Alkylation carbonium ion

Aquilante and Volpi indicate (2) that propanium ions formed by proton transfer from H3 + are not collisionally stabilized at propane pressures as great as 0.3 mm. and that they decompose by elimination of hydrogen or a smaller saturated hydrocarbon to form an alkyl carbonium ion. Others (16, 19) have proposed one or the other of these fates for unstabilized propanium ions. Our observations can be rationalized within this framework by the following mechanisms ... 

First, the rates of carbonylation of secondary and tertiary alkyl carbonium ions can now be compared quantitatively with the known rates of competing intramolecular rearrangements of these ions. The product distribution in the Koch synthesis of carboxylic acids depends, amongst other things, on these relative rates. 

This absorption is in fact due to the ions derived from l-methyl-3-phenylindane (the cyclic dimer of styrene) and its higher homologues (oligostyrenes with indanyl end groups). There can be no doubt that the ions formed at the end of the polymerisation of styrene belong to the same families of compounds (indanyl and various phenyl alkyl carbonium ions [7]). Our evidence showed that the 1-phenylethyl cation is absent from the ions formed from styrene by excess of acid its dimeric homologue, the l,3-diphenyl- -butyl cation, is a minor component of the ion mixture. We refer to this mixture of ions formed rapidly from styrene by excess acid, or at the end of a styrene polymerisation, as SD (styrene-derived) ions. 

Several methods for cleaving oxetanes by reaction with alkyl carbonium ions have been discovered. It is reported that 2-methyloxetane reacts with a-chloroethers in ether solution containing zinc chloride to form a mixture of two isomeric chloro-substituted acetals (equation 28) (72IZV125). 

Recent studies on simple haloalkanes, especially iodoalkanes. suggest that the initial process is always homolytic carbon-halogen bond cleavage, but that iodo-systerns are especially susceptible to subsequent electron transfer from the alky) radical to the fairly unreactive iodine atom. This gives the alkyl carbonium ion that reacts with, for example, a nucleophilic solvent. The operation of this mechanism provides for the generation and reaction of vinyl carbonium ions from vinyl iodides fS.66), and this offers one of the lew ways of generating such intermediates. 

The alkyl carbonium ions which result from these reversible, relatively unselective hydride abstractions then undergo a series of 1,2- (Wagner-Meerwein) or 1,3- (protonated cyclopropane) rearrangements which eventually result in the formation of the thermodynamically most stable products. The number of different reaction sequences by which one may rationalize the formation of a given products is, of course, necessarily large. A variety of independent pathways are generally available for the interconversion of the isomers of a given species by successive alkyl shifts. 

Carmustine (BCNU), lomustine (CCNU), and semustine (methyl-CCNU) generate alkyl carbonium ions and isocyanate molecules and hence are able to interact with DNA and other macromolecules. These agents, which are lipid soluble, cross the blood-brain barrier and are therefore effective in treating brain tumors. They are bone marrow depressants. 

On the other hand, when either alkyl radicals or alkyl carbonium ions (or their precursors) are used as alkylating agents, the metal center undergoes a change in the formal oxidation state of either one or two, respectively. 

On the other hand, there is additional evidence (of a kind we cannot go into here) that makes it very likely that there is a second mechanism for Friedel-Crafts alkylation. In this mechanism, the electrophile is not an alkyl carbonium ion, but an acid-base complex of alkyl halide and Lewis acid, from which the alkyl group is transferred in one step from halogen to the aromatic ring. 

It is found that a vinyl carbonium ion is slightly less stable than the corresponding alkyl carbonium ion. In particular, a secondary vinyl carbonium ion is less stable than a secondary alkyl carbonium ion. Bearing all these factors in mind, it is apparent that vinyl carbons tend not to undergo the SN1 reaction unless conditions are so arranged as to stabilise the resultant carbonium ion intermediate. Suggest two possible ways in which this could be done. 

The order of the ease of formation of alkyl carbonium ions is tertiary > secondary > primary. This sequence follows as a consequence of the hyperconjugation resonance structures which can be written. In tertiary butyl carbonium ions, for example, there are nine hyperconjugation forms which contribute to the stability of the ion. With the isopropyl carbonium ion there are six such structures and with ethyl there are only three. 

Although there is no proof for the mechanism, many of the data known about the reaction can be interpreted in terms of three general steps (1) sulfonation (XXII to XXIV) (2) formation of a m- or p-disulfonic acid with the elimination of an alkyl carbonium ion if one of these positions is occupied (XXIII to XXVII and XXIII to XXIV) and (3) displacement of the more hindered sulfonic acid group by the eliminated carbonium ion (XXIV to XXVI and XXVII to XXVIII) or an oxonium ion (XXVII to XXIX). In agreement with (3) it has been found that the most hindered sulfonic acids are the most readily desulfonated.67... 

Three major types of cationic species that can be derived from saturated hydrocarbons are alkyl carbenium ions (R+), alkane radical cations (RH +) and alkyl carbo-nium ions (RH2+). The term carbocations is usually reserved to denote alkyl carbenium and carbonium ions only. Pentacoordinated alkyl carbonium ions (proton-ated alkanes) are the species that result from protonation of alkane molecules they are of paramount importance as reactive intermediates/transition states in the initiation of (Br0nsted) acid-catalyzed conversions of saturated hydrocarbons. Upon dissociation of alkyl carbonium ions, trivalent alkyl carbenium ions are formed and these are responsible for the further progression of acid-catalyzed conversions of alkanes. Alkyl carbenium ions may also be formed by ionization of neutral alkyl radicals and by proton addition to olefins. In both carbenium and carbonium ions, the positive charge is very much located on a particular part of the cation. 

By analogy with the results obtained with the phenyl-substituted analogues, chemisorption of isobutane might be expected to proceed by hydride ion abstraction to form a tertiary-butyl carbonium ion and it would indeed be convenient to assign the 3000 A band to this ion. Although such alkyl carbonium ions have been useful for the mechanistic description of a variety of reactions of hydrocarbons over heterogeneous acid catalysts, in recent years numerous observations have been reported which apparently are not in accord with this concept. Notable in this respect is the remarkable degree of stereoselectivity associated with the acid-catalyzed double bond isomerization of butenes... 

The possibility that this chromophore was a tertiary-alkyl carbonium ion formed in a rapid isomerization of the other ion precursors in the acid solution cannot be correct for the following reasons (a) Cryoscopic data for alcohols in sulfuric acid indicate a process more complex than that expected for ionization of these compounds as secondary bases (236) (b) evolution of SO 2 from sulfuric acid solutions of octene-1 was observed to closely parallel the appearance of the 3000 A band in this system (137), indicating that an oxidation process was involved in forming the chromophore from olefins (c) the lowest electronic transition for the isoelectronic alkylborines occurs in the 2000 A-2200 A region (138). 

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