1,721,073 research outputs found
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
Proof of existence of cyclic C4H7+ ions in the dilute gas state
No full textDaughter C4H7-nTn+ ions from the β decay of multitritiated cyclobutane were allowed to react in the gas phase with suitable nucleophiles (H2O, NH3), and the products formed were analyzed by radio gas chromatog. Isolation of comparable amts. of cyclobutyl and cyclopropylcarbinyl derivs. among other products provides direct evidence for the existence of cyclic C4H7+ cations. On the other hand, large yields of linear C4 derivs. demonstrate the facile isomerization of the strained c-C4H7+ cations, either directly, or following their exothermic reaction with the nucleophile
Aromatic substitution in the gas phase. Mechanism of the dehalogenation reactions of halobenzenes and dihalobenzenes promoted by gaseous Broensted acids
No full textThe attack of CH5+ and C2H5+ ions, obtained in the dil. gas state from the γ radiolysis of CH4, on halo- and dihalobenzenes causes extensive dehalogenation via 2 distinct channels, leading resp. to protodehalogenated and methyldehalogenated products, whose relative rate depends primarily on the nature of the leaving halogen. Kinetic and mass spectrometric evidence suggest that direct attack of the Broensted acid to the halogen substituent leads to formation of the correspondent arylium cation, via hydrogen halide elimination, at a rate which decreases in the order F » Cl ≥ Br. The subsequent electrophilic attack of the arylium cations on CH4 yields methylated arenium ions, and eventually the obsd. methyldehalogenated products. On the other hand, attack of the Broensted acid to the arom. ring of halobenzenes promotes protodehalogenation, at a rate increasing in the order F « C
Concerning the Basicity of Ozone
No full textA comprehensive view on the basicity order of simple mols., including ozone, is reported and contrasted with recent ests. in heterogeneous systems
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 textNuclear-decay techniques
No full textThe role of radiochem. methods, in particular of those based on nuclear decay, in the study of gaseous ions, is reviewed with 104 refs
Aromatic substitution in the gas phase. Ambident behavior of halo- and dihalobenzenes toward D2T+. Tritiodeprotonation vs. tritiodehalogenation
No full textThe gas-phase electrophilic attack of radiolytically formed D2T+ ions on halo- and dihalobenzenes was investigated in homogeneous systems formed by a large excess (760 torr) of D2 labeled with DT and contg. at low concns. (∼1 torr) the arom. substrate and a thermal radical scavenger. Tritiodeprotonation, to give the tritiated substrate or its isomers, and tritiodehalogenation occurred. The relative rate of the 2 competing reactions was detd. by the nature of the halogen atom(s) contained in the substrate; the extent of dehalogenation increased in the order Br ≤ Cl << F. Tritiodeprotonation involved the attack of D2T+ on the arom. π system. Tritiodehalogenation involved the attack of the gaseous electrophile to the n center(s) of the substrate. These channel mechanisms were independently supported by ion cyclotron resonance and chem. ionization mass spectrometric data. The results obtained with different gaseous electrophiles and in related soln. reactions were discussed
Stereochemical course of a gas-phase electrophilic attack at saturated carbon. Reaction of meso-1,2-dichloro-1,2-difluoroethane with gaseous Broensted acids
No full textThe gas-phase electrophilic attack of He3HS ions from the @ decay of molecular tritium on nieso-1,2-
dichloro-l,2-difluoroethanlee ads to the predominant formation of a tritiated product retaining the original configuration
of the substrate, with a considerably smaller yield of the tritiated dl form. The effect of the substrate
pressure on the meso/dl ratio and the use of radiolytically produced methanonium and ethyl ions as milder gasphase
Bransted acids show that the minor yield of the dl form is likely to arise from a racemization process, rather
than from a discrete mode of attack, leading to inversion. The conclusion that the gas-phase tritiodeprotonation
proceeds with retention of the substrate configuration is compared with the theoretical predictions on the course of
the SE2 substitution processes
Gas-phase aromatic substitution. Isomerization of gaseous arenium ions from the attack of D2T+ on dihalobenzenes
No full textThe isomerization of gaseous, unsolvated arenium ions from the attack of radiolytically formed D2T+ on dihalobenzenes was investigated in the gas phase at atm. pressure, irradiating with the 60Co γ radiation homogeneous systems contg. an excess of D2 gas labeled with DT and low concns. of the arom. substrate and a thermal radical scavenger (O2). The isomeric composition of the tritiated products from RC6H4R1 (R = R1 = F, Cl; R = F, R1 = Cl, Br) was detd. Gas-phase tritiodeprotonation of dihalobenzenes by D2T+ was accompanied by isomerization, the rate of which depended on the nature of the substituents. The migratory aptitude of the halogens increased in the order: F << Cl < Br. The position of the substituents was also significant as the meta isomer of a given dihalobenzene exhibited less extensive halogen migration than did the ortho and para isomers. The isomerization process is intramol. and involved consecutive 1→2 halogen shifts of the gaseous arenium ions formed from the exothermic triton transfer from D2T+ to dihalobenzenes. The mechanism of the isomerization was compared with those of similar reactions promoted in soln. by strong Broensted acids
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