Preliminary reactions

In an indirect analysis the analyte participates in one or more preliminary reactions that produce or consume acid or base. Despite the additional complexity, the stoichiometry between the analyte and the amount of acid or base produced or consumed may be established by applying the conservation principles outlined in Section 2C. Example 9.3 illustrates the application of an indirect analysis in which an acid is produced. 

Vor-reaktion, /. preliminary reaction, -rech-nung, /. calculation coimting up. -recht, n. privilege, -rede, /. preface preamble, -reduktion,/. prereduction previous reduction. -reduktionsansatz, m. (Calico) pre reduction paste. 

Investigation into the effect has been mainly devoted to reactions with red fuming nitric acid . It seems that in red fuming nitric acid a preliminary reaction results in the formation of a surface deposit of finely divided metallic titanium ignition or pyrophoricity can then be initiated by any slight impact or friction. The tendency to pyrophoricity increases as the nitrogen dioxide content of the nitric acid rises from zero to maximum solubility at about 20%, but decreases as the water content rises, the effect being nearly completely stifled at about 2% water. 

Magnitudes of n have been empirically established for those kinetic expressions which have found most extensive application e.g. values of n for diffusion-limited equations are usually between 0.53 and 0.58, for the contracting area and volume relations are 1.08 and 1.04, respectively and for the Avrami—Erofe ev equation [eqn. (6)] are 2.00, 3.00 etc. The most significant problem in the use of this approach is in making an accurate allowance for any error in the measured induction period since variations in t [i.e. (f + f0)] can introduce large influences upon the initial shape of the plot. Care is needed in estimating the time required for the sample to reach reaction temperature, particularly in deceleratory reactions, and in considering the influences of an induction period and/or an initial preliminary reaction. 

As we have shown, the discussion of the product curves, in analogy to classical titration curves, affords information on the number of the intermediate complexes, their relation to each other and their steady-state concentration leading to a preliminary reaction scheme. 

Other nucleophiles than amines which have been employed in the reaction are malononitrile and cyanoacetic ester anions. Both of these anions undergo a preliminary reaction with the aryl halide to form the C-aryl derivatives before they attack the ir-allylpalladium intermediate, so that diarylated products are formed (equation 31).86 Phenylmalononitrile anion reacts with iodobenzene and butadiene to give the same product in 70% yield. 

Quiroga et al. [189] suggested that the preliminary reactions generated the appropriate arylidenecyclodiones, which, in turn, reacted with aminopyrazole, forming the intermediates 223 and 224. The cyclization of the latter with water elimination led to pyridine moiety formation and isolation of compounds 225 and 226 as reaction products (Scheme 3.61). 

A ns. The rate-determining step for all three reactions must be the preliminary reaction of the ketone with OH-, probably leading to the conjugate base of the ketone. The conjugate base subsequently reacts very rapidly with (a) Hr2, (b) some acid in the medium or (c) the solvent with deuterium. 

Alkylation of N-unsubstituted tetrazoles 24 with 0- r/-butyl-iV,iV -dicyclohcxylisourca provides mixtures of isomeric 1- and 2- vt-butyltetrazoles 217 and 218 and seems to be the only practical method for synthesis of 1 -tert-butyltetrazoles. The isourea is prepared by a preliminary reaction of tert-butyl alcohol with /V,/V-dicyclohexylcarbo-diimide in the presence of a catalytic amount of copper(i) chloride (Equation 24) <1976JHC391>. 

Most of the recent synthetic applications of M-RCM involve one of the above catalysts, particularly G1 or G2, chosen as a function of its own reactivity profile, generally after preliminary reaction assays on the genuine substrate or specific model compounds. The sensitivity of the RCM reaction to steric hindrance is well established. These ruthenium catalysts exhibit high affinity for carbon-carbon double bonds and are compatible with the presence of many functional groups, even the presence of free polar hydroxyl or amino groups. Their use does not require special conditions such as glove boxes, which are required when using Schrock s molybdenum catalyst. 

For the cations, a limited number of preliminary reactions should be studied first, followed by an analysis of a mixture or solution containing a member or members of each group. This will give practical experience in the routine operations of semimicro analysis and also provide practice from the very outset in the use of Group Separation Tables. Particular attention should be paid to ... 

Mannich reaction on nylon is carried out according to a method consisting of the preliminary reaction with formaldehyde/methanol followed by amine condensation on the resultant N-methoxymethyl derivative." ... 

It is a palladium-catalyzed cross-coupling reaction between organosilanes (vinyl, ethynyl and allylsilanes) and organic halides (aryl, vinyl and allyl halides). Allylpal-ladium chloride dimmer [( ri -C3H5PdCl)2] and either tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-n-butylammonium fluoride (TBAF) are used as catalysts. Fluoride ion acts as an activator for the coupling, forming an intermediate hypervalent anionic silicon species, which can then transmetallate with palladium as a preliminary reaction to coupling. 

Transferases require the sugar to be transferred as a nucleotide-activated compound, which can be prepared either chemically or enzymatically in preliminary reactions. 

G>) If you were to take more data, where would you place the points Why GalnAs films are important materials in fiber optic communication and in high-speed microelectronic devices. A preliminary reaction between trieth-ylindium and arsine is carried out to form an intermediate, which is then used in the deposition to form GalnAs. The reaction is... 

Principle The test principle includes a preliminary reaction in which salivary amylase is inhibited by the synergistic effect of two monoclonal antibodies. Non-inhibited pancreatic amylase hydrolyses the substrate indolyl-a- >-mal-toheptaoside yielding indolylmaltotetraoside, indolylmaltotrioside and in-dolylmaltoside. The two last derivatives are cleaved by a-glucosidase to release the indoxyl residue. 

Reducing components of the sample are eliminated via oxidation by iodate in the preliminary reaction. 

These direct titrations form the basis of more complicated analytical procedures. Many analytical procedures are indirect and involve additional preliminary reactions of the sample before the titration can be carried out. For example, a soluble calcium salt will not take part in a redox reaction with potassium permanganate. But adding ammonium oxalate to the solution containing Ca causes the quantitative precipitation of calcium oxalate ... 

The most generally satisfactory interpretation of this reaction 14 involves addition of the carbonium ion (V), formed by preliminary reaction of the olefin with sulfuric acid, to a molecule of unchanged olefin. 

The equipment and techniques used in conducting the preliminary reactions are, as a rule, sufficiently simple, and some of the techniques are considered below. 

Another important and undeveloped promising area in this field is the use of preliminary reactions together with concentration for the analysis of impurities. Developments in this area will make it possible to increase the selectivity and reduce the detection limit for toxic compounds [474,475]. 

Preliminary reactions are widely used in GC for protecting functional groups [17,18] in order to obtain stable, more volatile and less polar derivatives. Owing to these reactions, the range of compounds that can be analysed becomes fairly large. At the same time, preliminary reactions can be considered as methods of functional group analysis. 

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