucleophile to form the substitution product or lose a proton to form the elimination product. 

An E2 reaction is a concerted, one-step reaction the proton and the halide ion are removed in the same step, so no intermediate is formed. In an El reaction, the alkyl halide dissociates, forming a carbocation. In a second step, a base removes a proton from a carbon that is adjacent to the positively charged carbon. Because the El reaction forms a carbocation intermediate, the carbon skeleton can rearrange before the proton is lost. 

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