Preparative experiments

Evidence from the viscosities, densities, refractive indices and measurements of the vapour pressure of these mixtures also supports the above conclusions. Acetyl nitrate has been prepared from a mixture of acetic anhydride and dinitrogen pentoxide, and characterised, showing that the equilibria discussed do lead to the formation of that compound. The initial reaction between nitric acid and acetic anhydride is rapid at room temperature nitric acid (0-05 mol 1 ) is reported to be converted into acetyl nitrate with a half-life of about i minute. This observation is consistent with the results of some preparative experiments, in which it was found that nitric acid could be precipitated quantitatively with urea from solutions of it in acetic anhydride at —10 °C, whereas similar solutions prepared at room temperature and cooled rapidly to — 10 °C yielded only a part of their nitric acid ( 5.3.2). The following equilibrium has been investigated in detail ... 

In addition to the initial reaction between nitric acid and acetic anhydride, subsequent changes lead to the quantitative formation of tetranitromethane in an equimolar mixture of nitric acid and acetic anhydride this reaction was half completed in 1-2 days. An investigation of the kinetics of this reaction showed it to have an induction period of 2-3 h for the solutions examined ([acetyl nitrate] = 0-7 mol 1 ), after which the rate adopted a form approximately of the first order with a half-life of about a day, close to that observed in the preparative experiment mentioned. In confirmation of this, recent workers have found the half-life of a solution at 25 °C of 0-05 mol 1 of nitric acid to be about 2 days. ... 

It was shown that in preparative experiments sulphuric acid markedly catalysed, and acetate ions markedly anticatalysed the nitration of anisole. ... 

Little is known quantitatively about substituent effects in the nitration of derivatives of azanaphthalenes. In preparative experiments 4-hydroxy-quinoline, -cinnoline, and -quinazoline give the 6- and 8-nitro compounds, but with nitric acid alone 4-hydroxyquinoline and 2,4-di-hydroxyquinoline react at With nitric acid, 4-hydroxycinnoline... 

The preparation of (Z)-2-hutenylpotassium from (Z)-2-butene is analogous to that described for the /T-reagent with the following modification upon completion of the butyllithium addition, the mixture is warmed to —20° to —25 r C for 30-45 min before being recooled to —78 C. This ensures near quantitative formation of (Z)-2-butenylpotassium. Temperature control is less critical since (Z)-2-butenylpotassium is highly favored at equilibrium (99 l)15. However, preparative experiments have not been performed in which ( )-2-butene is metalated under conditions that permit complete isomerization of ( )-2-butenyl-potassium to (Z)-2-butenylpotassium. 

Literally hundreds of aldehydes have so far been tested successfully by enzymatic assay and preparative experiments as a replacement for (18) in rabbit muscle FruA catalyzed aldol additions [16,25], and most of the corresponding aldol products have been isolated and characterized. The rabbit FruA can discriminate racemic dl-(18), its natural substrate, with high preference for the D-antipode, but kinetic enantioselec-tivity for nonionic aldehydes is rather low [84,89]. 

The ionic radii at, a were calculated from bond lengths and covalent radii [6, 7, 8], It should be noted that for a cation, such as CH3 - C = O, the C=0 and C-C bonds are shorter than those of the isoelectronic neutral species Observed solubilities are derived from as yet unpublished preparative experiments f Prom reference [9] ... 

Table I records the results obtained in the preparation of 30 cycloaddition products from the acridizinium cation. As was demonstrated by Fields, Regan, and Dignan, even preparative experiments done at different temperatures and in different solvents are adequate to prove the inverse electron demand character of the reaction. Nucleophilic alkenes, like ketene diethyl acetal, reacted in minutes at room temperature while the strongly electrophilic alkene, tetracyanoethylene, failed to react under any conditions.

Some aryl iodides are known to generate the diaryImercury at a mercury cathode. In the case of 4-iodoaniso e, reduction at more negative potentials in dimeth-ylfonnamide leads to the formation of less di(4-methoxyphenyl)mercury. At glassy carbon, anisole is the only reduction product. 4-Bromoanisole gives only anisole at either mercury or carbon [143]. Mercur> has been used as cathode material for many preparative experiments with aryl halides but glassy carbon and also stainless steel are very satisfactory alternatives. 

Scheme 6.10 Range of representative acetals prepared from the 9-catalyzed acid-free acetalization of various aldehydes and ketones. The yields refer to preparative experiments (20mmol scale).

Calculation of S for the kinetic resolution of the cyclohexenyl carbonate rac-laa with 2-pyrimidinethiol and lithium t-butylsulfinate according to Eq. (1) gave large values for pairs of ee versus c (compare Tables 2.1.4.5 and 2.1.4.6). This is in accordance with the isolation of ent-laa with a high ee at approximately 50% conversion in the preparative experiments (compare Tables 2.1.4.1 and 2.1.4.4). However, Tables 2.1.4.5 and 2.1.4.6 also reveal major and irregular changes of S with conversion. Since our measurements of ee and c had a precision of only 0.5% and 1.0%, respectively, we ascribe the change of S with conversion mainly to errors in the determination of both values. 

Our standard procedure in preparative experiments of isotactic polymer is that used in earlier work, where the temperature during initiation was carefully controlled. We initiated the polymerization by mixing initiator solution and monomer solution already thermostatted to a predetermined temperature, usually that chosen for the polymerization. At 230 with t-BuMgBr in THF-toluene mixtures this led to a trimodal distribution with the high molar-mass peak in the 106 range (l.,2). More recent work, shown in Figure 2, has confirmed that trimodal distributions arise when the mol fraction of THF is below ca 0.3. 

Copolymerization has been used for evaluating the reactivity of the monomeric, anhydro sugar derivatives, and also to prepare stereoregular polysaccharides of structures more complex than those of those prepared from a single monomer.98-104,107 The procedure adopted has been first to determine the reactivity ratios of the monomers, and then to perform preparative experiments under conditions that provide polysaccharides having the desired, copolymer composition. 

There are two reasons for considering the mechanisms in this detail. Firstly, the potential at which preparative experiments are carried out is determined by which is the basic species. Secondly, methods for determining the rate of protonation (k ) depend on a knowledge of the mechanism. The rate of protonation of an EGB by an acid of known pK is the most convenient measure of basicity (kinetic basicity). 

Catalyst stability. Under the conditions used for preparative experiments, the optical yield remains constant up to complete conversion, suggesting that the modified catalyst is rather stable [62]. However, experiments at low modifier concentration indicate that the cinchona alkaloid deteriorates slowly and its enantioselective effect is lost [33,63]. 

Representatives of all kinds have been explored for synthetic applications while mechanistic investigations were mainly focussed on the distinct FruA enzymes isolated from rabbit muscle [196] and yeast [197,198]. For mechanistic reasons, all DHAP aldolases appear to be highly specific for the donor component DHAP [199], and only a few isosteric replacements of the ester oxygen for sulfur (46), nitrogen (47), or methylene carbon (48) were found to be tolerable in preparative experiments (Fig. 7) [200,201], Earlier assay results [202] that had indicated activity also for a racemic methyl-branched DHAP analog 53 are now considered to be artefactual [203]. Dihydroxyacetone sulfate 50 has been shown to be covalently bound via Schiff base formation, but apparently no a-deprotonation occurred as neither H/D-exchange nor C-C... 

Besides preparative experiments, kinetic studies have also been made on such systems 78-81>,... 

Unfortunately, preparative experiments of Iwaoka and Kondo (35) are of no direct relation to mechanistic investigations the use of a low pressure mercury lamp provides no selectivity as far as excitation of substrate or products is concerned. However, the fact that photolysis in strong acid solution decreased the bleaching rate would indicate the absence of an anchimeric effect and the results of their investigations by flash photolysis are in agreement with the electron (Equation 2) and energy transfer (Equation 4) reactions upon direct excitation. 

In most preparative experiments under high pressure, the procedure is as follows pressure is applied at room temperature (rt) to a sample tube containing the reagents and, if necessary, catalysts and solvent, before the temperature is raised, if required. After a suitable time, the heater is switched off. After cooling to rt, the pressure is carefully released, and the sample tube is removed from the vessel. When the reaction at high pressure does not take place at ambient temperature, according to GC, TLC, NMR, or other analytical techniques, an increase of pressure and/or temperature might be effective. In certain cases, the use of a catalyst may lead to success. 

For further understanding the performance of the SCISR by comparison, the preparation experiments are also carried out simultaneously in a stirred tank reactor (STR) with an effective volume of 0.6x10 m3, the structure of which is indicated in Fig. 13.1. In order to mimic industrial conditions, the STR is equipped with three dampers distributed uniformly along the circle the stirrer is a flat paddle. 

Little is known quantitatively about substituent effects in the nitration of derivatives of azanaphthalenes. In preparative experiments 4-hydroxy-quinoline, -cinnoline, and -quinazoline give the 6- and 8-nitro compounds, but with nitric acid alone 4-hydroxyquinoline and 2,4-di-hydroxyquinoline react at C(3).31 With nitric acid, 4-hydroxycinnoline still gives mainly 4-hydroxy-6-nitrocinnoline, but some of the 3-nitro compound can also be isolated.81 51 The change of orientation with reagent could be due to a change to free-base nitration in the more weakly acidic medium, or to the occurrence in nitric acid of nitration via nitrosation.31... 

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