Allowed process

Themiodynamics is a powerful tool in physics, chemistry and engineering and, by extension, to substantially all other sciences. However, its power is narrow, since it says nothing whatsoever about time-dependent phenomena. It can demonstrate that certain processes are impossible, but it caimot predict whether thennodynamically allowed processes will actually take place. [Pg.322]

This is the allowed process by the Woodward-Hoffmaim rules (see Tedder, Part 3, pp. 383-387 or Norman p. 292 ff if you want to know more). There are obviously two disconnections for any given cyclobutane but it is often easy to see the better at once. How would you make ... [Pg.94]

The reaction of carbon atoms with A-unsubstituted aziridines leads to alkenes and hydrogen cyanide (72IA3455), probably via extrusion from the initially formed adduct (285). The fragmentation does not appear to be concerted, although this would be a symmetry-allowed process, since only about half the alkene formed retains the aziridine stereochemistry in the case of cM-2,3-dimethylaziridine. [Pg.75]

Contamination from Process Gas. Contamination from process gas can occur when there is insufficient supply of sealing gas into the seal, allowing process gas to come into direct contact with the seal ring faces. Contaminants existing within the process gas can then damage the seal. [Pg.518]

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. [Pg.611]

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... [Pg.622]

Don t allow process improvements to be made in isolation without assessing their impact on the system. [Pg.156]

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. [Pg.364]

The steam valve at the other end of the hose was closed just before the column inlet valve, thus allowing process material to enter the hose. The operators knew this was not normal practice. [Pg.150]

Dithiadiazolyl radicals are typically prepared by reduction of the corresponding cations with SbPh3. They are unstable with respect to isomerization to the 1,2,3,5-isomers both in solution " and in the solid state. The isomerization is a photochemically symmetry-allowed process, which is thermally symmetry forbidden. A bimolecular head-to-tail rearrangement has been proposed to account for this isomerization (Scheme 11.1). This rearrangement process is conveniently monitored... [Pg.222]

The Diels-Alder Reaction. A Symmetry Allowed Process 273... [Pg.271]

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. [Pg.221]

For given feed composition, eluent, and stationary phase, the flowrates of a TMB to allow processing a given flowrate of feed are calculated based on the knowledge of the adsorption isotherms. [Pg.264]

Finally, there is active interest in developing catalyst systems, both ballistic and polymerization, that would promote combustion stability at high pressures (especially in metal-free systems for smokeless applications) and allow processing lattitude for relatively large motors. The ferric-based systems currently being used fall short of these performance measures. Compounds that form complex structures with the metal chelate to reduce its activity to acceptable levels seem to be most promising. Interestingly, the use of an antibiotic has been cited in this context [19],... [Pg.707]

The pressure at which the valve is expected to open (set pressure) is usually selected as high as possible consistent with the effect of possible high pressure on die process as well as the containing vessel. Some reactions have a rapid increase in temperature when pressure increases, and this may fix the maximum allowable process pressure. In other situations the pressure rise above operating must be kept to some differential, and the safety valve must relieve at the peak value. A set pressure at the maximum value (whether maximum allowable working pressure of vessel, or other, but insuring protection to the weakest part of the system) requires the smallest valve. Consult manufacturers for set pressure compensation (valve related) for temperatures >200°F. [Pg.425]

Catalyst cooler(s). Installing a catalyst cooler(s) is a way to control and vary regenerator heat removal and thus allow processing of a poor quality feedstock to achieve increased product selectivity. [Pg.204]

In addition, we have established that there is a sense of direction to the location of the inaccessible states. State 2, the state reached from 1 by a reversible adiabatic path, represents the division between the states on the second isotherm that are accessible and inaccessible from state 1. We represent this schematically in Figure 2.1 lb, where the reversible adiabatic path separates states that are accessible from state 1 from those that are inaccessible. The observation that the reversible path serves as the boundary between the two sets of states will be useful later when we show the direction of allowed processes in terms of the sign of A5(universe). [Pg.70]

Thus, the process of hydride ion abstraction from a primary position is approximately thermoneutral, and hence we must conclude that it is an energetically allowed process, although possibly with a relatively small reaction rate. A process competing with primary H abstraction (Reaction 13) is methide ion abstraction (Reaction 11, loss of CH4 from the... [Pg.186]

The primary goal of the work described here is to provide a means whereby specifiers can become implementers that is, to provide automation systems tools (for the specific case of batch polymerizations) allowing process and production experts to design, implement, maintain, and enhance their own "state-of- the-art" control systems. [Pg.470]

OS 68] [R 19] ]P 50] The lower yield of the industrial batch process (6 m stirred vessel) of only 72% is due to limitations of the cooling system allowing processing only at -20 °C, and not at —40 °C as is possible for the better laboratory-scale batch process (0.5 1 flask 0.5 h 88% yield) [134], The industrial process had a surface-to-volume ratio of 4 m m , the laboratory-scale process of 80 and the micro... [Pg.519]

Cycloadditions of ketenes and alkenes have synthetic utility for the preparation of cyclobutanones.163 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.164 To be an allowed process, the [2ir + 2-tt] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the TS is a 2irs + (2tts + 2tts) addition.165 Figure 6.13 illustrates these combinations. Note that both representations predict formation of the d.v-substituted cyclobutanone. [Pg.539]

Several of the spin-allowed processes are listed below ... [Pg.147]

The use of trifluoroethanol as solvent or absorption of the dienone on silica gel promotes the photoconversion of dienones into bicyclic ketenes.<47) For the photolysis<48 60) of (63) it has been shown by low-temperature infrared and ultraviolet spectroscopy that the initial photolysis gives a ketene which can be efficiently trapped by cyclohexylamine or, in the absence of a good nucleophile, thermally rearranges by a OA, + 20) allowed process to a bicyclic ketone (64) ... [Pg.468]

Supercritical C02 extraction coupled with a fractional separation technique is used by producers of flavours and fragrances to separate and purify volatile flavour and fragrance concentrates. Like any solvent, supercritical C02, it allows processing chemicals by predpita-tion or recrystallisation, obtaining partides of controlled size and shape, without excessive fines without thermal stresses and controlling the shape of a polymorphic substance. [Pg.101]

Within the isolobal formalism, the conversion of 47 to 48 is a symmetry-allowed process, if it were to proceed as a concerted reaction (50). Structure 47 represents a transoid-2-meta.Wa-1,3-butadiene. In the bonding description, complex 48 represents formally a 1-metalla-bicyclo[1.1.0]butane. Therefore, the conversion of 47 to 48 represents a thermally allowed, concerted [ 2a + 2S] ring closure, in analogy to the pericyclic ring opening of bicyclo[1.1.0]butanes to give trans,trans-, 3-butadienes. [Pg.65]

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