Electrophilic addition reactions description

All compounds with a particular functional group react similarly. Due to the cloud of electrons above and below its TT bond, an alkene is an electron-rich molecule, or nucleophile. Nucleophiles are attracted to electron-deficient atoms or molecules, called electrophiles. Alkenes undergo electrophilic addition reactions. The description of the step-by-step process by which reactants are changed into products is called the mechanism of the reaction. Curved arrows show which bonds are formed and which are broken and the direction of the electron flow that accompany these changes. 

This description should remind you of the first step in an electrophilic addition reaction of an alkene the nucleophilic alkene reacts with an electrophile and forms a carbocation intermediate (Section 6.0). In the second step of the reaction, the carbocation reacts with a nucleophile (Z ) to form an addition product. 

The second class of TAM RE AC s inventory includes the reactions between the coordinated ligands and external organic reagents. We divide these reactions into nucleophilic and electrophilic attacks and consider them as acid-base interactions. Table III presents their general description. The nucleophilic attacks are either addition reactions to unsaturated coordinated ligands (Reactions 44-46) or abstraction reactions (usually a proton abstraction, Reactions 47-50). The electrophilic attacks are similarly addition reactions (Reactions 51 and 52) and abstraction reactions (usually a hydride abstraction, Reactions 53-59). Reactions 60 to 63 represent some other intermolecular reactions. 

Addition of bronrine to an alkene is much more complicated than the simple representation in Figure 9.2 would suggest. The classical bromonium ion description of electrophilic addition of bromine to an alkene is useful only as a beginning point to describe the mechanistic options. The structure of the intermediate, the kinetics of the reaction, and both the stereochemistry and the regiochemistry of the products are all complex functions of the nature and concentration of tiie brominating agent, the solvent, any added nucleophiles, and the structure of the alkene. 

The Rumer basis of spin functions has found extensive use in organic chemistry in the description of the mechanisms of a great variety of organic reactions such as aromatic electrophilic and nucleophilic attack and a host of addition reactions. Even after 40 or more years of development of molecular orbital methods, this mode of description obstinately remains a major part of theoretical organic chemistry. 

Fig. 8-1) have done, therefore, is to show that this is indeed an entirely appropriate qualitative description. The HMO-treatment has, however, done more than this—it has given us some magnitudes, some quantitative information. It has also rationalised the empirical observation that attack by an N02 -ion to form a nitroaniline takes place preferentially in the ortho- and para-positions—in other words, that the —NH2 group is ortho/para-directing in an electrophilic reaction. (In addition to these electronic influences, there will of course also be some steric effects in the ortho-position but we are not considering these for the present they may be brought into the discussion as a separate consideration later on ( 8.2)). 

It is convenient to categorize reactions with concise descriptive labels. For substitution reactions we often use the notation SxM, in which the letter S indicates a substitution reaction. The subscript x indicates something of the mechanism, such as N for nucleophilic or E for electrophilic. M usually indicates the molecularity of the reaction, the nature of the reacting species, or additional information. The most familiar terms for substitution reactions are SnI (for substitution nucleophilic unimolecular ), as shown in equation 8.4,... 

There are several examples of zeolite-catalyzed reactions in organic synthesis include Friedel-Crafts alkylations and acylations and other electrophilic aromatic substitutions, additions and eliminations, cyclizations, rearrangements and isomerizations, and condensations are available in literature. A short description of these transformations has been summarized in the following section. 

Formaldehyde is a strong electrophile, allowing acetal to polymerize by nucleophilic, anionic, or cationic addition of an alcohol to ketene carbonyl groups. Relatively weak bases such as pyridine initiate anionic addition polymerization cationic addition polymerization is catalyzed by strong acids. When the cyclic trimer trioxane is used as a copolymer to polymerize acetal copolymers, Lewis acids such as boron tiifluoride promote copolymerization. A more fundamental description is polymerization of an aldehyde or ketone -l- alcohol -i- an acid or base catalyst to form hemiac-etal, which further converts to acetal. The hemiacetal reaction is reversible to aldehyde and alcohol. 

Cobalt-Complexed Propargyl Cations and Their Reactions. The stability and synthetic utility of hexacarbonyldicobalt complexed cations was first reported in 1971. They are readily formed by protonation of propargyl alcohol complexes or addition of electrophiles (protons, or alkyl or acyl cations) to vinylacetylene complexes. They can be isolated in pure form, usually by precipitation as hexafluorophosphates or tetrafluoroborates, but are more commonly generated in situ and used directly. Most of the early work is due to Nicholas, who has comprehensively reviewed the work up to 1986. A detailed description of the procedure for generating the l-methyl-2-propynyl complex and for its reaction with trimethylsilyloxycyclohexene (eq 51) has been given. ... 

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