Bond breaking reagent

There are many reactions that result in the formation of a new covalent bond and the rupture of an old one. The bond-breaking reagent may be either an acid or base. The two types of such reactions (called displacements) are illustrated below, first in general terms, then by use of specific examples ... 

In hydroxyUc solvents, the reaction with aniline follows a bi-molecular course but is complicated by competing solvolysis. This is a striking result when compared with the behavior of picryl chloride, which is much more selective with regard to the same reagents (aniline and alcohol), and has been interpreted to mean that bond-breaking has made appreciable progress in the rate-determining step of the reaction of phosphonitrilic chloride. Furthermore, the same indication is obtained from the fact that in the reactions of the halides, the fluorine chlorine ratios are less than one. ... 

CN/CC replacement has also been observed on treatment of pteridine with malonitrile or cyanoacetamide 6-amino-7-R-pyrido[2,3,-h]pyrazine (R = CN, CONH2) beingformed (73JCSP(1)1615) (Scheme 15). The reaction involves initial addition of the reagent to the N-3-C-4 bond, scission of the dihydro bond between N-3 and C-4 in the covalent adduct, and recycli-zation. This mechanism is fundamentally different from the mechanism mentioned in Scheme 14, where two molecules of the reagent were used for addition and where the bond breaking takes place between N-1 and C-2. 

The direction of cleavage in unsymmetrical ethers is determined by the relative ease of O-R bond breaking by either SN2 (methyl, benzyl) or SW1 (r-butyl) processes. As trimethylsilyl iodide is rather expensive, alternative procedures that generate the reagent in situ have been devised. 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

Over the past fifty years or more, archetypal cases within each of these reaction types have been studied in great detail. These extensive studies have enabled ideas on the nature and stabilisation of transition states and on the timing of bond-breaking and bond-forming processes to be formulated. Many of the results have led to modifications in techniques, and to the discovery and design of new reactions and reagents, i.e. to the development of the methodology of synthesis. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Reactions of pyrazoline derivatives with electrophilic reagents very often become complicated owing to the oxidative destruction and/or processes related to the C5—Ni bond breaking. 

The vast majority of organic methodology involves solution-phase chemistry. Reagents involved in a reaction are typically dissolved in the same liquid phase. Collisions between the reagent molecules result in bond-breaking and bond-forming reactions. 

According to Semenov [11] at thermolysis of chain process initiators the energy of the weakest bond break in molecules of almost all substances fits the range of 209-419 kJ. In this regard, the chain initiation by initial reagents themselves may progress at a low rate. Therefore, substances promoting initiation of chain reactions are often used. For example,... 

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