Kinetically controlled structures

The above computations for the fiee energy changes for composite latex particles have shown that only core-shell, inverted core-shell and hemispherical particles are stable in a thermodynamic sense. All of the other reported morphologies (e.g. sandwich-like , raspberry or confetti-like particles or occluded domains) are non-equilibrium, kinetically controlled structures, prepared... 

For example, an aminofunctional silane can condense almost instantaneously after hydrolysis due to the high pH of the amine group, leading to a kinetically controlled structure which is thermodynamically unequilibrated. Provided that a sufficiently high concentration was used, the solution turns into a gel. With time, in the presence of water, the catalytic action of the amine reorganizes the siloxane structure by rehydration and condensadon reactions, and redissolves the silane. Thus, the time factor is also important when silane stmctures are to be studied. 

It should be noted that all non-ideal, kinetically controlled structures like snowflakes represent out-of-equilibrium objects. That means that they depend on the conditions under which they were formed and also on how they evolved in time after growth (history dependence). Qualitatively, one may state that the faster the patterns were formed, the more they consist of small subunits (thinner branches or smaller objects constituting the pattern [6-8]), the more fragile they are and the more they tend to change with time ( ageing effect ) [16,17],... 

Being kinetically controlled, structures of partially crystalline samples are always strongly aflFected by the processing and show a memory of the thermal history, i.e. temperatures and times of crystallization, cooling rates, etc. A first requirement for the analysis are measurements enabling a characterization of the evolving structures. In this section, we will deal with the main observations and some of the applied techniques. 

Structural effects on the rates of deprotonation of ketones have also been studied using veiy strong bases under conditions where complete conversion to the enolate occurs. In solvents such as THF or DME, bases such as lithium di-/-propylamide (LDA) and potassium hexamethyldisilylamide (KHMDS) give solutions of the enolates in relative proportions that reflect the relative rates of removal of the different protons in the carbonyl compound (kinetic control). The least hindered proton is removed most rapidly under these... 

Early work established that S4N4 forms di-adducts with alkenes such as norbornene or norbomadiene. Subsequently, structural and spectroscopic studies established that cycloaddition occurs in a 1,3-S,S"-fashion. The regiochemistry of addition can be rationalized in frontier orbital terms the interaction of the alkene HOMO with the low-lying LUMO of S4N4 exerts kinetic control. Consistently, only electron-rich alkenes add to S4N4. 

Reactant and product structures. Because the transition state stmcture is normally different from but intermediate to those of the initial and final states, it is evident that the stmctures of the reactants and products should be known. One should, however, be aware of a possible source of misinterpretation. Suppose the products generated in the reaction of kinetic interest undergo conversion, on a time scale fast relative to the experimental manipulations, to thermodynamically more stable substances then the observed products will not be the actual products of the reaction. In this case the products are said to be under thermodynamic control rather than kinetic control. A possible example has been given in the earlier description of the reaction of hydroxide ion with ester, when it seems likely that the products are the carboxylic acid and the alkoxide ion, which, however, are transformed in accordance with the relative acidities of carboxylic acids and alcohols into the isolated products of carboxylate salt and alcohol. 

Another way to assess thiophene s reactivity is to compare the intermediate ions formed by addition of N02. Examine the structures, charge distributions and electrostatic potential maps of thiophene+nitronium at C2 and thiophene+nitronium at C3. Draw all of the resonance contributors needed to describe these structures. Which, if either, better delocalizes the positive charge Compare the energies of the two intermediates. Which product should form preferentially if the reaction is under kinetic control Are these results consistent with FMO theory ... 

The reactivity of the methyl group in 4-methylcinnoline ethiodide indicates that the structure of this compound is 5, and this evidence has also been interpreted to mean that N-1 is the basic group in cinnolines. However, evidence of this type is only indicative since the formation of quaternary salts is subject to kinetic control, whereas protonation yields predominantly the thermodynamically more stable cation. The quinazoline cation has been shown to exist in the hydrated, resonance-stabilized form 6 7 by ultraviolet spectro-... 

In contrast to the results of the reaction of tertiary and secondary alkyl cations with carbon monoxide (Figs. 1-5), which were obtained under thermodynamically controlled conditions, the results of the carbonylation with the vinyl cations were obtained under kinetically controlled conditions. This presents a difficulty in explaining the occurrence of the 1,2-CH3 shift in the reaction 16->-17, because it involves a strong increase in energy. The exclusive formation of the Z-stereoisomer 18 on carbonylation of the 1,2-dimethylvinyl cation 16 is remarkable, but does not allow an unambiguous conclusion about the detailed structure— linear 19 or bent 20—of the vinyl cation. A non-classical structure 21 can be disregarded, however, because the attack... 

In comparing nucleophiles whose attacking atom is in the same row of the periodic table, nucleophilicity is approximately in order of basicity, though basicity is thermodynamically controlled and nucleophilicity is kinetically controlled. So an approximate order of nucleophilicity is NH2 >RO > OH > R2NH > ArO > NH3 > pyridine > F > H2O > CIO4, and another is R3C > R2N > RO > F (see Table 8.1). This type of correlation works best when the structures of the nucleophiles being compared are similar, as with a set of substituted phenoxides. Within such a series, linear relationships can often be established between nucleophilic rates and pAT values. 

Write the structures of all possible enolates for each ketone. Indicate which you expect to be favored in a kinetically controlled deprotonation. Indicate which you would expect to be the most stable enolate. 

The aldol reaction is also important in the synthesis of more complex molecules and in these cases control of both regiochemistry and stereochemistry is required. In most cases, this is accomplished under conditions of kinetic control. In the sections that follow, we discuss how variations of the basic mechanism and selection of specific reagents and reaction conditions can be used to control product structure and stereochemistry. 

The first element of stereocontrol in aldol addition reactions of ketone enolates is the enolate structure. Most enolates can exist as two stereoisomers. In Section 1.1.2, we discussed the factors that influence enolate composition. The enolate formed from 2,2-dimethyl-3-pentanone under kinetically controlled conditions is the Z-isomer.5 When it reacts with benzaldehyde only the syn aldol is formed.4 The product stereochemistry is correctly predicted if the TS has a conformation with the phenyl substituent in an equatorial position. 

The dialin donor solvents were also used directly in coal liquefaction studies. Inasmuch as details of coal structure are unknown, the present theory can only be tested in a qualitative way, as follows. First, if the liquefaction of coal occurs under kinetic control with hydrogen-transfer from the donor solvent involved in the rate-determining step, then we should expect the dialin donors to be more effective than the control solvent T.et-ralin (and also Dfecalin). This is suggested by the theory because the dialins possess higher energy HOMOs than Tetralin and... 

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