Small substitution effects

Stereoisomerism. The first trials to use the XPS valence band spectra to distinguish between two stereoisomers were unsuccessful. Cis- and trans- poly(isoprene) - with short branched chain and small substitution effects-, as well as cis- and trans-poly(l,4dichloro-2,3epoxybutene) - with longer branched chain and more intense substituent effects- did not show in our first measurements significant differences in their valence band spectra that could be attributed to the searched effect. Before... 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

The use of carbon-14 is bringing to light many unexpected topological complexities. It is a more subtle label, in spite of the existence of small isotope effects, than a substituent alkyl group. Any considerable substitution may alter the mechanism of a reaction from that of the unsubstituted compound, while carbon-14 is unlikely to do this. An example of newly discovered topological complexity is the ionization of... 

In the preceding sections, the bond to the isotopic atom is broken or formed in the rate-determining step of the reaction. In these cases, the change in rate is referred to as a primary kinetic isotope effect. Isotopic substitution at other sites in the molecule has much smaller effects on the rate. These small isotope effects are collectively referred to as secondary kinetic isotope effects. 

A similar system, (CH3)2C=CH X, was studied by Endrysova and Kraus (55) in the gas phase in order to eliminate the possible leveling influence of a solvent. The rate data were separated in the contribution of the rate constant and of the adsorption coefficient, but both parameters showed no influence of the X substituents (series 61). A definitive answer to the problem has been published by Kieboom and van Bekum (59), who measured the hydrogenation rate of substituted 2-phenyl-3-methyl-2-butenes and substituted 3,4-dihydro-1,2-dimethylnaphtalenes on palladium in basic, neutral, and acidic media (series 62 and 63). These compounds enabled them to correlate the rate data by means of the Hammett equation and thus eliminate the troublesome steric effects. Using a series of substituents with large differences in polarity, they found relatively small electronic effects on both the rate constant and adsorption coefficient. 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

As reported in numerous studies 61 -66), the reactivity of the glycidyl groups in Bisphenol A diglycidyl ether is virtually the same. At the same time, a small negative 54 61) or positive 51 substitution effect has been observed. However, in some other cases this effect can be substantial 20). 

Since the carbonyl group has gone in R2C(OH)CN, we would expect very little effect from any of these substitutents. There can t be any conjugation, and inductive effects on the distant 0 atom will be small. What effect would an inductively withdrawing substituent have on the equilibrium for cyanohydrin formation ... 

In this case a large isotope effect would be expected. If, however, k2 [B] k u then Equation 7.71 cannot be simplified, and the magnitude of k2 will affect the overall rate albeit in a less than linear way. Then, even if k2 > ku some isotope effect should be observed. The small isotope effect of Melander s experiments make it appear that the first step is slower than the second, but that k 1 competes favorably with k2. When the second step becomes kinetically important in spite of the first step being the slow step, we have an example of the partitioning effect— so-called because the kinetic significance of the second step arises from the way in which the intermediate partitions itself. Since 1950 a very large number of electrophilic substitutions have been examined for isotope effects in the absence of special circumstances (see below), the isotope effects found are usually very small.158... 

Silver acetate has a small catalytic effect on the alkene substitution reaction but 5 equiv. of the salt only give 140% of stilbene in the styrene phenylation, based upon palladium.15 The same reaction carried out at 80 C under 300 lbf in-2 (1 lbf in-2 = 6.89 kPa) of oxygen gives stilbene in 248% yield, based upon palladium.16 The best reoxidation reagent is f-butyl perbenzoate, which yields 10-14 turnovers of the palladium in the vinyl substitution of cinnamaldehyde and similar alkenes with benzene.17... 

Kinetic studies of the addition of substituted phenyl azide to norbornene show a small substituent effect with a Hammett p value of +0.84, a large... 

In the past, the equivalence between the size distribution generated by the Smoluchowski equation and simple statistical methods [9, 12, 40-42] was a source of some confusion. The Spouge proof and the numerical results obtained for the kinetics models with more complex aggregation physics, e.g., with a presence of substitution effects [43,44], revealed the non-equivalence of kinetics and statistical models of polymerization processes. More elaborated statistical models, however, with the complete analysis made repeatedly at small time intervals have been shown to produce polymer size distributions equivalent to those generated kinetically [45]. Recently, Faliagas [46] has demonstrated that the kinetics and statistical models which are both the mean-field models can be considered as special cases of a general stochastic Markov process. 

The polymerization temperature, often called the cure temperature, affects both transitions in different ways. In Chapter 3 it was shown that the gel conversion does not depend on temperature for ideal stepwise polymerizations but may show a small dependence on temperature for the case where unequal reactivity of functional groups or substitution effects vary... 

Since the moment of inertia ratio will be nearly unity, the major source of the isotope effect at a given energy is the activated complex ratio. A pure intramolecular statistical secondary effect can scarcely arise, since by definition no net differential effect on the competing reaction coordinates arises due to isotopic substitution. However, in practice, small mechanistic effects mentioned earlier can exist for isotopic substitution which preferentially affects one of the competing bond-rupture sites. 

Methyl- and 2-phenylindolizine are each sulfenylated by CF3SC1 at both the 1- and the 3-positions giving the SCF3 derivatives. With an acetyl substituent at the 3-position, substitution still takes place at the 1-position (81JFC67), so acetyl does not deactivate appreciably, which is consistent with the small substituent effects in this very reactive system. 

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