Protonation of alkenes

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

Protonation of alkenes yields carbocations, as we have seen, and in the absence of other effective nucleophiles (e.g. HaO, p. 187) these ions can act as electrophiles towards as yet unprotonated alkene (c/. p. 108), e.g. with 2-methylpropene (41) ... 

Protonation of alkenes, cycloalkanes and alkanes lead to the formation of carbe-nium ions, carbonium ions and protonated cyclopropane rings (also called cyclo-... 

Mechanism for protonation of alkenes was previously discussed in Section 13.5.1. In general, protonation of alkenes is an exothermic process. Protonation of alkanes was discussed in Section 13.5.2. There wiU be further discussion on this step in Section 13.8.4 within the context of alkane cracking mechanisms. The formation of a penta-coordinated carbonium ion from alkane protonation is typically an endothermic process, the reverse being true for deprotonation. 

The mechanism of the reaction involves the following three steps Step 1 Protonation of alkene to form carbocation by electrophilic... 

The acidic centers, both the Br0nsted and Lewis types, are generated by the isomorphous substitution of trivalent aluminum for a tetravalent silicon in the silica lattice.61-63 Additionally, carbocations may be formed by protonation of alkenes,43,52,56,61 which explains their higher reactivity in catalytic cracking. 

Carbocations formed through protonation of alkenes by proton acids are usually assumed as intermediates in alkylation with alkenes. Metal halides, when free of protic impurities, do not catalyze alkylation with alkenes except when a cocatalyst is present. It was shown that no neat conjugate Friedel-Crafts acids such as HA1C14 or HBF4 are formed from 1 1 molar compositions in the absence of excess HC1 or HF, or another proton acceptor.163-166 In the presence of a proton acceptor (alkene), however, the Lewis acid halides—hydrogen halide systems are readily able to generate carbocations ... 

TABLE 4. Rate accelerations and free energy differences in the protonation of alkenes and alkynes... 

In so far as values of pATn2o for the hydration of alkenes are known or can be estimated,47 values of pATR can be derived by combining rate constants for protonation of alkenes with the reverse deprotonation reactions of the carbocations. The protonation reactions seem much less likely to be concerted with attack of water on the alkene than the corresponding substitutions. Indeed arguments have been presented that even protonation of ethylene in strongly acidic media involves the intermediacy of the ethyl carbocation.97,98... 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

A carbonyl group can be regarded as a very stabilized carbenium ion, and so the same sort of treatment should apply to deprotonation of carbenium ions to give alkenes (or protonation of alkenes to give carbenium ions). This is currently being actively pursued. The goal is to have a uniform mechanistic model for all proton transfers from a carbon next to an atom with a formal positive charge. 

Friedel—Crafts Alkylation Using Other Carbocation Sources We have seen several ways of generating carbocations, and most of these can be used for Friedel-Crafts alkylations. Two common methods are protonation of alkenes and treatment of alcohols with BF3. 

The protonation of alkenes to give carbocations is quite general. The carbocations may trap a nucleophile, as you have just seen, or they may simply lose a proton to give back an alkene. This is just the same as saying the protonation is reversible, but it needn t be the same proton that is lost. A more stable alkene may be formed by losing a different proton, which means that acid can catalyse the isomerization of alkenes—both between Zand-E geometrical isomers and between regioisomers, isomerization of an alkene in acid... 

In order to reduce the reaction temperature of acid catalysed alkane conversion reactions one can reduce the temperature by replacing carbonium formation by a route via the carbenium ion by protonation of alkenes generated by metal-catalysed (group 8-10 metals, e.g. Pt, Pd) dehydrogenation of alkanes. The metals can be readily dispersed in the micropores of a zeolite. A lowering of the reaction temperature is especially useful for alkane isomerization. A low temperature favours the branched product and inhibits consecutive reactions. 

Tidwell and coworkers studied extensively the rates of protonation of alkenes (equation 49)103126. The rates for over one-hundred 1,1-disubstituted ethylenes, ranging from ethylene (and other simple alkenes) to 1,1 -diethoxyethylene, were well correlated by equation 50. Improvements in the correlation coefficient could be obtained by use of selected data points, but neither the slope nor the intercept was significantly affected. 

The protonation of alkenes, the essential first step in acid-catalyzed alkene isomerization, alkylation, hydration, and so on, is considered to involve the initial overlap of the carbon p-orbital forming the tc bond with the vacant s-orbital of the proton, forming a three-center bonded or tc complex" "" (Scheme 6.40). Taft seems to be the first to have suggested the initial formation, in acid-catalyzed hydration of isobutylene, of a proton tc complex of the type described originally by Dewar, in which the proton is embedded in the TC orbitals which extend above (or below) the plane of the C-C double bond. ... 

Tidwell and coworkers studied extensively the rates of protonation of alkenes equation 49)103.126 jjjg one-hundred 1,1-disubstituted ethylenes, ranging... 

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both the observation of general acid catalysis and solvent isotope effect are consistent with rate-limiting protonation of alkenes such as 2-methylpropene and 2,3-dimethyl-2-butene. 

Allylic protons of alkenes undergo base-catalyzed exchange much more rapidly than either vinylic or saturated aliphatic protons . The enhanced kinetic acidity of such protons is due to the fact that their removal results in formation of resonance-stabilized allylic carbanions. These carbanions react with proton donors to form the original alkenes and sometimes one or more isomeric alkenes, viz. 

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