Carbonium ions, addition reactions from olefins

These energy values are calculated from thermochemical tables (11) and the ionization potentials of hydrocarbons obtained by Stevenson (15) using mass spectrometric methods. The union of an olefin and a proton from an acid catalyst leads to the formation of a positively charged radical, called a carbonium ion. The two shown above are sec-propyl and fer -butyl, respectively. [For addition to the other side of the double bond, A 298 = —151.5 and —146 kg.-cal. per mole, respectively. For comparison, reference is made to the older (4) values of Evans and Polanyi, which show differences of —7 and —21 kg.-cal. per mole between the resultant n- and s-propyl and iso-and tert-butyl ions, respectively, against —29.5 and —49 kg.-cal. per mole here. These energy differences control the carbonium ion isomerization reactions discussed below.]... 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

A carbonium ion is formed by proton-transfer from the complex acid to the olefin. The polymerisation is initiated by the carbonium ion, and the growing end of the polymer consists of an ion-pair. For reactions in alkyl halide solvents the situation was less clear. Early experiments [8] suggested that the addition of water had little or no effect. This prompted Pepper [8] to suggest that the alkyl halide solvent itself was acting as co-catalyst ... 

Chain Propagation. In the chain propagation step, an olefin molecule reacts with a tertiary butyl carbonium ion as postulated by Whitmore (1934). This addition reaction produces a larger carbonium ion which then either undergoes isomerization or abstracts a hydride from an isobutane molecule. (Under some circumstances, the larger carbonium ion may add a second molecule of olefin this reaction will be discussed under "Polymerization".) Hydride abstraction regenerates a chain-carrying, tertiary butyl carbonium ion and also forms a molecule of isoparaffin. Reactions follow ... 

Excess Polymerization. A small amount of high-boiling heavy "tail" or residue Is formed in Isobutane alkylation, even urxJer the most favorable reaction conditions. The polymer molecule is in reality on isoparaffin formed from two or more molecules of olefin plus one molecule of Isobutane. Polymer is formed because of the inherent tendency of larger carbonium ions, e.g., Cj or C0 ions, to complete with tertiary butyl carbonium ions for addition of olefin molecules before abstracting hydride ions and becoming isoparaffin molecules. Reactions follow ... 

In this section rate-equilibrium correlations for proton transfer to olefins and aromatic systems will be discussed. Although the kinetic behaviour varies from one unsaturated system to another some general features will become apparent. Most results for proton transfer involving unsaturated carbon have been obtained by studies of an overall reaction in which proton transfer to carbon is involved as a rate-determining step. The mechanisms of reactions of this type were discussed in Sects. 2.2.3 and 2.2.4. In these cases the rate coefficient for proton addition to form a carbonium ion is obtained. However, a few examples will be described where the equilibrium between an unsaturated system and a carbonium ion has been measured giving rate coefficients in both directions. 

A comparable study of bromine addition to steroidal A -derivatives with varied substitution at C(3> and/or C(x ) also revealed rate retardation by electronegative substituents [103], and an analysis of data by the Taft method [104] yielded a reaction constant in the range —2.0 to — 2.7f. Although the Hammett and Taft reaction constants ( and q ) are not strictly comparable, these values indicate a similarity of reaction mechanism between the two types of olefin. Since chlorination in saturated aliphatic systems is believed to involve chloronium ions rather than classical carbonium ions, we would expect chlorine addition to A -steroids to produce a -value not greatly different from bromination. This does not appear to have been studied. 

The reaction scheme involves initial formation of an ammine complex that can provide amide ions for reaction with benzyl carbonium ions formed by hydride ion abstraction from toluene. The resulting coordinated benzylamine is displaced from the complex by NH3, and subsequently rapidly dehydrogenated to benzonitrile. In support of this mechanism, it has been demonstrated that the events depicted in Eq. (6) occur very rapidly over ZnX catalyst at 500°. In addition, the use of an olefin to provide carbonium ions for abstraction of hydride ions from toluene enhances the overall reaction rate. 

Here a surface Lewis acid (denoted by j) abstracts a hydride ion from the methylene group adjacent to the double bond. This mechanism is in accord with the essential Lewis acid nature of the silica-alumina surface and is consistent with the previously demonstrated ability of this surface to abstract hydride ions from tertiary hydrocarbons. Since an alkenyl carbonium ion is stabilized by resonance to a greater extent than is a saturated carbonium ion, it may well be the most stable species which could form in the chemisorption of an aliphatic olefin or its precursor. It seems reasonable, therefore, to presume that such species may be involved in heterogeneous acid catalysis to a greater extent than has been generally recognized. This chemisorption process does not, of course, exclude the more conventional acid addition to the double bond which may occur under suitable circumstances but rather, it introduces an alternate path which may well exert a considerable influence on the overall course of catalytic reactions. Thus, for example, since a substituted ally lie carbonium ion may be converted to a conjugated diene by loss of a proton, it may be an important intermediate in the formation... 

Trialkylsubstituted ethylenes such as limonene (57) and 1-methylcyclohexene (50) give rise to ratios of tertiary-secondary hydroperoxides of about 44 to 56, while open-chain olefins such as trimethyl-ethylene, l,l-dimethyl-2-ethylethylene, 2,6-dimethyl-2-octene, myrcene, jS-citronellol, linalool, and l,l-dimethyl-2-benzylethylene give ratios of tertiary-secondary hydroperoxides between 54 to 46 and 60 to 40 31, 43, 47, 60, 63, 66). Since there is no hydrogen abstraction prior to oxygen addition to one of the double bond carbons, this addition must be the first step if a multistep reaction takes place. Whatever the so-formed intermediates may be, however, diradical species such as 8 a or 8 b, or ionic species such as 9a or 9b [the latter has been suggested b> some authors (37, 67)], secondary hydroperoxides should be produced almost exclusively from the nonsterically hindered olefins since in the case of the peroxy intermediates, 8a and 9a, the most stable (tertiary) alkyl radical or carbonium ion, respectively, should be formed, and in the case of the... 

The question of carbonium ion formation from saturated hydrocarbons was considered in (,1) by the writer when the possibility of participation by olefins from thermal cracking was mentioned. However, it was only somewhat later that this suggestion was seriously adopted ( ). Then it was postulated that even traces of unsaturated hydrocarbons can activate saturated hydrocarbons by first forming a carbonium ion by proton addition. This ion can then extract a hydride ion by hydrogen transfer from the paraffin or cycloparaffin. This initiates a sort of chain reaction in which new carbonium ions are formed by hydrogen transfer with a steady-state population of ions on the catalyst surface. 

Formation of carbonium ions from olefins alkenes). Many industrial reactions of olefins involve protonation to give a carbonium ion, which is subject to nucleophilic attack, followed by proton transfer from the product to olefin. The ease of protonation follows the stability of the carbonium ion formed in the sequence tertiary > secondary > primary. Additional proton exchanges can occur at any stage in the overall process, leading to doublebond shifts in the olefinic feedstock and mixed products in some cases. (At high temperatures, products with terminal substituents may also be detectable). 

The acid-induced reaction of aryldiazomethanes with olefins gives arylcyclo-propanes in addition to olefins and esters. The cyclopropanes are formed stereo-specifically and their yields are largest in reactions with olefins which on cation addition give secondary carbonium ion centres. The use of deuteriated acids leads to partial incorporation of deuterium in the cyclopropane adducts, whereas the use of [a- H]-phenyldiazomethane leads to partial loss of deuterium, suggesting a slow proton transfer from the acid to the diazo-compound a carbenoid rather than a free carbene appears to be involved. 

For preparative purposes, Ritter s original procedure of generating the carbonium ion from the alcohol (or olefin) and sulphuric acid is preferred under these conditions, hydrogen cyanide, produced in situ from sodium cyanide, can replace the nitrileSecondary and tertiary alcohols can be used" , and the reaction, providing essentially for substitution by the 5 1 mechanism, is a valuable addition to the methods already discussed. It must be borne in mind, however, that under certain conditions carbonium ion rearrangement is possible, and has indeed been observed in some applications of this reaction... 

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