Bimolecular Reactions between Intermediates

If we also consider the role of adsorption on the distribution of products, then we should note that surface concentrations of substrate and intermediate(s) must be taken into account, i.e. their concentrations in the inner Helmholtz layer (cf. for example, Wendt, 1973). One effect of this would possibly be to 

The very success of electrochemical reactions that give products resulting from coupling between two fragments originating from two molecules is an indication that locally high concentrations of intermediates must play an important role. Owing to the difficulty of 

Stabilization of Kolbe-generated radicals by —M substituents has the effect of decreasing the difference in concentration dependence between the electrochemical reaction and another Kolbe-duplicating 

Organic Products0 Formed by Decomposition of Propionyl Peroxide and by Kolbe Electrolysis of Potassium Propionate in Propionic Acid  

Product Composition of Composition of product from the anode products from decomposition of electrolysis of 2 moles of 1 mole of peroxide potassium propionate  

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Stoichiometry (28) is followed under neutral or in alkaline aqueous conditions and (29) in concentrated mineral acids. In acid solution reaction (28) is powerfully inhibited and in the absence of general acids or bases the rate of hydrolysis is a function of pH. At pH >5.0 the reaction is first-order in OH but below this value there is a region where the rate of hydrolysis is largely independent of pH followed by a region where the rate falls as [H30+] increases. The kinetic data at various temperatures both with pure water and buffer solutions, the solvent isotope effect and the rate increase of the 4-chloro derivative ( 2-fold) are compatible with the interpretation of the hydrolysis in terms of two mechanisms. These are a dominant bimolecular reaction between hydroxide ion and acyl cyanide at pH >5.0 and a dominant water reaction at lower pH, the latter susceptible to general base catalysis and inhibition by acids. The data at pH <5.0 can be rationalised by a carbonyl addition intermediate and are compatible with a two-step, but not one-step, cyclic mechanism for hydration. Benzoyl cyanide is more reactive towards water than benzoyl fluoride, but less reactive than benzoyl chloride and anhydride, an unexpected result since HCN has a smaller dissociation constant than HF or RC02H. There are no grounds, however, to suspect that an ionisation mechanism is involved. 

The mechanism of the reaction was shown by Eaborn and Taylor to be (60JCS3301) an acid-catalyzed version of the SE2 mechanism (A-SE2), which applies to most electrophilic substitutions (Scheme 2.1). This involves a bimolecular reaction between an acid (HA) and the aromatic to give a Wheland intermediate, which then loses a hydrogen ion to give... 

Recall that Van Leeuwen et al. (24) had shown that only the most efficient radical scavengers would intercept the photogenerated CH3 radicals. A more probable mechanism involves bimolecular reaction between the transition metal complex and a photogenerated Ti species such as 15 in Scheme 2. This could lead to CHs/Br exchange via a bridged intermediate such as 25. Yields of these reactions were not reported and thus their... 

The similarity of behavior between the meneidic dihydrodiazepines and their salts and activated benzene derivatives, such as anilines and phenols, extends to reaction mechanisms, as indicated by kinetics. In aqueous solution bromination is a bimolecular reaction between dihydrodiazepinium cations and bromine molecules.27,41 The leaving bromide ion is still present in the transition state and, as in many (but not all) brominations of activated benzene derivatives, the rate-determining step is the initial attack which leads to formation of the intermediate v-complex (13). Steric factors play a large part in the rates of bromination of dihydrodiazepinium salts in methanol, reaction being slower when large substituents are present at the adjacent 5- and 7-positions.47a Substituents further removed from the site of bromination may also affect the rate.47 ... 

Photodecomposition studies indicate that the initial step in the decomposition reaction is the absorption of light at 254 nm, which creates excited states of the azide ion whose lifetime is probably impurity controlled. Intensity data suggest that the decomposition reaction occurs by a bimolecular reaction between (N3) states, although details concerning energy transport and the nature of the decomposition site (lattice distortions, impurity centers, crystal surfaces, etc.) remain unanswered. Conjectures concerning probable reaction paths have been based on information about the intermediate products, identified by the optical and ESR measurements previously discussed. 

The feasibility of the rate-limiting formation of a jc-radical cation intermediate, II, can be assessed by examining the linear free-energy correlation between the second-order rate constants ki) for the bimolecular reaction of alkenes with the relatively stable Cr(V)-oxo tetraarylporphyrins and the potentials for the le" oxidation (AE1/2) of the alkenes. The linear correlation between log k2 and A i/2 for the reaction of 16 alkenes with (BrgTPP)Cr (0)(X) is shown in eq 19 and Figure 4. Also, the bimolecular reactions between five variously... 

In the bimolecular cydization process, the ring-closure process involves a one-pot two-step reaction (1) the bimolecular reaction between one of the polymer ends and one function of the difunctional coupling agent to form an a,co-heterodifunctional chain and (2) the unimolecular ring closure corresponding to the readion between the a- and co-polymer ends. Indeed, the Mgh dilution that is required to favor cydization versus chain extension is not favorable to the quantitative formation of the heterodifunctional polymer intermediate (see Scheme 5). To overcome this difficulty, another approach involves the dired synthesis of an... 

For reasons that will become apparent later in this chapter (Section 5.26), reactions of this type never take place by a single concerted process. The groups X and Y add in two separate steps, either as followed by Y (electrophilic addition) or as X" followed by Y" (nucleophilic addition) or as X- followed by Y (radical addition). Here we will consider electrophilic additions in which the rate-determining step is a bimolecular reaction between the olefin and XY to form an intermediate cation and Y ,... 

According to this mechanism, the reaction intermediate is the fluorine atom. It should be expected that the reactions with reactive atoms, as is the fluorine atom, are fast. It is therefore probable that the bimolecular reaction between NO and F2 is the slowest step, the bottleneck of the reaction, and therefore rate-determining. Thus, the overall reaction rate should be given by the expression ... 

A careful distinction must be drawn between transition states and intermediates. As noted in Chapter 4, an intermediate occupies a potential energy minimum along the reaction coordinate. Additional activation, whether by an intramolecular process (distortion, rearrangement, dissociation) or by a bimolecular reaction with another component, is needed to enable the intermediate to react further it may then return to the starting materials or advance to product. One can divert an intermediate from its normal course by the addition of another reagent. This substance, referred to as a trap or scavenger, can be added prior to the start of the reaction or (if the lifetime allows) once the first-formed intermediate has built up. Such experiments are the trapping experiments referred to in Chapters 4 and 5. 

The mechanism of these bimolecular nucleophilic substitution reactions is shown in Scheme 11.3 for the reaction between a primary amine and the intermediate dichlorotriazine. A corresponding scheme can be drawn for reaction of a secondary amine, an alcohol or any other nucleophile in any of the replacement steps. It follows from this mechanism that the rate of reaction depends on ... 

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