The rates of reactions

The acid, if monobasic, can usually be distilled directly from the reaction mixture. If this procedure is not possible, the reaction mixture is poured into excess of crushed ice, and the acid is isolated by ether extraction or by other suitable means. The acid is then characterised (Section 111,85). The addition of hydrochloric acid (as sodium chloride say 5 per cent, of the weight of sulphuric acid) increases the rate of the reaction. 

Diels-Alder reactions can be divided into normal electron demand and inverse electron demand additions. This distinction is based on the way the rate of the reaction responds to the introduction of electron withdrawing and electron donating substituents. Normal electron demand Diels-Alder reactions are promoted by electron donating substituents on the diene and electron withdrawii substituents on the dienophile. In contrast, inverse electron demand reactions are accelerated by electron withdrawing substituents on the diene and electron donating ones on the dienophile. There also exists an intermediate class, the neutral Diels-Alder reaction, that is accelerated by both electron withdrawing and donating substituents. 

The way the substituents affect the rate of the reaction can be rationalised with the aid of the Frontier Molecular Orbital (FMO) theory. This theory was developed during a study of the role of orbital symmetry in pericyclic reactions by Woodward and Hoffinann and, independently, by Fukui Later, Houk contributed significantly to the understanding of the reactivity and selectivity of these processes. ... 

The solvents listed in Table 2.1 were chosen to cover a broad range in solvent properties. In fact hexane was initially also among them, but unfortunately the rate of the reaction in this solvent is extremely low. It turned out that in this solvent spontaneous decomposition of 2.4a competes with the Diels-Alder reaction. 

The rate of the uncatalysed reaction in all four solvents is rather slow. (The half-life at [2.5] = 1.00 mM is at least 28 hours). However, upon complexation of Cu ion to 2.4a-g the rate of the Diels-Alder reaction between these compounds and 2.5 increases dramatically. Figure 2.2 shows the apparent rate of the Diels-Alder reaction of 2.4a with 2.5 in water as a lunction of the concentration of copper(II)nitrate. At higher catalyst concentrations the rate of the reaction clearly levels off, most likely due to complete binding of the dienophile to the catalyst. Note that in the kinetic experiments... 

The effect of substituents on the rate of the reaction catalysed by different metal ions has also been studied Correlation with resulted in perfectly linear Hammett plots. Now the p-values for the four Lewis-acids are of comparable magnitude and do not follow the Irving-Williams order. Note tlrat the substituents have opposing effects on complexation, which is favoured by electron donating substituents, and reactivity, which is increased by electron withdrawirg substituents. The effect on the reactivity is clearly more pronounced than the effect on the complexation equilibrium. 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

As anticipated from the complexation experiments, reaction of 4.42 with cyclopentadiene in the presence of copper(II)nitrate or ytterbium triflate was extremely slow and comparable to the rate of the reaction in the absence of Lewis-acid catalyst. Apparently, Lewis-acid catalysis of Diels-Alder reactions of p-amino ketone dienophiles is not practicable. 

The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentration of the reacting species in or at the micelle. Also for higher-order reactions the pseudophase model is usually adopted (Figure 5.2). However, in these systems the dependence of the rate on the concentration of surfactant does not allow direct estimation of all of the rate constants and partition coefficients involved. Generally independent assessment of at least one of the partition coefficients is required before the other relevant parameters can be accessed. 

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

The rate of the reaction decreases with increasing number of substituents in the acetylenic halide, and it is higher with acetylenic bromides than with the corresponding chlorides. Methyl magnesium iodide gives equal amounts of 1,1- and 1,3--substitution products, whereas tert.-butylmagnesium bromide does not react. However, for some tert.-butyl substituted allenes there exists an attractive com-... 

Because the carbon-halogen bond breaks m the slow step the rate of the reaction depends on the leaving group Alkyl iodides have the weakest carbon-halogen bond and are the most reactive alkyl fluorides have the strongest carbon-halogen bond and are the least reactive... 

The addition of hydrogen cyanide is catalyzed by cyanide ion but HCN is too weak an acid to provide enough C=N for the reaction to proceed at a reasonable rate Cyanohydrins are therefore normally prepared by adding an acid to a solution containing the carbonyl compound and sodium or potassium cyanide This procedure ensures that free cyanide ion is always present m amounts sufficient to increase the rate of the reaction... 

As carried out the rate of the reaction is pseudo-first-order in picrate and... 

If measurements are made early in the reaction, the product s concentration is negligible, and the step described by the rate constant k 2 can be ignored. Under these conditions the rate of the reaction is... 

The reaction is now first-order in substrate, and the rate of the reaction can be used to determine the substrate s concentration by a fixed-time method. 

Deming and Pardue studied the kinetics for the hydrolysis of p-nitrophenyl phosphate by the enzyme alkaline phosphatase. The progress of the reaction was monitored by measuring the absorbance due to p-nitrophenol, which is one of the products of the reaction. A plot of the rate of the reaction (with units of pmol mL s ) versus the volume, V, (in milliliters) of a serum calibration standard containing the enzyme yielded a straight line with the following equation... 

A variation on the use of pseudo-ordered reactions is the initial rate method. In this approach to determining a reaction s rate law, a series of experiments is conducted in which the concentration of those species expected to affect the reaction s rate are changed one at a time. The initial rate of the reaction is determined for each set of conditions. Comparing the reaction s initial rate for two experiments in which the concentration of only a single species has been changed allows the reaction order for that species to be determined. The application of this method is outlined in the following example. 

The order of the rate law with respect to the three reactants can be determined by comparing the rates of two experiments in which the concentration of only one of the reactants is changed. For example, in experiment 2 the [H+] and the rate are approximately twice as large as in experiment 1, indicating that the reaction is first-order in [H+]. Working in the same manner, experiments 6 and 7 show that the reaction is also first-order with respect to [CaHeO], and experiments 6 and 8 show that the rate of the reaction is independent of the [I2]. Thus, the rate law is... 

The foregoing conclusion does not mean that the rate of the reaction proceeds through Table 5.1 at a constant value. The rate of reaction depends on the concentrations of reactive groups, as well as on the reactivities of the latter. Accordingly, the rate of the reaction decreases as the extent of reaction progresses. When the rate law for the reaction is extracted from proper kinetic experiments, specific reactions are found to be characterized by fixed rate constants over a range of n values. 

The rate of the reaction in such case is R = k [PH ], where P is the reactant (ie, a repeat unit bearing the acid-labile protecting group). 

Iodine dissolves without reaction in concentrated sulfuric acid and with concentrated nitric acid it reacts to form iodine pentoxide (47). Iodine reacts with alkah metal hydroxide solutions to form the corresponding hypoiodite and the rate of the reaction increases with the alkaU concentration and temperature. At 50°C, the reaction is almost instantaneous ... 

Particle Activity. Particle activity determines the type and rate of the reaction of a powder particle with its environment. 

Specific Surface. The total surface area of 1 g of powder measured ia cm /g is called its specific surface. The specific surface area is an excellent iadicator for the conditions under which a reaction is initiated and also for the rate of the reaction. It correlates in general with the average particle size. The great difference in surface area between 6-p.m reduced iron powder and 7-p.m carbonyl iron powder (Table 3) cannot be explained in terms of particle size, but mainly by the difference between the very inregular-shaped reduced and the spherical carbonyl iron powders. 

Equation 4 can be classified as S, , ie, substitution nucleophilic bimolecular (221). The rate of the reaction is influenced by several parameters basicity of the amine, steric effects, reactivity of the alkylating agent, and solvent polarity. The reaction is often carried out in a polar solvent, eg, isopropanol, which may increase the rate of reaction and make handling of the product easier. 

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