Pressure effects reaction

Pressure Effects. Reactions in supercritical media typically involve elevated pressures that could have either an enhancing or inhibiting effect on rate and equilibrium constants. Based on transition-state theory, the pressure dependence on the rate constant is given by the following equation ... 

Reductive alkylations and aminations requite pressure-rated reaction vessels and hiUy contained and blanketed support equipment. Nitrile hydrogenations are similar in thein requirements. Arylamine hydrogenations have historically required very high pressure vessel materials of constmction. A nominal breakpoint of 8 MPa (- 1200 psi) requites yet heavier wall constmction and correspondingly more expensive hydrogen pressurization. Heat transfer must be adequate, for the heat of reaction in arylamine ring reduction is - 50 kJ/mol (12 kcal/mol) (59). Solvents employed to maintain catalyst activity and improve heat-transfer efficiency reduce effective hydrogen partial pressures and requite fractionation from product and recycle to prove cost-effective. 

Reactive Chemicals Reviews The process chemistry is reviewed for evidence of exotherms, shock sensitivity, and other insta-bihty, with emphasis on possible exothermic reactions. It is especially important to consider pressure effects— Pressure blows up people, not temperature The pumose of this review is to prevent unexpected and uncontrolled chemical reactions. Reviewers should be knowledgeable people in the field of reactive chemicals and include people from loss prevention, manufacturing, and research. 

Refers to the elapsed time after an adhesive is applied until pressure effects curing. Intermediate-stage reaction step for various thermosetting resins. During this stage the material swells when in contact with certain liquids and... 

Spectator ion An ion that, although present, takes no part in a reaction, 279,82-83, 372-373,399 Spontaneity of reaction concentration and, 465-467,475-476q entropy and, 453-458 free energy and, 458-471 pressure effects, 465-467,475-476q process, 451-453 redox, 489-490... 

These results indicate that, during thermolyses of fructose-containing saccharides, di-D-fructose dianhydrides are formed readily, but subsequent isomerization is extremely slow—even in the presence of added acid. However, under these conditions, the protonating power of any acid is moot. At the high temperatures used, residual water would be driven off rapidly, unless the reaction vessel is pressurized therefore, reaction occurs in the anhydrous melt. It is presumably protonation of one of the ring oxygen atoms in the dianhydrides that constitutes the first step in isomerization, followed by scission of a C-O bond to yield one of the oxocarbenium ion intermediates postulated in Refs. 31 and 80. Such ions have also been postulated as intermediates in the isomerization of spiroketals to a more-stable product. This latter isomerization can be extremely facile 104 dilute aqueous acid,120 or non-aqueous Lewis-acid conditions121 have been used to effect such transformations. 

The thermal instability of 37 reduces its applicability with poorly reactive dienes such as vinylcyclohexene and its derivatives 38, unless high pressure (HP) is employed. Ultrasound is not only effective in promoting the cycloaddition of 37 with 38, but sometimes also improves the regioselectivity. Some data are illustrated in Table 4.8 and compared with cycloadditions in refluxing benzene and under high pressure. The reactions of 37 with reactive dienes such as cyclopentadiene and l-(trimethylsiloxy)-1,3-butadiene give a good yield of type D adducts under mild conditions, while with less reactive dienes, such as isoprene and butadiene, poor results are obtained. 

As shown in Ch. 2, the effect of pressure on the nature of the deposit is considerable. At high pressure (i.e., ca. atmospheric), the deposition is diffusion limited and, at low pressure, surface reaction is the determining factor. In practical terms, this means that low pressure generally provides deposits with greater uniformity, better step coverage, and improved quality. 

At extreme pressures, liquid-phase reactions exhibit pressure effects. A suggested means for correlation is the activation volume, A Vaco Thus,... 

In order to obtain reasonable concentrations of CHj- radicals in the low pressure studies, reactions were carried out at temperatures > 700 C whereas, the conventional catalytic studies of Vannice and co-workers have been carried out at temperatures 700 °C. Therefore, for comparison with our low pressure results, the effect of temperature on the rate of Nj formation at a total pressure of 760 Torr has been investigated up to 875 °C for three gas compositions (i) 1 % NO, (ii) 1 % NO + 0.25 % CH4 and (iii) 1 % NO + 0.25% CH4 -f- 0.5% Oj. The results are summarized in Figure 7. At 700 °C, the trends with respect to the effect of gas composition are qualitatively the same as those reported by Vannice i.e., the addition of CH enhanced the rate of Nj formation, and the addition of O2 enhanced the rate even more. At 700 C the effect of adding CH4 was to increase the rate two-fold, while the effect of adding CH4 O2 was to increase the rate five-fold. 

Table 3.1 Effect of Hydrogen Pressure on Reaction Rate and Time at Absolute...

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

It should be emphasized that the enthalpy change includes not only sensible heat effects but also a heat-of-reaction term and in some cases pressure effects. If there are multiple inlet and outlet streams, appropriate averaging techniques must be used to employ this equation. 

A runaway self heating reaction occurs 3.8 s after mixing at 201°C or above, and at that temperature an exotherm to 304°C at a rate above 20007s was observed, but no pressure effects were seen. 

Scheeren and coworkers used the pressure effect for a powerful domino process consisting of three cycloadditions [2], to form up to six bonds and eight stereogenic centers in a single operation. At 15 kbar and 50 °C, the reaction of a mixture of 10-1 (1 equiv.) and 10-2 (3 equiv.) led to tricyclic nitroso acetals 10-5,10-6 and 10-7 in a 1 3 1 ratio via the first [4+2]-cycloadduct ( )-10-3 and the second [4+2]-cycloadduct ( )-10-4. The final step in this sequence is a [3+2]cycloaddition (Scheme 10.1). 

As might be expected, 02 impurities were very effective scavengers of Cr(CO)5, but more unexpectedly C02 had no effect. Reactions carried out under pressures of C02 (97) were indistinguishable from those carried out under Ar. This contrasts sharply with matrix studies (101) where C02 was found to photooxidize metal carbonyls. 

Farhataziz et al. (1974a, b) studied the effect of pressure on eam and found that as the pressure is increased from 9 bar to 6.7 Kbar at 23° (1) the primary yield of e decreases from 3.2 to 2.0 (2) hv increases from 0.67 to 0.91 eV (3) the half-width of the absorption spectrum on the high-energy side increases by 35% and (4) the extinction coefficient decreases by 19%, which is similar to eh. The pressure effects are consistent with the large volume of ean (98 ml/M), whereas the reduction in the observed primary yield at 0.1 ps is attributable to the reaction eam + NH4+. Some of the properties of eam have been discussed by several authors in Solvated Electron (Hart, 1965). 

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