1,721,008 research outputs found
Liquid-phase reactions of free methyl cations from the decay of methane-t4. Alternative alkylation sites in halobenzenes.
No full textThe reaction of free methyl cations, generated from the decay of multitritiated methane dissolved in liquid halobenzenes, gives the corresponding labeled halotoluenes together with methyl halides, toluene, and minor amounts of decomposition products. The attack of the reagent on the n-type nucleophilic centers displays a remarkable selectivity, following the trend Br > CI > F, in agreement with the HSAB principle. Ring alkylation is characterized by an appreciable positional selectivity, which is in contrast with the scarce substrate discrimination. The reactivity ratios deduced from the competition experiments depend on the composition of the system, the selectivity of the reagent increasing with the concentration of halobenzenes. This trend is explained by the intervention of methylphenylhalonium ions, which, besides being intermediates for the formation of methyl halides. are also methylating agents and are more selective than free methyl cations
Gas phase reactions of free methyl cations from the decay of [3H4]-methane. A study of their attack on the halobenzenes.
No full text
Aromatic substitutions by methyl-3H3 decay ions. A comparative study of the gas and liquid phase attack on benzene and toluene
No full textA nuclear technique based on the spontaneous decay of tritiated precursors that allows the generation of free carbonium ions of exactly the same nature in different environments, has been exploited in a comparative study of aromatic alkylation by CT3+ ions, both in the gas phase at various pressures and in the liquid phase. The differences between the reactivity pattern of the methyl cation in the two environments can be essentially reduced to the much greater efficiency of collisional stabilization in the condensed phase, allowing a larger fraction of the excited arenium ions, from the highly exoenergetic (ΔH° < 335 kJ mol–1) attack of the CT3+ ions on benzene and toluene, to survive dissociation and/or isomerization. The mechanism of the major competitive processes promoted by CT3+ attack on arenes, i.e. methylation, tritiodemethylation, and methyldemethylation are discussed, and the substrate and positional selectivity of the CT3+ ions from the decay of CT4 is compared with that of more conventional alkylating reagents
Reactions of "free" methyl cations with aromatic substrates. A study of their attack on tert-butylbenzene in the gaseous and liquid phase.
No full textIsopropylation of halobenzenes by sec-C3H7+ cations. Evidence for a participation of the substituent in a gas-phase alkylation
No full textlsopropylation of the halobenzenes (PhF, PhCI, and PhBr) by isopropyl cations, generated in the diluted gaseous phase from the radiolysis of propane, leads to formation of the corresponding halocumenes. The regioselectivity of the substitution changes from apparent kinetic control, i.e., ortho >SO% at 760 Torr, to thermodynamic control Le., meta, as the overall pressure is lowered to 20 Torr. The results of competitive alkylation experiments with toluene show the reactivity trend PhF > PhCl N PhBr. Appreciable yields of anisole, increasing in the order PhF > PhCl N PhBr, are measured when the system is radiolyzed in the presence of CH30H. The formation of PhOCH3 is traced to the attack of methanol on the aromatic ring, activated toward nucleophilic displacement by a specific interaction of sec-C3H7+ ions with the halogenated substrates. The same interaction helps also to explain the exceptionally high reactivity of the ring positions ortho to a n-donor substituent
Gas-phase alkylation of xylenes by tert-butyl(1+) ions.
No full textThe gas-phase alkylation of xylenes by t-C4H9+ ions from the radiolysis of neopentane has been investigated in the pressure range from 20 to 771 Torr. The gaseous cation is entirely unreactive toward p-xylene and alkylates o-xylene, yielding exclusively 1,2-dimethyl-4-tert-butylbenzene.R eaction with m-xylene yields a mixture of 1,3-dimethyl-4-tert-butylbenzene and 1,3-dimethyl-5-tert-butylbenzene,w hose isomeric ratio depends on the experimental conditions, ranging from the 8:92 value observed at 20 Torr in the absence of added bases to the 89: I 1 value measured at 770 Torr in the presence of NH3. The substrate selectivity of the gaseous electrophile has been studied by competition experiments with respect to toluene. The observed ko.xylene:ktoluenrea tio of 2.0 & 0.2 appears fairly independent of the experimental conditions, while the apparent km-xylene:ktoluenrae tio is considerably affected by the nature and the concentration of gaseous bases, ranging from 0.65 & 0.1 in the absence of NH3 to 1.9 f 0.2 in the presence of excess NH3. These results are interpreted assuming the reversible tert-butylation at the ring positions ortho to one methyl group of m-xylene in order to explain both the effect of the pressure and of NH3 on the isomeric composition of products and the apparent changes of the reactivity of m-xylene with respect to that of toluene. The reaction provides an example of an aromatic alkylation by a gaseous, unsolvated carbenium ion and allows the direct evaluation of the reactivity and the steric requirements of the electrophile, unaffected by the complicating effects of the solvent and the counterion invariably present in solution. The unusual substrate and positional selectivity of the gaseous t-C4H9+ reagent, in particular the remarkable extent of the substitution ortho to one methyl group of m-xylene, are discussed in comparison with the conventional Friedel-Crafts alkylation
Aromatic substitution in the liquid phase by bona fide free methyl cations. Alkylation of benzene and toluene.
No full textFormation and reactivity of gaseous acetylium ions from the methylation of carbon monoxide.
No full textFree acetylium ions have been prepared in the diluted gas state at atmospheric pressure, via two different routes, i.e., the methylation of carbon monoxide by CT3+ ions from the decay of fully tritiated methane, and the nucleophilic displacement by CO on CH3M+ (M = HF, CH3F) ions, produced from the y-radiolysis of methyl fluoride. Independently of the preparation methods, the acetyl cation displays a much higher reactivity toward n-type than toward r-type nucleophiles. The reaction of thermalyzed CHjCO+ ions with alcohols or phenols yields the corresponding acetates, while no acylation of benzene and toluene was detected. Low yields of ring-acetylated products were obtained from the attack of acetylium ions on anisole and phenol, the reactivity being, however, strongly biased (83.8%) in favor of the oxygen atom of the latter substrate. From these results it appears that the intrinsic intramolecular selectivity of the free acetylium ion, measured in the gas phase, in the absence of the complicating phenomena, such as solvation and ion pairing, invariably affecting solution-chemistry studies, is characterized by a high ortho/para ratio in the acylated products
Reactivity and Selectivity of the Methoxycarbonyl Cation in the Gas-Phase Electrofilic Aromatic Substitution
No full text- …
