Substitution gas phase

Hase W L 1994 Simulations of gas-phase chemical reactions applications to S j2 nucleophilic substitution Science 266 998-1002... 

Eleetron energy-loss speetroseopy is used for obtaining speetroseopie data as a eonvenient substitute for optieal speetroseopy, and, taking advantage of diflferenees in seleetion rules, as an adjimet to optieal speetroseopy. In addition, eleetron speetroseopy has many applieations to ehemieal and stnietural analysis of samples in the gas phase, in the solid phase, and at the solid-gas interfaee. 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

Infrared absorption properties of 2-aminothiazole were reported with those of 52 other thiazoles (113). N-Deuterated 2-aminothiazole and 2-amino-4-methylthiazo e were submitted to intensive infrared investigations. All the assignments were performed using gas-phase studies of the shape of the vibration-rotation bands, dichroism, isotopic substitution, and separation of frequencies related to H-bonded and free species (115). With its ten atoms, this compound has 24 fundamental vibrations 18 for the skeleton and 6 for NHo. For the skeleton (Cj symmetry) 13 in-plane vibrations of A symmetry (2v(- h, 26c-h- Irc-N- and 7o)r .cieu.J and... 

The gas phase chlorination of methane is a reaction of industrial importance and leads to a mixture of chloromethane (CH3CI) dichloromethane (CH2CI2) trichloromethane (CHCI3) and tetrachloromethane (CCI4) by sequential substitution of hydrogens... 

A simjlai treatment is possible in terms of an overall gas-phase driving force by substituting equation 29 into equation 38 ... 

The question of whether adsorption should be done ia the gas or Hquid phase is an interesting one. Often the choice is clear. Eor example, ia the separation of nitrogen from oxygen, Hquid-phase separation is not practical because of low temperature requirements. In C q—olefin separation, a gas-phase operation is not feasible because of reactivity of feed components at high temperatures. Also, ia the case of substituted aromatics separation, such as xylene from other Cg aromatics, the inherent selectivities of iadividual components are so close to one another that a simulated moving-bed operation ia hquid phase is the only practical choice. 

Control of addition vs substitution by free radicals can be effected by the reaction conditions, ie, radical concentration, temperature, and phase. Using halogens as propylene reactants, high temperatures and the gas phase favor high radical concentrations and substitution reactions cold, Hquid-phase conditions favor addition reactions. 

It is also of significance that in the dilute gas phase, where the intrinsic orientating properties of pyrrole can be examined without the complication of variable phenomena such as solvation, ion-pairing and catalyst attendant on electrophilic substitution reactions in solution, preferential /3-attack on pyrrole occurs. In gas phase t-butylation, the relative order of reactivity at /3-carbon, a-carbon and nitrogen is 10.3 3.0 1.0 (81CC1177). 

Diels-Alder reactions, 4, 842 flash vapour phase pyrolysis, 4, 846 reactions with 6-dimethylaminofuKenov, 4, 844 reactions with JV,n-diphenylnitrone, 4, 841 reactions with mesitonitrile oxide, 4, 841 structure, 4, 715, 725 synthesis, 4, 725, 767-769, 930 theoretical methods, 4, 3 tricarbonyl iron complexes, 4, 847 dipole moments, 4, 716 n-directing effect, 4, 44 2,5-disubstituted synthesis, 4, 116-117 from l,3-dithiolylium-4-olates, 6, 826 electrocyclization, 4, 748-750 electron bombardment, 4, 739 electronic deformation, 4, 722-723 electronic structure, 4, 715 electrophilic substitution, 4, 43, 44, 717-719, 751 directing effects, 4, 752-753 fluorescence spectra, 4, 735-736 fluorinated derivatives, 4, 679 H NMR, 4, 731 Friedel-Crafts acylation, 4, 777 with fused six-membered heterocyclic rings, 4, 973-1036 fused small rings structure, 4, 720-721 gas phase UV spectrum, 4, 734 H NMR, 4, 7, 728-731, 939 solvent effects, 4, 730 substituent constants, 4, 731 halo... 

This is the gas-phase mass-transfer limited condition, which can be substituted into Eq. (14-71) to obtain the following equation for calculating the height of packing for a dilute system ... 

Another example is the acidities of a series of carboxylic acids. It is known that the substitution effect on these compounds also depends on the environment. The behavior of the halo-substituted acetic acids is one of the prototype problems for the solvent effect on acidity The order in strength of the haloacetic acids in the gas phase is... 

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

This is opposite from the order in solution as revealed by the pK data in water and DMSO shown in Table 4.14. These changes in relative acidity can again be traced to solvation effects. In the gas phase, any substituent effect can be analyzed directly in terms of its stabilizing or destabilizing effect on the anion. Replacement of hydrogen by alkyl substituents normally increases electron density at the site of substitution, but this effect cannot be the dominant one, because it would lead to an ordering of gas-phase acidity opposite to that observed. The dominant effect is believed to be polarizability. The methyl... 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

The relative stabilities of 1-phenylvinyl cations can be measured by determining the gas-phase basicity of the corresponding alkynes. The table below gives some data on free energy of protonation for substituted phenylethynes and 1-phenylpropynes. These give rise to the corresponding Yukawa-Tsuno relationships. 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

Theoretical studies of the relative stabilities of tautomers 14a and 14b were carried out mostly at the semiempirical level. AMI and PM3 calculations [98JST(T)249] of the relative stabilities carried out for a series of 4(5)-substituted imidazoles 14 (R = H, R = H, CH3, OH, F, NO2, Ph) are mostly in accord with the conclusion based on the Charton s equation. From the comparison of the electronic spectra of 4(5)-phenylimidazole 14 (R2 = Ph, R = R3 = H) and 2,4(5)-diphenylimidazole 14 (R = R = Ph, R = H) in ethanol with those calculated by using ir-electron PPP method for each of the tautomeric forms, it follows that calculations for type 14a tautomers match the experimentally observed spectra better (86ZC378). The AMI calculations [92JCS(P1)2779] of enthalpies of formation of 4(5)-aminoimidazole 14 (R = NH2, R = R = H) and 4(5)-nitroimidazole 14 (R = NO2, R = R = H) point to tautomers 14a and 14b respectively as being energetically preferred in the gas phase. Both predictions are in disagreement with expectations based on Charton s equation and the data related to basicity measurements (Table III). These inconsistencies may be... 

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