Electrophilic substitution, chemical reactivity

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally sim liar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different The principal site of reaction m dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group Pyrophosphate is a reasonably good leaving group m nucleophilic substitution reactions especially when as in dimethylallyl pyrophosphate it is located at an allylic carbon Isopentenyl pyrophosphate on the other hand does not have its leaving group attached to an allylic carbon and is far less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents The principal site of reaction m isopentenyl pyrophosphate is the carbon-carbon double bond which like the double bonds of simple alkenes is reactive toward electrophiles... 

Beside being acidic, a significant industrial chemical property of phenol is the extremely high reactivity of its ring toward electrophilic substitution. If steric conditions permit, the substitution leads first to the formation of the 2- or 4-mono derivative, then to the 2,4- or 2,6-diderivative, and finally to the 2,4,6-triderivative. The halogenation of phenol produces mono-, di-, and tribal ophenols. 

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

The reactivity of Ce, C7, Cg aromatics is mainly associated with the benzene ring. Aromatic compounds in general are liable for electrophilic substitution. Most of the chemicals produced directly from benzene are obtained from its reactions with electrophilic reagents. Benzene could be alkylated, nitrated, or chlorinated to important chemicals that are precursors for many commercial products. 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

Thus, in spite of its lack of reactivity, iodine reacts chemically with unsaturated compounds, whereby the silica gel of the TLC layer can sometimes be assigned a catalytic role [11, 12]. Irreversible oxidations and electrophilic substitution and addition reactions have been observed on the interaction of iodine with tertiary nitrogen compounds such reactions possibly depend on particular steric relationships or are favored by particular functional groups [13, 14]. 

More than just a few parameters have to be considered when modelling chemical reactivity in a broader perspective than for the well-defined but restricted reaction sets of the preceding section. Here, however, not enough statistically well-balanced, quantitative, experimental data are available to allow multilinear regression analysis (MLRA). An additional complicating factor derives from comparison of various reactions, where data of quite different types are encountered. For example, how can product distributions for electrophilic aromatic substitutions be compared with acidity constants of aliphatic carboxylic acids And on the side of the parameters how can the influence on chemical reactivity of both bond dissociation energies and bond polarities be simultaneously handled when only limited data are available ... 

In a study of quantum chemical calculations of reactivity, it was found that charge densities give only qualitative agreement with experimental reactivities in electrophilic substitution, whereas semiquantitative agreement is obtained with the localization energies.7... 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

From the investigation of all these data it is clear that the aromaticity of phosphinine is nearly equal to that of benzene. Their chemical reactivity, however, is different. Most important is the effect of the in-plane phosphorus lone pair, which (i) is able to form a complex and (ii) can be attacked by electrophiles to form A -phosphinines. Thus, electrophilic substitution reaction on the ring carbon is impossible. In Diels—Alder reactions, phosphinines behave as dienes, providing similar products to benzene but under less forcing condition (the reaction with bis(trifluoromethyl) acetylene takes place at 100 °C with 3, while for benzene 200 °C is required). 

The main aspects of the chemical reactivity of helicenes (e.g. electrophilic substitution) equally not deviate from those of planar aromatic compounds, and remarkable reactions of helicenes, which are incidentally found (e.g. the transannular bond formation between a C(l)-substituent and a part of the inner helix) can ultimately be reduced to known principles of aromatic reactivity. 

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

In terms of chemical reactivity, cycl[3,2,2]azine shows the normal behavior of a stable aromatic system, undergoing substitution reactions smoothly and in good yields. From MO calculations it was predicted that electrophilic substitution should occur most readily at the 1-position.lb Reaction at positions 2, 5, and 6 have not yet been observed. 

The chemical reactivity of coumarin to a large extent resembles that of pyran-2-one. Electrophilic substitution occurs preferentially in the carbocyclic ring at C-6. Substitution at C-3 can also occur when more vigorous conditions are employed. Coumarin is readily attacked by nucleophiles, giving rise to a variety of ring-opened products. 

In view of the highly reactive character of pyrrole, the controlling factor for electrophilic substitution is considered to be the v-electron densities. To obtain an agreement between Hiickel MO calculations and experimental observations of chemical reactivity, recourse has to be made to the use of the auxiliary inductive parameter. Satisfactory results are obtained when h = 2 and = 0.25. 

A quantitative description of the reactivity of monosubstituted benzenes to electrophilic substitution based on considerations of inductive effect parameters and con-jugative effect parameters from the 13 C chemical shifts of the aromatic compounds has been proposed.3 MO calculations on the proton migration in the ipso adducts formed in the reaction of CH3+ and SiH3+ with benzene have been described.4 With SiH3+ the ipso adduct is the most stable of possible isomers, whereas for CH3+ the >ara-protonated isomer is the most stable. 

Whereas various reactions of phenotellurazines and particularly of phe-noxatellurine have been thoroughly studied, information on the reactivity of tellurantrene is rather scarce and reactions of phenothiatellurine are practically unstudied. The chemical behavior of heterocyclic compounds 92 is determined by the presence in these tricyclic systems of two reaction centers represented by a tellurium and a second heteroatom M (NR, O, S) and also by a tendency of the activated benzene rings to enter into electrophilic substitution reactions. We shall follow this classification of reactions of tricyclic systems 92. 

It will be seen that the 7r-electron distribution seems indeed to be uneven, the nitrogen being estimated to have 1.8 n electrons and the boron but 0.2. Electrophilic substitution takes place very much more readily in the 6- and 8-positions than anywhere else the 2-position is the third most reactive. The 8-position is somewhat more reactive than the 6-position but the difference is very much less than that between the 6- and 2-positions. It will be seen that the simple PMO estimates of 6E give a very good account of substitution in this system, given that the 4-position, like that in phenanthrene, must be sterically hindered. These calculations correctly predicted the orientation of the products for us before we had established this experimentally indeed we were assisted in devising chemical proofs of their structures by this foreknowledge. 

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