Isomerization reaction time

From stochastic molecnlar dynamics calcnlations on the same system, in the viscosity regime covered by the experiment, it appears that intra- and intennolecnlar energy flow occur on comparable time scales, which leads to the conclnsion that cyclohexane isomerization in liquid CS2 is an activated process [99]. Classical molecnlar dynamics calcnlations [104] also reprodnce the observed non-monotonic viscosity dependence of ic. Furthennore, they also yield a solvent contribntion to the free energy of activation for tlie isomerization reaction which in liquid CS, increases by abont 0.4 kJ moC when the solvent density is increased from 1.3 to 1.5 g cm T Tims the molecnlar dynamics calcnlations support the conclnsion that the high-pressure limit of this unimolecular reaction is not attained in liquid solntion at ambient pressure. It has to be remembered, though, that the analysis of the measnred isomerization rates depends critically on the estimated valne of... 

Example 1 is repeated in exactly the same way, with the exception that in the isomerization step 250 mg of LiBr instead of 340 mg of Lil is used, and that the reaction time results to be of 10 hours,... 

Kote 1. The reaction time at room temperature was 2-3 h. At 40 C the isomerization was finished within 30 min. With a dilute (1 g per 10 ml) aqueous solution of K2C03.only about 5 min were required. 

A mild procedure which does not involve strong adds, has to be used in the synthesis of pure isomers of unsymmetrically substituted porphyrins from dipyrromethanes. The best procedure having been applied, e.g. in unequivocal syntheses of uroporphyrins II, III, and IV (see p. 251f.), is the condensation of 5,5 -diformyldipyrromethanes with 5,5 -unsubstituted dipyrromethanes in a very dilute solution of hydriodic add in acetic acid (A.H. Jackson, 1973). The electron-withdrawing formyl groups disfavor protonation of the pyrrole and therefore isomerization. The porphodimethene that is formed during short reaction times isomerizes only very slowly, since the pyrrole units are part of a dipyrromethene chromophore (see below). Furthermore, it can be oxidized immediately after its synthesis to give stable porphyrins. 

Carbonylation of propargylic carbonates proceeds under mild neutral conditions (50 °C, I-10 atm) using Pd(OAc)2 and Ph ,P as a catalyst, yielding the 2,3-alkadienoates 18 in good yields[9,10]. The 2.3-alkadienoates isomerize to 2,4-dienoates during the reaction depending on the solvents and reaction time. 2-Decynyl methyl carbonate is converted into methyl 2-heptyl-2,3-butadienoate (19) in 82% yield. 

Prolonged reaction time causes the isomerization of (Z)-e-[2-(N,N-dimethylam1no)phenyl]styrene to the (E)-isomer. 

Isomerization of fluoroolefins by a shift of a double bond is catalyzed by halide 10ns [7] The presence of crown ether makes this reaction more efficient [74] Prolonged reaction time favors the rearranged product with an internal double bond (equations 3-5) Isomerization of perfluoro-l-pentene with cesium fluoride yields perfluoro-2-pentenes in a Z ratio of 1 6 [75] Antimony pentafluoride also causes isomenzation of olefins leading to more substituted products [76]... 

Similar isomerization reactions were observed in CDCI3 solutions of N-( -aminomethyl)-l,2,4-triazoles and -tetrazoles (90T633). As for the analogous benzotriazoles, these reactions are-intermolecular and slow at 20°C in the NMR time scale. 

In contrast, 1,5-bis(trifluoromethyl)-3-azaquadricyclanes, e.g. 4, isomerize to a mixture of the 3,6- and 4,5-bis(trifluoromethyl)-l //-azepines, e.g. 5 and 6, respectively. Although the relative yields of the two isomers vary only modestly with temperature, the reaction times are drastically reduced with increasing temperature (7d at O C 10 min at 100 C).122,129... 

Bromo-3//-2-bcnzazepines, e.g. 38, undergo palladium(0)-catalyzed carboalkoxylation with carbon monoxide in the presence of copper(I) iodide and methanol to give mixtures of isomeric methyl esters, e.g. 39 and 40.78 The function of the copper(I) iodide is not understood, but its presence lowers significantly (18 h to 3 h) the reaction time. 

Where X is phenyl, the result of irradiation (sunlight, mercury lamp) is the formation of Ru(NO)X3(PPh3)(OPPh3) (X = Cl, Br) in the case of the diethyl-phenylphosphine complex, irradiation causes isomerization to the cis,mer-isomer. The frans,mer-isomer is the usual synthetic product, but in the case of dimethylphenylphosphine the/ac-isomer was obtained using short reaction times it isomerized to the usual mer,trans-isomer on heating [123],... 

When halogens add to Pt(PPh3)3, the initial product is tnms-Pt(PPh3)2X2, isolable after a short reaction time (in the presence of excess X2, which removes free PPh3, catalyst for the isomerization to the m-form). 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

X-ray dififtaction (XRD) analysis of the freshly calcined catalysts as well as samples used for several hours in the isomerization reaction, only presented the peaks corresponding to the tetragonal form of zirconia. At the same time, for the silica series, XRD confirmed the presence of NiO on the unsulfated catalysts and NiS04 on the sul ted ones. However, XRD did not show any evidence of any of these species for the zirconia series, probably due to their high state of dispersion. Similarly, the XPS data clearly showed the presence of NiO and NiS04 on the unsulfated and sulfated silica-supported catalysts, respectively, but they were not conclusive in the case of zirconia series since both sulfate and oxide species were observed. 

Activity in n-butane isomerization reaction of various alkaline salts of H3PW12O40 and H4SiWi2O40 was shown to be strongly dependent on the strength and number of accessible protons whereas the stability with time on stream was correlated to the presence of mesoporosity. For the liquid iC4/C4 continuous alkylation reaction, the strength and the number of acid sites appeared less important than the existence of mesoporosity indicating that the diffusion of the reactants and of the products plays an important role in this reaction. 

Recently, Eisenthal and coworkers have developed time-resolved surface second harmonic techniques to probe dynamics of polar solvation and isomerization reactions occurring at liquid liquid, liquid air, and liquid solid interfaces [22]. As these experiments afford subpicosecond time resolution, they are analogous to ultrafast pump probe measurements. Specifically, they excite a dye molecule residing at the interface and follow its dynamics via the resonance enhance second harmonic signal. 

Isomerization of the epoxide (IV) with pyrrolidine was carried out as described by Sih (8) and consistently gave yields of 35-60% rather than the 73% reported. Changes in experimental conditions including longer reaction times at lower temperatures, use of freshly distilled pyrrolidine, use of NaOH dried pyrrolidine, and use of distilled epoxide (IV) had little effect on the yield. The only variation that improved the yield was to allow the reaction to proceed at ambient temperature for a longer period of time than the recommended 3 hours. Allowing the reaction to proceed for 40 hours provided a maximum 67.5% yield. Other bases such as sodium carbonate, tri ethyl amine, diethylamine l,5-diazabicyclo[4.3.0]non-5-ene(DBN), and sodium methoxide all gave lower yields of distilled product than pyrrolidine. It is important to use the hydroxyaldehyde (V) as soon as possible since it is a very unstable material. 

Fig. 4. Variation of autocorrelation function with changes in the equilibrium constant in the fast reaction limit. A and B have the same diffusion coefficients but different optical (fluorescence) properties. A difference in the fluorescence of A and B serves to indicate the progress of the isomerization reaction the diffusion coefficients of A and B are the same. The characteristic chemical reaction time is in the range of 10 4-10-5 s, depending on the value of the chemical relaxation rate that for diffusion is 0.025 s. For this calculation parameter values are the same as those for Figure 3 except that DA = Z)B = lO"7 cm2 s-1 and QA = 0.1 and <9B = 1.0. The relation of CB/C0 to the different curves is as in Figure 3. 

If one desires to utilize this reactor to carry out a first-order isomerization reaction of the type A - R, and if the rate constant for the reaction is 7.5 ksec-1, determine the average conversion that one expects in this reactor. Compare this value with those one would obtain in an ideal PFR and in an ideal CSTR having the same average residence time as our actual reactor. 

There is an absence of cis-to-trans isomerization with conversion or time for the C8 (1,5-cyclooctadiene) polymer. This is shown from 52 to 58% conversion after 1 to 16 hours reaction time in Table II and III. The above review (A0, A2, A3, A5) shows that the cis structure in polymers from 1,5-cyclooctadiene using various chloride catalysts fell below 50% cis even to 20% cis units this means that the second cis double bond from the monomer underwent extensive cis-to-trans isomerization following the ring-opening of the first cis bond. Where cis-2-butene isomerizes to trans structure using other catalyst preparations, there is no evidence of this for cis-2-butene using the iodine system. However, polymer molecular... 

One exception to the preservation of selectivity in microwave reactions was the C2 arylation of 2,3-dihydrofuran, which yielded an isomeric byproduct under the action of microwaves (2-phenyl-2,3-dihydrofuran/byproduct = 71/29), in contrast with the reported procedure using conventional heating (Eq. 11.4). The desired product could be isolated in 58% yield. Attempts to reduce the reaction time by using oil baths (125 °C or 150 °C) did not result in similar yields, but instead furnished complicated reaction mixtures, in definite contrast with the microwave procedure [17]. 

---