Carbo cations tertiary

As carbocations go CH3" is particularly unstable and its existence as an inter mediate m chemical reactions has never been demonstrated Primary carbocations although more stable than CH3" are still too unstable to be involved as intermediates m 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 m support of tertiary carbo cation intermediates is stronger yet... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

The reactions of alcohols with hydrogen halides to give alkyl halides (Chapter 4) are nucleophilic substitution reactions of alkyloxonium ions m which water is the leaving group Primary alcohols react by an 8 2 like displacement of water from the alkyloxonium ion by halide Sec ondary and tertiary alcohols give alkyloxonium ions which form carbo cations m an S l like process Rearrangements are possible with secondary alcohols and substitution takes place with predominant but not complete inversion of configuration... 

Both alcohols are formed from the same carbocation Water may react with the carbo cation to give either a primary alcohol or a tertiary alcohol... 

In the present instance, protonation of the C1-C2 double bond gives a carbo-cation that can react further to give the 1,2 adduct 3-chloro-3-methylcyclohexene and the 1,4 adduct 3-chloro-L-methylcyclohexene. Protonation of the C3-C4 double bond gives a symmetrical carbocation, whose two resonance forms are equivalent. Thus, the 1,2 adduct and the 1,4 adduct have the same structure 6-chloro-l-methyl-cyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a tertiary allylic cation rather than a secondary allylic cation. 

Steps 1-2 of Figure 27.14 Epoxide Opening and Initial Cyclizations Cyclization is initiated in step 1 by protonation of the epoxide ring by an aspartic acid residue in the enzyme. Nucleophilic opening of the protonated epoxide by the nearby 5,10 double bond (steroid numbering Section 27.6) then yields a tertiary carbo-cation at CIO. Further addition of CIO to the 8,9 double bond in step 2 next gives a bicyclic tertiary cation at C8. 

When the enone 182 was heated with trimethylsilyl chloride (TMSC) and sodium iodide in acetonitrile, a mixture of the 3,1-benzoxazines 183 and 184 in a ratio of 1 5 was formed stereospecifically. The a,/3-unsaturated ketones in TMSC/Nal first gave jS-iodoketones thereafter a tertiary carbo-cation was formed, and subsequent acetonitrile addition resulted in the oxazines 183 and 184 (89TL4741). 

Substituted olefins that are capable of forming secondary or tertiary carbo-nium ion intermediates polymerize well by cationic initiation, but are polymerized with difficulty or not at all free radically. In general, vinyl or /-alkenes that contain electron donating groups (alkyl, ether, etc) polymerize well via a carbo-cationic mechanism. 

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

The detethering of one such trans cycloadduct (208), containing a tertiary ether linkage, was carried out. An El process, mediated by a Lewis acid, effected selective C-0 bond cleavage to give the most stable of the alternative regioisomeric carbo-cations, and finally olefin (209) (Scheme 54). 

Saunders and Rosenfeld (1969) extended their H-nmr investigation to temperatures above 100°C and discovered another, slower process which exchanges the two methylene protons with the nine methyl protons, resulting in coalescence of these bands above 130°C. The band shape analysis gave an activation energy of 18.8 1 kcal mol" for this new process. Since any mechanism involving primary alkyl cations is expected to have a barrier of ca 30 kcal mol" (the enthalpy difference between tertiary and primary carbo-cations), the formation of a methyl-bridged (corner protonated cyclopropane)... 

Only alcohols that give rise to fairly stable carbocations react (secondary, tertiary, benzylic, etc.) non-benzylic primary alcohols do not give the reaction. The carbo-cation need not be generated from an alcohol, but may come from protonation of an alkene or from other sources. In any case, the reaction is called the Ritter reac-Lewis acids, such as Mg(HS04)2, have been used to promote the reac-... 

Note that in the S -l reaction, which is often carried out under acidi conditions, neutral water can act as a leaving group. This occurs, for exam pie, when an alkyl halide is prepared from a tertiary alcohol by reaction with HBr or HCl (Section 10,7). The alcohol is first protonated and then spontaneously loses HgO to generate a carbocation. Reaction of the carbo cation with halide ion yields the alkyl halide (Figure 11.14). Knowing that an S l reaction is involved in the conversion of alcohols to alkyl halides makes it clear why the reaction works well only for tertiaiy alcohols Tertiary alcohols react fastest because they give the most stable carbocation intermediates. 

The best substrates yield the most stable carbo-cations. As a result, SmI reactions are best for tertiary, allylic, and benzylic halides. 

Other substitution reactions we ve seen include some of the reactions used for preparing alkyl halides from alcohols. We said in Section 10.7, for example, that alkyl halides can be prepared by treating alcohols with HX—reactions now recognizable as nucleophilic substitutions of halide on the protonated alcohols. Tertiary alcohols react by an S>jl pathway involving unimolecular dissociation of the protonated alcohol to yield a carbo-cation, whereas primary alcohols react by an 8 2 pathway involving direct bimolecular displacement of H2O from the protonated alcohol (Figure 11.23). 

Section 5.12 Secondary and tertiary alcohols undergo dehydration by way of carbo-cation intermediates. 

The notion that carbocation formation is rate-determining follows from our previous experience and by observing how the reaction rate is affected by the strncture of the alkene. Table 6.2 gives some data showing that alkenes that yield relatively stable carbo-cations react faster than those that yield less stable carbocations. Protonation of ethylene, the least reactive alkene in the table, yields a primary carbocation protonation of 2-methyl-propene, the most reactive in the table, yields a tertiary carbocation. As we have seen on other occasions, the more stable the carbocation, the faster is its rate of formation. 

A hydride shift produces a tertiary carbocation a methyl shift produces a secondary carbo-cation. 

Of various mechanisms that may be proposed, the only acceptable one is that summarized in Equation 5. It is assumed that the attack on the cyclopropane system by the active site leads to the formation of a 7r complex, which later rearranges to a carbo cation. The rupture of the bond of carbons 1 and 2 and the rotation between A and carbon 2 involves the appearance of a positive charge on carbon 1. The primary carbo cation formed will be able to rearrange into a more stable tertiary carbo cation by hydride shift. The polymers obtained by such a mechanism would have structures P2a to P4b. They are the only ones having one methyl group in the side chain per monomer unit and two in the case of l-methylbicyclo[n.l.O]alkanes. It must, therefore, be assumed that this is the mechanism to be considered, and that structures P a to P4b are the only ones that agree with the data. 

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