Alkenes carbocation rearrangements during

Carbocation rearrangement during electrophihe addition to alkener, (Section 6.12)... 

Carbocation rearrangement during electrophilic addition to alkenes... 

This IS a frequently used proce dure for the preparation of alkenes The order of alcohol reactivity paral lels the order of carbocation stability R3C > R2CH > RCH2 Benzylic al cohols react readily Rearrangements are sometimes observed... 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

Detailed wide-ranging studies are available on the addition of HC1 and HBr to alkenes. The most useful procedure is to react dry HC1 gas and the alkene neat or in an inert organic solvent. Water or acetic acid may also be used. Alkenes yielding tertiary or benzylic alkyl chlorides react most readily. Styrene, however, adds HC1 only at — 80°C to give a-chloroethylbenzene without polymerization.101 At more elevated (room) temperature polymerization prevails. HBr adds to alkenes in an exothermic process more rapidly than does HC1. Rearrangements may occur during addition indicating the involvement of a carbocation intermediate 102... 

In addition to the primary alkylated product formed according to the above mechanism (eqs. 48-51), various other products may also be formed during alkane-alkene alkylation. These may be isomeric alkylated products resulting from the rearrangements of carbocations. Carbocation 13 may participate in a series of hydride and alkyde shifts, and each carbocation thus formed may react via hydride ion transfer to form isomeric alkanes. In addition, alkenes often undergo isomerization prior to participating in alkylation. It was observed, for example, that in the alkylation of isobutane with n-butenes in the presence of protic acids product distributions are very similar. This may be explained by a fast equilibration of n-butenes prior to participating in the alkylation step. 

Rearrangements of alkyl groups (such as methide, iCHs and ethide, CH2CH3) and hydrogen (as hydride, H ) are often observed during the dehydration of alcohols, especially in the presence of very strong acid where carbocations can exist for longer periods of time. For example, when 3,3-dimethyl-2-butanol is treated with sulfuric acid, the elimination reaction yields the mixture of alkenes shown below. Can you predict which alkene is the principal product ... 

You might compare the product of oxymercuration-reduction of 3,3-methyl-l-butene with the product formed by acid-catalyzed hydration of the same alkene (Section 6.3C). In the former, no rearrangement occurs. In the latter, the major product is 2,3-dimethyl-2-butanol, a compound formed by rearrangement. The fact that no rearrangement occurs during oxymercuration-reduction indicates that at no time is a free carbocation intermediate formed. 

Simpler cases of Wagner-Meerwein processes are also known. For example, alkyl migration during addition of HX to alkenes (Chapter 6, Scheme 6.19) has already been noted. Similarly, to the extent that the same carbocations are generated during nucleophilic substitution reactions, the same processes occur Indeed, almost identical rearrangments will be encountered again in Chapter 8 in the discussion of derivatives of alcohols as they are here with alkyl halides. 

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

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