Product ratios, conversion dependence

Scheme 6 depicts a typical penicillin sulfoxide rearrangement (69JA1401). The mechanism probably involves an initial thermal formation of a sulfenic acid which is trapped by the acetic anhydride as the mixed sulfenic-acetic anhydride. Nucleophilic attack by the double bond on the sulfur leads to an episulfonium ion which, depending on the site of acetate attack, can afford either the penam (19) or the cepham (20). Product ratios are dependent on reaction conditions. For example, in another related study acetic anhydride gave predominantly the penam product, while chloroacetic anhydride gave the cepham product (7lJCS(O3540). The rearrangement can also be effected by acid in this case the principal products are the cepham (21) and the cephem (22 Scheme 7). Since these early studies a wide variety of reagents have been found to catalyze the conversion of a penicillin sulfoxide to the cepham/cephem ring system (e.g. 77JOC2887).

In case 2, the lowest AG is that for formation of A from R, but the AG for formation of B from A is not much larger. System 2 might be governed by either kinetic or thermoifynamic factors. Conversion of R to A will be only slightly more rapid than conversion of A to B. If the reaction conditions are carefully adjusted, it will be possible for A to accumulate and not proceed to B. Under such conditions, A will be the dominant product and the reaction will be under kinetic control. Under somewhat more energetic conditions, for example, at a higher temperature, A will be transformed to B, and under these conditions the reaction will be under thermoifynamic control. A and B will equilibrate, and the product ratio will depend on the equilibriiun constant determined by AG. 

The value of E - y is called the open-circuit voltage of the cell, which is related to the composition of the product. Note that the steam conversion ratio, X, depends on the open-circuit voltage, and is not affected by the pressure or flow rate of the reactant. Also, the open-circuit voltage decreases with increasing temperature because of the endothermic nature of the reaction. However, due to the temperature dependence of the logarithmic term in Equation 4.5, this effect decreases with the value of X. 

Samples of 1 (200 mg) were sealed in evacuated Pyrex ampoules (inner diameter 4 mm) and immersed in a 500-mL Pyrex beaker filled with ice and water in such a way that no ice blocked the laser beam. The beam of an excimer laser (Lambda Physics, EMC 201 XeCl 17 ns pulses 50 Hz repetition rate 3 h X = 308 nm) was positioned vertically using two dielectric mirrors and focused to the desired intensity by a quartz-lens with a focal length of 20 cm. For low intensity irradiations, the ampoules were placed in front of a mercury arc at a distance of 5 cm. The product ratio depended on the light intensity. The compounds 1, 2, 3 and 4 were separated by gas chromatography or HPLC on RP18 and spectroscopically characterized after 93-97% conversion to 3 and 4. 

Stereochemical integrity may also be lost when the reaction of a stereoisomer occurs via an intermediate which retains a stereogenic element, but whose bonding permits interconversion of stereoisomers faster than its conversion of stereoisomeric products. [2 + 21-Cycloaddition of TCNE and cis-propenyl methyl ether [30] yields cis- and trans-adducts, 22 in Scheme 9.13, in ratios which depend on the solvent (84 16 in favour of the cis-adduct in acetonitrile). The dipolar 23 was proposed as an intermediate. The initial bonding destroys the double bond character between Cl and C2 of the enol ether reactant, and the much... 

Nitroquinoline 209 enters into a direct cyclocondensation with aromatic hydra-zones in NaOH/DMF giving pyrazolo[3,4-/]quinolines 210 and (or) triazino[6,5-/ quinolines 211 in low to moderate yield (Scheme 62) (OOOL413). Their ratio mainly depends on the structure of the starting hydrazone. For example, electron-donating substituents in its aryl moiety assist triazine ring closure. Evidently, pyrazoles 210 are products of two consecutive SNH and SN ipso reactions, whereas conversion of 209 into 211 looks rather complicated and better corresponds with cascade hetero-cyclizations considered in Section III.D.l. 

The sensitizer-dependence of these product ratios has been attributed to triplet energies that differ for the s-cis and s-trans conformers (Sch. 6). Conversion of the dienes to triplet biradicals 24 and 25 lock in the initial conformations. Subsequent bond formation with a second diene leads to two allylic radicals 26 and 27, again preserving the geometries. Spin inversion and bond formation then forms the cyclobutane 6 from 26 and the cyclohexene 7 from 27. The higher population of s-cis conformer for isoprene 16 is reflected in the higher proportion of [4+2] cycloadducts 20 and 21 as well as the appearance of [4+4] cycloadducts 22 and 23 in the product mixture (Sch. 5) [32]. 

Situation (I) corresponds to a fluid isotropic solution where a uniform time averaged environment should exist. Under such conditions single exponential decay would be expected for the guest excited states and the photoreactivity should be predictable on the basis of a single effective reaction cavity. In situation (II) there should be two kinetically distinct excited states in two noninterconverting sites resulting in nonexponential decay of the excited state of A. The quantum efficiency of product formation and the product distribution may depend upon the percent conversion. An example of mechanism (II) is provided in Sch. 22 [137]. The ratio of products A, B, and C has been shown to depend on the crystal size. With the size of the crystal the ratio of molecules present on the surface and in the interior changes which results in different extents of reactions from two the distinct sites namely, surface and interior. 

For this case, since the desired product B does not react further, its yield versus conversion depends only on the ratio... 

In the case of propane, the two chief products are ethylene and propylene. In practice, operations arc conducted to 70 to 90 per cent conversion depending on the desired nhyiene to propylene ratio. At 90 per cent once-through conversion, a hnal ethylene yield of about 45 molar per cent is obtained after ethane recycling, and the propylene ieid varies from 26 per cent for 75 per cent once-through conversion to 16 molar per cent for 90 per cent conversion. 

A detailed study of the photooxygenation of hexene isomers showed that, depending on the reaction temperature (25c or 50 C). the allylic hydroperoxides are produced E selectively38, in diastereomeric ratios from 78 22 to > 99 1. In cases with a high alkene concentration in ethanol as solvent and rose bengal as sensitizer, the product ratios are determined after 5-10% conversion. 

The dilution technique makes use of the different concentration dependence of the S—T conversion (a) and the carbene addition to the olefin (/S). The decay of the metastable singlet state is unimolecular, while the stereospecific addition rate is first-order in olefin concentration 78). The dilution technique has not yielded a common ratio in the experiments with cis- or iraws-butene and 2c (see Table 11). Extrapolation of the data to infinite dilution gives a product ratio of 0.16, suggesting that... 

Further insight into the reaction mechanism can be obtained from consideration of the conversion dependence of product ratios. These can provide more reliable information than normal selectivity plots since, although these ratios are subject to the uncertainties in the individual product yields, their accuracy is not affected by the larger uncertainty in the calculated conversion levels. Moreover, the variation in product yield ratios with conversion can provide a clearer insight into the course of secondary reactions. 

The most significant differences between perfectly mixed and segregated flow in a CSTR occur in copoly merizalions. In a batch reaction, the copolymer composition varies with conversion, depending on the reactivity ratios and initial monomer feed composition. In a perfectly mixed CSTR, there will be no composition drifts but the distribution of product compositions will broaden as mixing in the reactor approaches segregated flow. 

The change in sample size has been shown to influence product ratios, fractional conversions, and rates of pyrolysis for several polymers [14]. However, the dependence of pyrolysis results on sample size does not have only a negative impact on data reproducibility. This effect may reveal mechanistic features of the pyrolysis process, such as secondary reactions that occur during the residence time of primary products in the melt and diffusion-restriction of the interaction of long-chain radicals, when the thickness of the degrading film is comparable with the diameter of the hydrodynamic volume of the polymer molecules. 

---