Kinetic condition

When the aldol reaction is carried Wt under thermodynamic conditions, the product selectivity is often not as high as under kinetic conditions. All the regioisomeric and stereoisomeric enolates may participate as nucleophiles. The adducts can return to reactants, and so the difference in stability of the stereoisomeric anti and syn products will determine the product composition. [Pg.469]

The dehydration reactions have somewhat higher activation energies than the addition step and are not usually observed under strictly controlled kinetic conditions. Detailed kinetic studies have provided rate and equilibrium constants for the individual steps in some cases. The results for the acetone-benzaldehyde system in the presence of hydroxide ion are given below. Note that is sufficiently large to drive the first equilibrium forward. [Pg.470]

Further bromination of the 4 -bromo compound, under thermodynamic or kinetic conditions, yields the 2, 4)S-dibromo compound (23) which apparently contains a little of the 2,2-dibromo isomer. Again, the 2,4-dibromo compound is best obtained directly from the parent ketone with two moles of bromine in acid. " With suitable methods of dehydrobromination (see... [Pg.277]

In this review we put less emphasis on the physics and chemistry of surface processes, for which we refer the reader to recent reviews of adsorption-desorption kinetics which are contained in two books [2,3] with chapters by the present authors where further references to earher work can be found. These articles also discuss relevant experimental techniques employed in the study of surface kinetics and appropriate methods of data analysis. Here we give details of how to set up models under basically two different kinetic conditions, namely (/) when the adsorbate remains in quasi-equihbrium during the relevant processes, in which case nonequilibrium thermodynamics provides the needed framework, and (n) when surface nonequilibrium effects become important and nonequilibrium statistical mechanics becomes the appropriate vehicle. For both approaches we will restrict ourselves to systems for which appropriate lattice gas models can be set up. Further associated theoretical reviews are by Lombardo and Bell [4] with emphasis on Monte Carlo simulations, by Brivio and Grimley [5] on dynamics, and by Persson [6] on the lattice gas model. [Pg.440]

It may happen that AH is not available for the buffer substance used in the kinetic studies moreover the thermodynamic quantity A//° is not precisely the correct quantity to use in Eq. (6-37) because it does not apply to the experimental solvent composition. Then the experimentalist can determine AH. The most direct method is to measure AH calorimetrically however, few laboratories Eire equipped for this measurement. An alternative approach is to measure K, under the kinetic conditions of temperature and solvent this can be done potentiometrically or by potentiometry combined with spectrophotometry. Then, from the slope of the plot of log K a against l/T, AH is calculated. Although this value is not thermodynamically defined (since it is based on the assumption that AH is temperature independent), it will be valid for the present purpose over the temperature range studied. [Pg.258]

It is worth considering each of these kinetic conditions in detail, as these are behaviors that are all observed experimentally and can be observed for the same antagonist under different experimental conditions. A summary of these various kinetic conditions is shown schematically in Figure 6.4. [Pg.102]

When 2-azaallylphosphonates are used in the addition reaction with 2-propcnoatcs only the cycloadducts were formed. Under kinetic conditions the more strained all-ri.s diastereomers are obtained in 95% yield 337. [Pg.996]

The products of the dimerization of l,6-dihydroxy-2-hexanone118 are the simplest models for the dihexulose dianhydrides. Under thermodynamic conditions, the isomer (20) was the sole product. Under kinetic conditions, compounds 20 and 21 are formed as a 1 1 mixture. Whereas 20 has the conformation illus-... [Pg.228]

To do this we developed a computer model to predict the kinetic conditions during the runaway stage. The kinetic model is used to estimate the reaction rates, temperatures, pressures, viscosities, conversions, and other variables which influence reactor design. [Pg.339]

Several of the claimed crystalline modifications of boron are not truly boron modifications but consist of phase mixtures or B-rich compounds formed under favorable kinetic conditions. The existence of the a- and /3-rhombohedral (a-rh and... [Pg.248]

Norbomenone 26 undergoes reduction by sodium borohydride under kinetic conditions to produce 5% exo- (i.e., endo attack) and 95% endo- (i.e., exo attack) 2-norbomeol. This leads to the partial rate constants of 11.4 for exo and 0.6 for endo attack (relative rate with respect to the rate of LiAlH reduction of cyclopen-tanone (1.00)) [80]. In the saturated 2-norbomanone 25, the values are 4.55 for exo and 0.74 for endo attack. Thus, the introduction of the double bond enhances the... [Pg.141]

Preparative scale synthesis of (3-D-glucopyranoside using primary alcohols (under kinetic condition)... [Pg.257]

In Chapter 3 we will provide experimental verification of expression obtained in this Section linking the concentration of active particles in ambient volume with the change in electric conductivity of adsorbent under stationary and kinetic conditions as well as experimental prove of validity assumptions made while deriving above expressions. [Pg.163]

Of all existing methods to monitor electrical properties while using semiconductor sensors, only two [5] have become widely implemented both in experimental practice and in industrial conditions. These are kinetic method, i.e. measurement of various electrical parameters under kinetic conditions, and stationary (equilibrium) method based on the measurement of steady-state parameters (conductivity, work function. Hall s electromotive force, etc.). [Pg.173]

Among Michael acceptors that have been shown to react with ketone and ester enolates under kinetic conditions are methyl a-trimethylsilylvinyl ketone,295 methyl a-methylthioacrylate,296 methyl methylthiovinyl sulfoxide,297 and ethyl a-cyanoacrylate.298 Each of these acceptors benefits from a second anion-stabilizing substituent. The latter class of acceptors has been found to be capable of generating contiguous quaternary carbon centers. [Pg.186]

Under kinetic conditions, iodolactonization reflects reactant conformation. Several cases illustrate how the stereoselectivity of iodolactonization can be related to reactant conformation. For example, the high stereoselectivity of 1 corresponds to proximity of the carboxylate group to one of the two double bonds in the preferred reactant... [Pg.313]

INTERMOLECULAR APPLICATIONS OF 0-QUINONE METHIDES (o-QMs) ANIONICALLY GENERATED AT LOW TEMPERATURES KINETIC CONDITIONS... [Pg.89]

Under field ionization kinetic conditions (FIK) the process of CH3 elimination from ionized 27 is not observed at t < 10-10 sec. This is a surprising result bearing in... [Pg.9]

Compounds will precipitate from a DMSO stock solution if the kinetic conditions are appropriate for example, if enough time passes or enough freeze-thaw cycles are encountered as part of the compound in DMSO storage process. Opera-... [Pg.219]

This temperature (4 °C) was intended to analyse the transformations in unfavourable kinetic conditions. It is also an adverse temperature to biologic... [Pg.380]

With time, schwertmannite progressed to a more crystalline mineral (low crystalline goethite), even under unfavourable kinetic conditions (T<4°C) ... [Pg.382]

Other anions such as nitronate anion,96 or the conjugate base of (3-ketoesters,97 give similar results. Usually, the 1,2-cw-C-glycosyl compound is formed under kinetic conditions. Thus, in the case of the reaction of the GlcNAc derivative 75 with the Wittig reagent Ph3P=CHCOOEt, the a-C-pyranosyl compound 77a is the major product under conditions of kinetic control. Under basic conditions, 77a is slowly epimerized into the more stable (3-C-pyranosyl derivative 77p (Scheme 25). [Pg.49]

The endo-spiro-OZT could be prepared through a reaction sequence similar to that applied for the exo-epimer, with spiro-aziridine intermediates replacing the key spiro-epoxides (Scheme 18). Cyanohydrin formation from ketones was tried under kinetic or thermodynamic conditions, and only reaction with the d-gluco derived keto sugar offered efficient stereoselectivity, while no selectivity was observed for reaction with the keto sugar obtained from protected D-fructose. The (R) -cyanohydrin was prepared in excellent yield under kinetic conditions (KCN, NaHC03, 0 °C, 10 min) a modified thermodynamic procedure was applied to produce the (S)-epimer in 85% yield (Scheme 18). [Pg.136]

Anodic oxide films formed under different kinetic conditions vary in structure, composition and property (e.g., etch rate) and they change with time during the anodization. [Pg.184]

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