Nature of the reagents

2 FACTORS THAT INFLUENCE THE VELOCITIES OF REACTIONS 2.2.1 Nature of the reagents 

The velocities of elementary chemical reactions depend on a great number of factors, in particular the nature of the reactants, concentrations or pressures, temperature, light, catalysts and the solvent used. The great variation observed in reaction velocities will be related first to the nature of the reagents. Many reactions, such as those between oppositely 

However, reactions involving structurally similar reactant molecules such as exchange reactions of electrons between two isotopically labelled transition metal complexes, which also do not involve breaking chemical bonds often show great differences in rates under similar conditions 

Reaction (2.IV) is 10 times faster than reaction (2.V). In contrast, although reactions involving bond breaking and bond formation are generally slow, there are some extremely fast reactions of this type such as the oxidation of iron (II) by permanganate ion in acid solution  

In this table, we stress the fact that if we keep a series of the parameters constant, it is possible to attribute the change in activation energy, and consequently, the velocity of reaction (eq (1.1)), to the change in a specific parameter. For example, in the reactions (i) and 

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Oxidations usually proceed in the dark at or below room temperature in a variety of solvents ranging from aqueous bicarbonate to anhydrous benzene-pyridine. Base is quite commonly used to consume the hydrogen halide produced in the reaction, as this prevents the formation of high concentrations of bromine (or chlorine) by a secondary process. The reaction time varies from a few minutes to 24 hours or more depending on the nature of the reagent and the substrate. Thus one finds that NBS or NBA when used in aqueous acetone or dioxane are very mild, selective reagents. The rate of these oxidations is noticeably enhanced when Fbutyl alcohol is used as a solvent. In general, saturated, primary alcohols are inert and methanol is often used as a solvent. 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

The close similarity in the isomer ratios obtained from different sources of the phenyl radical suggests that the mechanism of aryla-tion is independent of the nature of the reagent which generates the radical. This principle has been used in reverse in that the constancy of isomer ratios has been cited as evidence that the decomposition of lead tetrabenzoate gives free phenyl radicals. 

Equation 1.7). A concerted synchronous transition state [15] (the formation of new bonds occurs simultaneously) and a concerted asynchronous transition state [16] (the formation of one reaction depends on the nature of the reagents and the experimental conditions [17]. 

Oximes can be alkylated by alkyl halides or sulfates. N-Alkylation is a side reaction, yielding a nitrone. " The relative yield of oxime ether and nitrone depends on the nature of the reagents, including the configuration of the oxime, and on the reaction conditions. For example, anri-benzaldoximes give nitrones, while the syn isomers give oxime ethers. " ... 

The nature of the reagent in 1,3-dipolar cycloaddition is such that almost all such reagents are not symmetric. This fact obviates the use of the mechanistic test described above for the Diels-Alder reaction. 

In order to predict the products of a reaction, the first step is determining the identity and nature of the reagent. That is, you must analyze the reagent and determine the category to which it belongs. Let s get some practice with this critical skill. 

Development of an industrial monitoring application for IMS requires extensive preparatory work, as well as optimal operational conditions for IMS, i.e. the nature of the reagent gas, calibration curves, evaluation of interferents, assessment of reliability. Analysis of mixtures with four or fewer components may be possible, but extension to more complex mixtures should be considered only in special cases, and generally would be unrealistic. Use of preseparators, such as GC columns, is the only known technical approach... 

The hydride-methyl complex OsH(Me)(CO)2(P Pr3)2 reacts with electrophilic reagents. The reaction products depend on the nature of the reagent (Scheme 39). Whereas the reaction with iodine gives almost quantitatively the diiodide OsI2(CO)2(P,Pr3)2, the reaction with a five-fold excess of phenylacetylene does not lead to the formation of the previously mentioned bis-alkynyl complex... 

Many of the reactions of the weak carbon acids are reactions of the carbanion, the rate being the rate of ionization and independent of the concentration or nature of the reagent that determines what the product will be. 

Cyclic Nitronates The chemistry of cyclic nitronates substantially differs from the chemistry of their acyclic analogs. Cyclic nitronates are involved predominantly in various rearrangements rather than in elimination reactions. The character and pathways of these rearrangements are determined not only by the nature of the reagent used but also by the character of the heterocycle and the nature of the substituents attached to the heterocycle. 

Cleavage of cyclobutane rings can occur easily. As indicated in the introductory section, the strain of the four-membered ring, the substituents on the ring, the nature of the reagents as well as the conditions of reaction are all responsible for the ease of cleavage of cyclobutanes 3). The substituents on the ring constitute one of the... 

The nature of the reagent eventually providing the central carbon atom of the allene unit, the ylid and the ketene intermediate have been varied considerably over the years, as shown by the following selection of examples. 

The behaviour of other aromatic nitrocompounds (e.g. m-nitroanisole, 3,5-dimethoxynitrobenzene and 4-nitrobiphenyl) follows the same pattern the same short-lived absorption is produced upon exciting an aromatic compound in the presence of a variety of nucleophilic agents, whereas the lifetime of the species formed depends on the nature of the reagent. 

Most addition reactions actually involve both steps, but the order in which these occur depends on the nature of the reagent and the reaction conditions. Under basic conditions, the nucleophile attacks... 

The position of equilibrium, i.e. whether the carbonyl compound or the addition product is favoured, depends on the nature of the reagents. The equilibrium constant is often less than 1, so that the product is not favoured, and many simple hemiacetals and hemiketals are not sufficiently stable to be isolated. However, stable cyclic hemiacetals and hemiketals... 

The acidic nature of the reagent is important the trifluoroacetic acid liberated in the reaction catalyzes hydrolysis of the intermediate isocyanate, and also ensures that the amine which is formed is protonated and cannot react with the isocyanate to give urea by-products. The reaction can be accelerated by addition of pyridine to an observed pH of about 3.5, and is retarded by added acid or trifluoroacetate ion. In the present procedure pyridine was not employed, since the reaction in its absence proceeds with a satisfactory rate. 

Reduction can lead to dihydro-, tetrahydro- and hexahydropy-ridines (piperidines), depending upon the nature of the reagents and the reaction conditions. 

A straightforward extension of DKR to polymer chemistry is the use of diols and diesters (AA-BB monomers) or ester-alcohols (AB monomers) as substrates (Scheme 10, routes A and B). Such reactions have been referred to as DKR polymerizations and lead to the formation of oligomers and/or polymers because of the bifunctional nature of the reagents. 

Reagent Reactivity. One of the most interesting aspects of substitution reactions of square planar complexes is that the reaction rates depend on the nature of the reagent. This permits a thorough investigation of the factors responsible for reagent reactivity towards these substrates. Note that this has not been possible for the reactions of most six-coordinated metal complexes, since their rates do not depend on the reagent. 

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