Enhanced reaction rates

Although the coimneicial process normally uses heat and pressure without a catalyst, acid or base catalysts can be used to enhance reaction rates or... 

Enhanced reaction rate, which can be used to increase plant capacity by up to 75%. 

In the synthesis of the tetracyclic intermediates for the synthesis of isoarborinol and its CDE-antipode femenol, the stereochemistry of the Diels-Alder reaction can be varied using various Lewis-acid catalysts in aqueous media (Eq. 12.36).97 Their results show that the hydrophobic effects play an important role in enhancing reaction rates and can control product distribution. Novel 2,4-dialkyl-1-alkylideneamino-3-(methoxycarbonylmethyl)azetidines were obtained from aldazines and... 

Recent research has demonstrated enhanced reaction rates in the ruthenium-catalyzed hydro-... 

Although the initial reason for the development of solvent-free conditions for micro-wave irradiation was safety, it soon became apparent that use of these conditions had many other benefits - simplicity, efficiency, easy work-up, very often higher yields, and enhanced reaction rates. The absence of solvent is, furthermore, time- and moneysaving and often enables elimination of waste treatment. 

If microwave heating leads to enhanced reactions rates, it is plausible to assume that the active sites on the surface of the catalyst (micro hot spots) are exposed to selective heating which causes some pathways to predominate. In the case of metal supported catalysts, the metal can be heated without heating of the support due to different dielectric properties of both catalyst components. The nonisothermal nature of the microwave-heated catalyst and the lower reaction temperature affects favorably not only reaction rate but also selectivity of such reactions. 

Structurally developed cyclodextrins. Effective procedures for the selective functionalization of peripheral hydroxyl groups on the cyclodextrins have been developed. A motivation for these studies has been to produce suitably functionalized hosts which will induce enhanced reaction rates... 

Qureshi N, Annous BA, Ezeji TC et al (2005) Biofilm reactors for industrial bioconversion processes employing potential of enhanced reaction rates. Microb Cell Fact 4(24) 1-21... 

Dual Photo/Thermal Initiation Studies. A series of studies were performed using reactive formulations containing both a photoinitiator and a thermal initiator dissolved in the Derakane resin. The objective of these studies was to investigate a dual cure strategy in which the heat liberated by the photo-induced polymerization leads to the production of additional active centers by the dissociation of a thermal initiator. In this way, the dual cure strategy could offer both the temporal control of the start of the reaction afforded by the photopolymerization, as well as enhanced reaction rate and completeness of cure provided by the thermal initiation. 

ORMOSIL are chemical sponges they adsorb and concentrate reactants at their surface, thereby enhancing reaction rates and sensitivity (in sensing applications). ORMOSIL-imprinted materials with a suitable chiral template such as a surfactant or a protein selectively adsorb (and detect) external reactants. A remarkable example is provided by thin materials that are generally enantioselective, namely where the chirally imprinted cavities can discriminate between enantiomers of molecules not used in the imprinting process, and completely different from the imprinting ones. 

S to N transfer of the nitroso group has also been postulated on a number of other occasions53,71 to explain enhanced reaction rates. Detailed kinetic studies on the nitrosa-tion of thioproline72 (12) and thiomorpholine73 (13) reveal that two pathways can exist, which depend on the reaction conditions, (a) a direct reaction at nitrogen and (b) a direct reaction at sulphur followed by S to N migration of the nitroso group. 

The steel burster tubes from projectiles are unaffected by the caustic in the COINS. To enhance reaction rates, burster tubes are sheared into three pieces by the burster size-reduction machine to provide more surface area for the reaction of tetryl with caustic. The... 

In general they found both enhanced reaction rates and polymers with lower poly-dispersities in the presence of ultrasound provided by both bath and probe systems. Higher ultrasonic intensities resulted in narrower molar mass distributions. 

Lewis acids have served as catalysts in numerous fields of organic synthesis. By temporary coordination to basic sites substrates can be activated resulting in enhanced reaction rates. Immobilization of Lewis acids to a solid support has been carried out in order to overcome work-up problems, for complexing ligands or for generating distinct reaction environments. 

The reaction with monomethyl malonate in acetic acid, which does not occur at 0-10°C, proceeds smoothly when sonication is applied (Allegretti et al. 1993). From cyclohexene, only the cis ring fusion in bicyclic lactone is observed the product is formed at 80% yield for 15 min at 10°C. The overall transformation, in brief, is shown in Scheme 6.16. The stereoselectivity of the sonochemical process probably reflects the enhanced reaction rate, which does not allow equilibration processes to take place. 

In Table 1 (pp. 251-254), IM rate constants for reaction systems that have been measured at both atmospheric pressure and in the HP or LP range are listed. Also provided are the expected IM collision rate constants calculated from either Langevin or ADO theory. (Note that the rate constants of several IM reactions that have been studied at atmospheric pressure" are not included in Table I because these systems have not been studied in the LP or HP ranges.) In general, it is noted that pressure-related differences in these data sets are not usually large. Where significant differences are noted, the suspected causes have been previously discussed in Section IIB. These include the reactions of Hcj and Ne with NO , for which pressure-enhanced reaction rates have been attributed to the onset of a termolecular collision mechanism at atmospheric pressure and the reactions of Atj with NO and Cl with CHjBr , for which pressure-enhanced rate constants have been attributed to the approach of the high-pressure limit of kinetic behavior for these reaction systems. 

Also the use of moisture stable ionic liquids as solvents in the Diels-Alder reaction has been carried out, and in all examples an enhanced reaction rate was observed [182,183]. The application of pyridinium-based ionic liquids allowed the utilization of isoprene as diene [184]. The chiral ionic liquid [bmim][L-lactate] was used as a solvent and accelerated the reaction of cyclopentadiene and ethyl acrylate, however, no enantiomeric excess was observed [183]. In addition several amino acid based ionic liquids have been recently tested in the Diels-Alder reaction. Similar exo. endo ratios were found but the product was obtained as racemate. The ionic liquids were prepared by the addition of equimolar amounts of HNO3 to the amino acids [185]. Furthermore, an enantiopure imidazolium salt incorporating a camphor motive was tested in the Diels-Alder reaction. No enantiomeric excess was found [186]. 

Ionic liquids represent a unique class of reaction media for catalytic processes, and their application in catalysis has entered a period of exploding growth. The number of catalytic reactions involving ionic liquids continues to increase rapidly. These liquids offer promising solutions to the problems associated with conventional organic solvents the potential advantages may include enhanced reaction rates, improved chemo- and regioselectivities, and facile separation of products and catalyst recovery. 

A remarkable increase in the rate was observed when the reaction was conducted in ionic liquids instead of CH3CN (Fig. 13). The rate increased by 11 times in [BMIM]BF4 and 34 times in [BMIM]PF6. Furthermore, the product yield was significantly enhanced in [BMIM]PF6 (65%), and this value is considerably higher than that in CH3CN (35%) and is comparable to that of characterizing the neat reaction. The enhanced reaction rates in the ionic liquids were attributed to a shift in the equilibrium favoring stabilization of the zwitterionic intermediate. 

Enhanced reaction rates, improved yields, and high functional group compatibility are the features demonstrated by these ionic liquids. The products are weakly soluble in the ionic phase, so that they are easily separated by simple extraction with ether. 

A.9. Enhanced Reaction Rate Due to Increased Substrate Solubility... 

Enhanced reaction rates have also been observed in some instances of decreased polymer solubility with conversion when the precipitating polymer absorbs the small-molecule reactant. Adsorption results in an increased concentration of the reactant at the actual polymerization site (the polymer coils). Similar effects are observed if a catalyst is adsorbed onto the precipitating polymer [Beresniewicz, 1959]. 

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