Alkenes relative acidity

A few measurements are available that relate to the ion pair acidity of ethylene and some other alkenes. Ethylene is difficult to metallate directly, but vinyl bromides and iodides undergo facile transmetallation with alkyllithium reagents. Applequist and O Brien determined the equilibrium constants of transmetallation exchange reactions as a measure of relative acidity (equations 6 and 7)25. 

The relative ease with which hydrogen halides react with alkenes is in the order HI > HBr > HCl > HF. This is the same as their relative acidities (see Section 4.3.2) and indicates that protonation of the alkene is the rate-limiting step for the addition reaction. 

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. 

In general, the ease of addition of H—X to simple alkenes follows their relative acidity, HI > HBr > HC1, but HF addition is often surprisingly easy. A diversity of mechanisms appear to be involved in these processes, ranging from relatively pure carbocation processes to those more reminiscent of four-center addition. Markovnikov addition is commonly observed when precautions are taken to exclude peroxide or other possible free radical initiators. Though strong acids are involved and rearrangements are not uncommon, the reaction conditions are otherwise quite mild and yields can be high. 

Gas phase basicities defined by the reaction of alkenes with acids (equation 8) can be used to determine the relative stabilities of the resulting carbenium ions. Tsuno and coworkers found in pulsed ICR gas phase protonation experiments of a-trimethylsilylstyrenes 39 that benzyl cation 40 is exclusively formed. The measured gas phase basicities for 39 are comparable to those of a-alkylstyrenes and they are significantly higher than for styrene18. 

We have drawn the substitution at the benzylic centre as an Sn2 reaction as it would normally be with a primary alkyl halide, though it could be S>jl in this case as the benzylic cation is stable. Friedel-Crafts alkylation works well with relatively stable cations especially tertiary cations. The cation can be generated in a number of ways such as the protonation of an alkene, the acid-catalysed decomposition of a tertiary alcohol, or the Lewis-acid-catalysed decomposition of a f-alkyl chloride. 

Carbon Acidity of Strained Hydrocarbons The difference in the formal hybridization at carbon accounts for the relative acidities of alkanes versus alkenes versus akynes. The greater the s-character of the carbon of Ihe C-H bond of interest, the more acidic is that proton. Of interest here is extension of this idea to strained compounds, which may employ greater or lesser s-character to accommodate the unusual geometries. Will these compounds have acidities that reflect their hybridization Are strained hydrocarbons more acidic than their unstrained analogs ... 

The industrial Wacker process is carried out in aqueous hydrochloric acid using PdClj/CuCh as the catalyst under oxygen pressure. The oxidation of higher terminal alkenes under the same conditions is slow and sometimes accompanied by undesired by-products formed by the chlorination of carbonyl com-poimds by CuCh, and isomerization of double bonds. Earlier examples of oxidation of various alkenes, mainly in aqueous solutions, have been tabulated.The pseudo-first-order rate constants for oxidation of various alkenes, relative to the value for cycloheptene, with PdCb in the presence of benzoquinone in aqueous solution have been rqwrted. An accelerating effect of surfactants such as sodium lauryl sulfate on the stoichiometric oxidation of higher alkenes in an aqueous solution has been reported. 

Because of the relatively acidic hydrogen on the carbon adjacent to the carbonyl group, this compound reacts by the Elcb mechanism to produce the alkene with the double bond conjugated with the carbonyl group. 

On the other hand, the direct arylation of carbanionic species generated from substrates having relatively acidic hydrogens such as active methylene compounds and ketones can occur (mechanism B) [5,6]. Aryl halides are also capable of coupling directly with appropriately functionalized aromatic substrates and five-membered heteroaromatic compounds as formal carbon nucleophiles via cleavage of their unactivated C-H bonds [5,7-9]. The Fujiwra-Moritani reaction, which is the arylation of alkenes with arenes, is also useful for preparing arylalkenes without employing any halides (mechanism D) [10,11]. 

Epoxidation in the absence of a metal catalyst is possible. Hydrogen peroxide, in the presence of a nitrile, aldehyde or ketone, or a relatively acidic alcohol (e.g. phenol), can effect epoxidation of an alkene. Peroxy-imidic acids RC(=NH)OOH, formed in situ by reaction of nitriles (RC=N) with hydrogen peroxide, react under mildly alkaline or neutral conditions. For example, 2-aUyl-cyclohexanone was readily converted into the corresponding epoxide with the alkaline reagent, whereas with peroxy-acetic acid Baeyer-Villiger ring-expansion intervenes. The perhydrate 39,... 

The (Z)-alkene selectivity seen in the RBR product 56 is unusual for stilbenes, in that isomerization of the relatively acidic episulfone intermediate t5 ically ensures that the (E)-isomers exclusively prevail (see Section 8.41. The reversal of alkene stereoselectivity in this case has been proposed to result from intramolecular promotion of the sulfur dioxide extrusion from the episulfone by the adjacent alkoxide. ... 

In Section 11.4.1, ionization of the C-Br bond in tertiary halide 64 gives carbocation 66. Carbocations were discussed in Chapter 10, Section 10.2, in connection with the acid-base reaction of an alkene with acids such as H-X (HCl, HBr, etc.). To understand formation of a carbocation in a substitution reaction, remember that the stability of a carbocation is related to the number of substituents attached to the positive carbon. The formation of carbocations from alkenes was described in Chapter 10, Section 10.2, as was the relative stability of carbocations. 

Aldehydes (RCHO) react with 1-diazo-l-lithioacetone to give a-diazo- -hydroxyketones (48). These latter compounds can then be converted to P-diketones by the addition of rhodium(il) acetate a procedure which has been applied in the synthesis of -damascone. 1,6,6A -Trithiapentalene and a metal salt catalyse the lithiation of unactivated alkenes to alkenyl-lithiums by lithium metal, constituting a very useful method for preparing such compounds since in the past the preparation of alkenyl-lithiums from lithium metal has been confined to alkenes containing relatively acidic protons. Finally, the allenyl-lithium reagent (49) can be converted to various functionalized allenes by simple electrophiles. ... 

The most striking difference in properties between alkenes and alkynes is that terminal alkynes (RC CH) are relatively acidic. When a terminal alkyne is treated with a strong base, such as sodium amide, Na" " NH2, the terminal hydrogen is removed and the corresponding acetylide anion is formed ... 

As might be expected, the hybridization at nitrogen itself also drastically affects basicity, in the order jNHs > R2C=NR > RC=N , a phenomenon that we already enconntered in the discussion of the relative acidity of alkanes, alkenes, and alkynes (Section 13-2). Thus, iminium ions (Section 17-9) have valnes estimated to be of the order of 7 to 9 A-protonated nitriles (Section 20-8) are even more acidic (pa s < -5). Table 21-2 summarizes the pAa values of the conjugate acids of some representative amines. 

Relative Rates of Acid-Catalyzed Hydration of Some Representative Alkenes... 

Relative Rates of Epoxidation of Some Representative Alkenes with Peroxyacetic Acid... 

Alcohol Substitution. In the early period of normal thiol production, the normal alcohols were utilized as feedstocks. The use of a strong acid catalyst results in the formation of a significant amount of secondary thiol, along with other isomers resulting from skeletal isomerization of the starting material. This process has largely been replaced by uv-initiation because of the higher relative cost of alcohol vs alkene feedstock. 

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