Electron-rich species

A diazonium salt is a weak electrophile, and thus reacts only with highly electron-rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequendy, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH > of phenol or naphthol typically pH 7—11), whereas aromatic amines such as N,N diaLkylamines are coupled in a slightly acid medium, typically pH 1—5. This provides optimum stabiUty for the dia2onium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

A comparison of the rate constant for photoisomerization of the unsubstituted 3-phenyl derivative (kT = 3 x 1010 sec-1) to that of the 3-(p-methoxy phenyl) derivative (kr = 1.5 x 1010 sec-1) indicates that the presence of the p-methoxy groups imparts no special stability to the intermediate responsible for isomerization even though cleavage of a cyclopropane bond is predominant. Clearly these results are inconsistent with an intermediate possessing electron-poor or electron-rich species such as would be obtained from heterolytic cleavage of the cyclopropane. On the other hand, the results are consistent with a biradical species as intermediate. Further evidence consistent with this conclusion was obtained in a study of trans-3-p-cyanophenyl-/ra w-2-phenyl-1 -benzoylcyclopropane,<82)... 

Reference has already been made to electron-donating and electron-withdrawing groups, their effect being to render a site in a molecule electron-rich or electron-deficient, respectively. This will clearly influence the type of reagent with which the compound will most readily react. An electron-rich species such as phenoxide anion (36)... 

Several compounds containing Tt-bonds show reactions with 1 which most likely proceed via [2+1] or [4+1] cycloaddition processes, but no detailed mechanistic studies have been performed so far. Not unexpectedly, the electron-rich species 1 preferentially reacts with electron-poor substrates, and ring-strained or dipolar intermediates rearrange or react further to more stable products in a sometimes rather complicated and surprising fashion. In a few cases even the pentamethylcyclopentadienyl substituents at silicon are involved in the reaction pathways. 

The opposite effects are found for the interaction of an electron-rich species with these two molecular fragments. 

Where we have reason to suspect the involvement of a particular species as a labile intermediate in the course of a reaction, it may be possible to confirm our suspicions by introducing into the reaction mixture, with malice aforethought, a reactive species which we should expect our postulated intermediate to react with particularly readily. It may then be possible to divert the labile intermediate from the main reaction pathway—to trap it—and to isolate a stable species into which it has been unequivocally incorporated. Thus in the hydrolysis of trichloromethane with strong bases cf. p. 46), the highly electron-deficient dichlorocarbene, CClj, which has been suggested as a labile intermediate (p. 267), was trapped by introducing into the reaction mixture the electron-rich species cis but-2-ene (11), and then isolating the resultant stable cyclopropane derivative (12), whose formation can hardly be accounted for in any other way ... 

In 1971, Coulson at DuPont reported the first example of an OHA reaction catalyzed by soluble Rh and Ir complexes [5]. Secondary amines such as dimethyl-amine, pyrrolidine and piperidine were effectively added to ethylene, while primary amines, ammonia and heavier olefins were essentially unreactive (see Equation 6.3). IrCl3-3H20 proved to be an equally effective catalyst precursor in these reactions. It is probable that, under the conditions employed in this study, the Rh(III) and Ir(III) salts are reduced to monovalent, electron-rich species such as 3 (see Equation 6.6). 

Curly arrows must start from an electron-rich species. This can be a negative charge, a lone pair, or a bond. 

Many reactions will involve both nucleophiles and electrophiles. These may then be classified as nucleophilic if the main change to the substrate involves attack of a nucleophile, or electrophilic if the principal change involves attack of the substrate onto an electrophile. This distinction will become clearer in due course (see Section 7.1). The electron-rich species is always regarded as the attacking agent. 

In this example, the student remembered that a series of curly arrows was required, and they are generally in the right places, but not coming from electron-rich species, and not flowing in the right direction. This is typical of trying to remember a mechanism, which then fails to obey the general rules. 

Note that if we choose not to put in all the curly arrows, we could write the mechanism in two ways either considering the radical as the attacking species or the double bond as the electron-rich species. The first version is perhaps more commonly used, but it is much more instmctive to compare the second one with an electrophilic addition mechanism (see Section 8.1). The rationalization for the regiochemistry of addition parallels that of carbocation stability (see Section 8.2). 

Wade also incorporates electron-rich species such as C5H5 and C4H42- as arachno species derived by removal of two nonadjacent high-coordination vertices from the pentagonal and tetragonal bipyramids, respectively, and has, moreover, correlated many aspects of seemingly unrelated classes of compounds beyond the recognized borders of carborane chemistry. 

The 6-phenyldihydrodiazepinium cation is readily brominated at room temperature to give the 6-p-bromophenyl derivative [81JCS(P1)726]. At a naive level this result would seem to indicate the unusual feature of activated substitution at the p-position of a benzene ring brought about by a substituent onium group, but the present onium group is in fact an electron-rich species. Analogs with substituents at the l-,2-,3, or 4-... 

Alkenes are electron-rich species. The double bond acts as a nucleophile, and attacks the electrophile. Therefore, the most important reaction of alkenes is electrophilic addition to the double bond (see Section 5.3.1). An outline of the electrophilic addition reactions of alkenes is presented here. 

A nucleophile is an electron rich species that reacts with an electrophile. The term electrophile literally means electron-loving , and is an electron-deficient species that can accept an electron pair. A number of nucleophilic substitution reactions can occur with alkyl halides, alcohols and epoxides. However, it can also take place with carboxylic acid derivatives, and is called nucleophilic acyl substitution. 

The chemistry of all fullerenes is dominated by their ability to react as poorly conjugated and electron-deficient 2ir alkenes they show very few properties typical of dienes or arenes (5). In addition, because of the high cage stability, they never undergo substitutions. C60 shows behavior similar to that of a monosubstituted alkene such as vinyl chloride or acrylate. All fullerenes readily add to electron-rich species such as nucleophiles, bases, radicals, or reducing agents. They are, for example, perfect dienophiles for Dieles-Alder reactions. The types of reactions undergone by fullerenes are illustrated in Scheme 1. 

The usual activation of carbon monoxide by coordination appears to involve complexes in which the caibon atom bonded to the metal is rendered slightly positive, and thus more readily attacked by electron rich species such as ethylemc or acetylenic linkages. An example is seen in the reaction of nickel carbonyl and aqueous acetylene, which results in the production of acrylic add. 

Hydrolysis is part of the larger class of chemical reactions called nucleophilic displacement reactions in which a nucleophile (electron-rich species with an unshared pair of electrons) attacks an electrophile (electron deficient), cleaving one covalent bond to form a new one. Hydrolysis is usually associated with surface waters but also takes place in the atmosphere (fogs and clouds), groundwater, at the particle-water interface of soils and sediments, and in living organisms. 

Because aluminum is an element, its atoms do not degrade in the environment. In addition, aluminum compounds occur in only one oxidation state, Al(+3). Aluminum can complex with electron-rich species that occur in the environment. The forms of aluminum encountered in a natural system are determined by the strength of the attraction between the positively charged aluminum and the anionic or negatively charged ligands, and the preponderance and types of ligands that are present. These factors will be influenced by pH. 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

The boron atom in BH3 is sf hybridized with a vacant p orbital perpendicular to the plane of the three boron-hydrogen bonds. Thus borane and its derivatives are electrophilic (Lewis acidic) and combine readily with electron-rich species. For example, borane interacts with one of the lone pairs on the oxygen atom of tetrahydrofuran as shown below. 

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