Solvent effects half-life

The enol can be observed by NMR spectroscopy and at —20°C has a half-life of several hours. At -1-20°C the half-life is only 10 minutes. The presence of bases causes very r id isomerization to acetaldehyde via the enolate. Solvents have a significant effect on the lifetime of such unstable enols. Solvents such as DMF and DMSO, which are known to slow rates of proton exchange by hydrogen bonding, increase the lifetime of unstable enols. ... 

A series of simulations were performed to study the effect of variables such as initiator concentration, initiator half-life and activation energy on the optimum temperature and optimum time. It was assumed that initially the polymerization mixture contained S volume percent monomer, the rest of the mixture being solvent and polymer formed earlier. It was required to reduce the monomer concentration from S volume percent to 0.S volume percent in the minimum possible time. The kinetic and tbeimodyamnic parameters used are similar to those of free radical polymerization of MMA. The parameter values are given in Appendix B. 

An important parameter that has to be considered during desulfurization as well as for subsequent biocatalyst separation and recycle is the impact of the oil phase on the biocatalyst activity and half-life. Additionally, the effect of the biocatalyst on forma-tion/breakage of the oil-water emulsions is also important. The latter will be discussed in Section 2.3.3. It becomes important for lower boiling feedstocks such as gasoline, which offers the most toxic solvent environment for the biodesulfurization catalyst. The effect of solvents on biocatalysts has been investigated in very few reports. A study by the Monot group reported effect of two solvents on several Rhodococcus strains [254], The strains contacted with the solvents and their desulfurization activity, growth, and... 

The published quantification of the rate of hydrogenation of the dienes COD and NBD of a large number of cationic rhodium(I) chelate complexes allows a good estimation of expected effects on the rate of enantioselective hydrogenation of prochiral alkenes. From the first-order pseudo-rate constants the time needed for complete hydrogenation of the diene introduced as part of the rhodium precursor can be easily calculated as six- to seven-fold the half life. It is recommended that the transfer into the solvent complex be followed by NMR spectroscopy. 

The observed half life at 100°C. of 23 hours for a dilute solution of hydroperoxide in benzene indicates that significant decomposition may occur in the autoxidation of butene, depending on reaction conditions. No reliable evaluation can be made because of the known complications introduced on hydroperoxide decomposition by the effect of the solvent, the hydroperoxide concentration (2), the presence of oxygen (12), and the possibility of a strong acceleration in rate in the presence of oxidizing olefin, observed in at least one system (8). However, using the data reported by Bateman for a benzene solvent at 100 °C. in the presence of air (2), l-butene-3-hydroperoxide decomposes 13 times faster than cyclohexene hydroperoxide, a product which may be formed in extremely high yield by the oxidation of cyclohexene. 

Diazoamides of type 300 rapidly cyclize to form aziridines 302 (342) (Scheme 8.73). It is conceivable that this reaction proceeds through a 1,2,3-triazoline intermediate 301, which is the consequence of a LUMO(dipole)— HOMO(dipolarophile) controlled intramolecular [3 + 2] cycloaddition. Some remarkable steric effects were encountered for this cyclization. While the piperidine derivative [300, R R2 = (CH2)4] readily cyclized by diazo group transfer at 0 °C in 88% yield, the pyrrolidine analogue [300, R, R2 = (CH2)3] had to be heated for 1-2 days in polar solvents. The corresponding acyclic diazoamide (300, R1 = R2 = H) possessed a half-life of >10 days at ambient temperature. The intramolecular aziridination reaction, however, could be readily achieved under catalysis using Rh2(OAc)4. 

The formation of organic fluorides has been a difficult undertaking. However, an investigation of the effect of the solvent, leaving group, and source of the fluoride ion has shown that using tetrabutylammonium fluoride or CsF in r-amyl alcohol is able to convert arene sulfonates into fluorides rapidly at 90 °C in excellent yields (>80%).65 Since the half-life of 18F is only 110 min, this is an important advance as it allows one to label compounds with 18F for PET studies. 

A number of mechanistic problems remain, after accepting that the encounter pair ArNH2.NO is formed by pre-association. One such problem concerns whether the rate-determining step is the proton transfer or the reaction of the nitronium ion with the free amine formed within the encounter pair. It seems likely that the latter view is true from consideration of the rate profile, the very short half-life of the free amine, and the fact that these reactions do not show a marked primary hydrogen isotope effect when the rates in H2S04 and D2S04 are compared the reduction in rate by a factor of 2-3 in the deuterated solvent (Hartshorn and Ridd, 1968) is consistent with the lower concentration of the free amine. The reaction path can then be written as shown in Scheme 7 where NOJ. ArNHJ. B is an encounter triplet . 

The photoisomerization of several copolymers was studied, in order to determine the effects of the structure and switching of the chiral side chain on the helicity of the main chain. A delicate balance of parameters was found, including separation and nature of the stereocenters, solvent, and concentration of azobenzene moieties J77 Stereoselectivity was often greatly enhanced if the chiral moieties were closer to each other. Accordingly, it was found that the incorporation of the stereocenter into a short, two-carbon spacer resulted in much more pronounced helical preference, as well as CD effects at lower chiral chromophore concentrations. The greater helical twist and improved thermal stability of the cis form (half-life 40h at RT) are notable features. 771 It was also found that the relationship between the trans-cis... 

Phenanthridine, like other monoazaaromatics which fluoresce only weakly in nonpolar solvents, is subject to marked fluorescence activation by hydroxylic solvents. Recent studies have shown this to result from the effects of solvent on vibronic interactions between re,77 and 77,77 electronic states, and the effect of solvent changes on the phosphorescence half-life has been similarly explained.237 Measurements of fluorescence and, more especially, phosphorescence characteristics have been proposed as analytical methods for mixtures containing phenanthridines238, 239 and detailed studies of the emission spectra of phenanthridine,240, 241 its cation,241 9-methylphenanthri-dine,242 and phenanthridine-iV-oxide243 have been reported. A... 

Solvent effects were found to be substantial for this [3+2] cycloaddition. Thus, the half-life of the thermal reaction of 21 with maleic anhydride at 105°C was reduced to 59 min in <73-MeN02 compared with 120 min in i/g-toluene. The authors proposed the key notion that a reactive, conformationally rigid metalated azomethine species 24 is responsible for the high diastereoselectivity observed in the reaction. 

Thus, two types of active esters are of interest those formed from an acid and a substituted phenol (12-15) and those formed from an acid and a substituted hydroxylamine (16-19). Both types are reactive by virtue of the electron-withdrawing properties of the OR moiety in 2. The level of activation of the substituted phenyl esters varies directly with the electronic effect going from 4-nitrophenyl to 2,4,5-trichlorophenyl, pentachlorophenyl, and pentafluorophenyl, which corresponds with the increasing acidity of the phenols. A diminution in the rate of aminolysis is caused by the presence of a substituent in the ortho position of the ring.f l An additional phenomenon contributes to the reactivity of the esters formed from substituted hydroxylamines, namely anchimeric assistance. Since the anoinolysis of active esters is a bimolecular reaction, it is dependent on concentration and can be forced to completion by an excess of one of the reactants. Aminolysis is also characterized by a pronounced dependence on the polarity of the solvent in particular for the esters formed from substituted phenols, the half-life of a 2,4,5-trichlorophenyl ester in the presence of benzylamine being one hundred times less in dimethylformamide than in benzene. Furthermore, aminolysis is catalyzed by mild acid such as acetic acid. The rate of anoinolysis is slowed if the side chain of the active ester contains a P-methyl substituent. 

Kinetic studies have revealed that aliphatic ketyl radical anions are very shortlived compared with aromatic (half life of acetone " in aqueous 2-propanol is 72 ps, whereas that for acetophenone " is 1.5 ms) [253]. The reductive dimerization of simple aromatic aldehydes has been studied in aprotic solvents, with the second order rate constant being larger in acetonitrile than in DMF, because of ion-pair effects [254]. Electron-withdrawing substituents reduce the speed of dimerization (benzaldehyde " k = 2.4x 10 m" s", p-cyanobenzaldehyde k = 5 M s" ) [255], whereas protic solvents lead to protonation before dimerization [256]. 

The latter was shown to be probably due to a medium effect on the exciplex lifetime. Addition, of small amounts of polar solvents to the solutions produced similar results. With further increases in the concentrations of polar additives, the transient half-life Increased to a constant value. The transitions in decay kinetics from first-order to second-order and in transient absorption spectra indicated that a new decay process of the exciplex was dominant ... 

The generation of ethoxycarbonylcarbene by thermal cleavage of ethyl diazoacetate is usually effected at 140-150 C. At 100°C this diazo ester has a half-life time of 109 hours in the inert solvent 1,3,5-trimethylbenzene. A second substituent at the diazo carbon can markedly influence the thermal stability of the diazo ester. For example, the phenyl group, as well as acyl and sulfonyl substituents reduce the thermal stability, whereas the diethoxyphosphoryl and trialkylsilyl groups enhance it. 

---