1,721,058 research outputs found
Facial selectivity of the (R)-1,3.dimethyl-1-cyclohexyl cation in the gas phase.
No full textThe model reaction between the (R)-1,3-dimethyl-1-cyclohexyl cation (I) and methanol has been investigated under gas-phase radiolytic conditions (750 Torr; 25–120 °C) with the aim of evaluating the intrinsic factors that govern the facial selectivity of biased carbocations. The peculiarity of the experimental approach allows the formation of different CH318OH⋅I ionic adducts. Subsequent conversion of these adducts to give the corresponding E/Z covalent products follows different reaction coordinates, which are characterized by their own activation parameters. On the grounds of density functional theory (DFT) results, several [CH3OH⋅I] structures have been located on the relevant potential-energy surface (PES). The experimental results point to a gas-phase facial selectivity, which is mainly governed by entropic factors that arise as a result of the occurrence of different noncovalent ion–molecule “facial adducts” (FA). The formation of FAs may also play an important role in both the reaction dynamics and the positional selectivity. The present results cannot be interpreted by any of the models based on solution-phase experiments
Wagner-Meerwein Rearrangements in the Gas Phase: Deuterium Isotope Effects on Acid-Induced Dissociation of Optically Active Phenylpropanols.
No full textThe D isotope effects on the kinetics of H2O loss from O-protonated (S)-PhCH2CHMeOH (1sH+) and (S)-PhCHMeCH2OH (2sH+) were studied in the gas phase at 750 Torr and 25. The measured primary and secondary .alpha.-D1, .beta.-D1, and .beta.'-D3 kinetic isotope effects provide conclusive evidence on the detailed mechanism of the gas-phase dissocn. of 1sH+ and 2sH+; this involves competing anchimeric assistance from all the groups adjacent to the reaction center (C.alpha.). Their anal., combined with that of the relevant activation parameters reported in the preceding paper, is consistent with: (i) a transition state (TS) structure involving Ph-group participation, which is placed rather early along the reaction coordinate and in which the partial C.alpha.-OH2+ bond cleavage is coupled with a weak Ph-C.alpha. interaction; (ii) a tight TS structure for .beta.-H participation, in which a limited C.alpha.-OH2+ bond cleavage is fully outweighed by the advanced H(D)-C.alpha. bonding;, and (iii) a borderline TS structure for .beta.-Me group participation in 2sH+, in which intense Me-C.alpha. bonding is coupled with pronounced C.alpha.-OH2+ bond elongation. The values of the primary and secondary .alpha.-D1, .beta.-D1, and .beta.'-D3 kinetic isotope effects are discussed and compared with those of related gas-phase and solvolysis reactions
Troposelective substitutions in microsolvated systems
No full textThe mechanism and the stereochemistry of the intracomplex "solvolysis" of the proton-bound complexes I-X between (CH3OH)-O-18 and (R)-(+)-1-aryl-ethanol (1(R)(X); aryl = phenyl (X = H); pentafluorophenyl (X = F)) have been investigated in the gas phase in the 25-100 degreesC temperature range. The results point to intracomplex "solvolysis" as proceeding through the intermediacy of the relevant benzyl cation IIIX (a pure S(N)1 mechanism), "Solvolysis" of I-H leads to complete racemization at T > 50 degreesC, whereas at T C the reaction displays a preferential retention of configuration. Predominant retention of configuration is also observed in the intracomplex "solvolysis" of I-F. This picture is rationalized in terms of different intracomplex interactions between the benzylic ion IIIX and the nucleophile/leaving group pair, which govern the timing of their reorientation within the electrostatic complex. The obtained gas-phase picture is discussed in the light of related gas-phase and solution data. It is concluded that the solvolytic reactions are mostly governed by the lifetime and the dynamics of the species involved and, if occurring in solution, by the nature of the solvent cage. Their rigid subdivision into the S(N)1 and S(N)2 mechanistic categories appears inadequate, and the use of their stereochemistry as a mechanistic probe can be highly misleading
Wagner-Meerwein Rearrangements in the Gas Phase: Anchimeric Assistance to Acid-Induced Dissociation of Optically Active Phenylpropanols.
No full textActivation entropy, rather than enthalpy, governs anchimeric assistance in gas-phase water loss from O-protonated (S)-1-phenyl-2-propanol and (S)-2-phenyl-1-propanol. Conformational factors and electrostatic interactions are responsible for a gas-phase frontside phenyl-group participation, with an activation energy lower than that of the competing backside participation
L'esame di chimica generale
No full textQuesto breve testo intende assistere gli studenti che si preparano ad affrontare l’esame orale di Chimica Generale ed Inorganica per tutti i Corsi di Laurea che prevedono tale esame o similari. Esso è concepito in modo da raccogliere in forma concisa i concetti essenziali di Chimica Generale, spesso riassunti in forma tabulare. Il libro è organizzato in brevi capitoli in cui viene fornita una rassegna sulla teoria atomica, sui fattori che determinano le interazioni fra atomi e molecole, sulle proprietà della materia, sugli scambi di energia nelle trasformazioni fisiche e chimiche, e sulla velocità di tali trasformazioni. Il testo, inoltre, presenta due appendici in cui sono riportate le unità di misura fondamentali e derivate, le costanti fondamentali, e la nomenclatura dei composti chimici. Alla fine di ogni capitolo sono proposti alcuni test di apprendimento che forniscono allo studente un base di autovalutazione. Inoltre, allo scopo di orientare correttamente il candidato nei confronti della prova da sostenere, sono riportate alla fine del testo alcune tipiche domande d’esame
Does a Chiral Alcohol Really Racemize when Its OH Group Is Protected with Boyer's Reaction?
No full textChiral reactants have been employed for assessing the real stereochemistry of the BiBr(3)-catalyzed synthesis of benzylic ethers, a very useful reaction for protecting alcoholic groups. The results of this investigation are in clear contrast with the conclusions of previous studies (Boyer et al., Tetrahedron 2001;57:1917-1921). Indeed, chiral GC-MS analysis of the ethereal products gives unequivocal evidence of the complete racemization of the benzylic moiety and the complete retention of configuration of the protected alcoholic substrate. Such findings make BiBr(3) a powerful and stereo-chemically preservative catalyst for benzylation of chiral alcohols, and a potential candidate for orthogonal protecting group strategies applicable to polyhydroxy compounds. Chirality 22:88-91, 2010. (C) 2009 Wiley-liss, Inc
KrOn+ (n=1, 2) AND KrOHn+ ARE STABLE SPECIES IN THE GAS PHASE?
No full textKrypton reacts with cationic ozone derivatives in the gas phase under Fourier-transform ion cyclotron resonance conditions to yield stableKrOn+ (n = 1,2) and KrOH+ ions. The stability properties of these species are contrasted with previous experimental data and with theoretical predictions. The persistency of gaseous KrO+ and KrOH+ ions at low pressures (4 × 10−8 Torr) and room temperature is justified by their pronounced krypton-oxygen bond dissociation enthalpy estimated to be as large as 56.8 and 40.5 kcal mol−1, respectively, on the grounds of ab initio density functional calculations
Hindered Inversion of Chiral Ion-Dipole Pairs
No full textO-Protonated S-(-)-1-phenyl-1-methoxyethane (IS) has been generated in the gas phase by (CH3)2Cl+ methylation of S-(-)-1-phenylethanol (1S). Detailed information on the reorganization dynamics of the intimate ion-dipole pair (IIS), arising from IS by C-O bond dissocn., is inferred from the kinetic study of the intramol. inversion of configuration of IS vs its dissocn. to .alpha.-methylbenzyl cation and CH3OH. The behavior of IIS in the gas phase is compared to that obsd. in aq. solns., where the loss of optical activity of IS is prevented by exchange of the leaving CH3OH with the solvent shell. Hindered inversion of IS in soln. is attributed to the operation of attractive interactions between the moving CH3OH moiety and the solvent cage which inhibit internal return in the intimate ion-dipole pair IIS. Similar interactions do not operate in the solvolysis of 18O-labeled 1S in aq. acids, whose loss of optical activity efficiently competes with exchange of the leaving H218O with the solvent shell
GAS-PHASE HETEROAROMATIC SUBSTITUTION .10. REACTION OF FREE PHENYLIUM ION WITH SIMPLE 5-MEMBERED HETEROARENES IN THE GASEOUS AND LIQUID-PHASE
No full textPhenylium ion, obtained from the spontaneous P decay of 1,4-ditritiobenzene, has been allowed to react with pyrrole, N-methylpyrrole, furan, and thiophene, in both the gaseous and liquid phases. The differences between the reactivity pattern of phenylium ion in the two environments can be essentially reduced to significant ion-neutral electrostatic interaction in the gas phase and to the much greater efficiency of collisional stabilization in the condensed phase, allowing a larger fraction of the excited ionic intermediates, from the highly exothermic attack of phenylium ion on the aromatic substrate, to survive dissociation and isomerization. The mechanism of the phenylation process and of the subsequent isomerization of the relevant . . ionic intermediates is discussed and the intrinsic substrate and positional selectivity of the phenylium ion evaluated. While the limited substrate discrimination of phenylium ion fully agrees with its well-known exceedingly high reactivity, its pronounced affinity toward the a carbons of the selected heteroarenes does not conform with the relatively "hard" character of the reactant, .~ . expected on the grounds of its STO-3G calculated LUMO energy. The conceivable occurrence of an intimate entropy-favored . . . . . two-step addition mechanism, involving a preliminary single-electron transfer (SET) from the heteroaromatic substrate to the ionic electrophile, which is thermochemically allowed only for phenylium and methyl cations and prevented for other alkylating electrophiles, is discussed
XeO+ and XeOH+ ions: a new class of powerful oxidative oxygenating agents in the gas phase.
No full textFourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to examine ion−molecule reactions in xenon/ozone and xenon/methane/ozone mixtures. The cationic ozone derivatives, formed in these mixtures, are found to react with xenon, yielding a variety of stable products containing the Xe−O bond, i.e., XeO+, XeO2+, XeO3+, XeOH+, and XeOH2+. Estimated values for the Xe−O bond strength in the XeO+ and XeOH+ ions range around 51 and 52 kcal mol-1, respectively. Both these ions exhibit distinct oxidative oxygenating properties toward some simple inorganic and organic substrates. In some representative cases, a stepwise oxidation mechanism is observed, involving the homolytic scission of a bond of the substrate by XeO+, followed by intracomplex ion−radical recombination with elimination of the Xe fragment
- …
