Substitution reactions characteristics

Since all the C atoms of fullerenes are quarternary, and therefore contain no hydrogens, substitution reactions characteristic for planar aromatics are not possible... 

Tertiary analogues 183 of 180 are also configurationally stable at -78 °C.80 182 can be deprotonated and kept for 8 h at -78 °C with little loss of enantiomeric purity. It is consequently possible to make either enantiomer of the protected amine 184 from the appropriate amine 182, of which both enantiomers are available by virtue of the invertive substitution reactions characteristic of tin electrophiles (see section 6.1.4).82 83 Tertiary O-substituted benzyllithiums also have high configurational stability (see above). 

Reactivity First and foremost, porphyrins are aromatic molecules. For example, they undergo some of the electrophilic substitution reactions characteristic of aromatic compounds—nitration, halogenation, sulphona-tion, formylation, acylation, and deuteration. Porph)nins differ from molecules such as benzene, in that there are two different sites on the macrocycle where electrophilic substitution can take place with different reactivities the meso-position and the pyrrole p-position. Which of these... 

Cyclobutadieneiron tricarbonyl (XVIII) has also been shown to undergo electrophilic substitution reactions characteristic of aromatic systems to give monosubstituted cyclobutadieneiron tricarbonyl complexes (CII) (39a) (see Appendix). 

The imides, primaiy and secondary nitro compounds, oximes and sulphon amides of Solubility Group III are weakly acidic nitrogen compounds they cannot be titrated satisfactorily with a standard alkaU nor do they exhibit the reactions characteristic of phenols. The neutral nitrogen compounds of Solubility Group VII include tertiary nitro compounds amides (simple and substituted) derivatives of aldehydes and ketones (hydrazones, semlcarb-azones, ete.) nitriles nitroso, azo, hydrazo and other Intermediate reduction products of aromatic nitro compounds. All the above nitrogen compounds, and also the sulphonamides of Solubility Group VII, respond, with few exceptions, to the same classification reactions (reduction and hydrolysis) and hence will be considered together. 

The hydroxyben2oic acids have both hydroxyl and the carboxyl groups and, therefore, participate in chemical reactions characteristic of each of these moieties. In addition, these acids can undergo electrophilic ring substitution. The following reactions are discussed in terms of saUcyhc acid, but are characteristic of all the hydroxyben2oic acids. 

Virtually all of the organo derivatives of CA are produced by reactions characteristic of a cycHc imide, wherein isocyanurate nitrogen (frequendy as the anion) nucleophilically attacks a positively polarized carbon of the second reactant. Cyanuric acid and ethylene oxide react neady quantitatively at 100°C to form tris(2-hydroxyethyl)isocyanurate [839-90-7] (THEIC) (48—52). Substitution of propylene oxide yields the hydroxypropyl analogue (48,49). At elevated temperatures (- 200° C). CA and alkylene oxides react in inert solvent to give A/-hydroxyalkyloxazohdones in approximately 70% yield (53). Alternatively, THEIC can be prepared by reaction of CA and 2-chloroethanol in aqueous caustic (52). THEIC can react further via its hydroxyl fiinctionahty to form esters, ethers, urethanes, phosphites, etc (54). Reaction of CA with epichlorohydrin in alkaline dioxane solution gives... 

If (A i[X ]/A 2[Y ]) is not much smaller than unity, then as the substitution reaction proceeds, the increase in [X ] will increase the denominator of Eq. (8-65), slowing the reaction and causing deviation from simple first-order kinetics. This mass-law or common-ion effect is characteristic of an S l process, although, as already seen, it is not a necessary condition. The common-ion effect (also called external return) occurs only with the common ion and must be distinguished from a general kinetic salt effect, which will operate with any ion. An example is provided by the hydrolysis of triphenylmethyl chloride (trityl chloride) the addition of 0.01 M NaCl decreased the rate by fourfold. The solvolysis rate of diphenylmethyl chloride in 80% aqueous acetone was decreased by LiCl but increased by LiBr. ° The 5 2 mechanism will also yield first-order kinetics in a solvolysis reaction, but it should not be susceptible to a common-ion rate inhibition. 

More interesting are the substitution reactions on the triazole ring where a characteristically different course can be expected from that of analogous reactions in the purine series. These reactions were studied in more detail only in connection with the preparation of the glycosyl derivatives, and the experimental material does not permit the drawing of general conclusions. 

The azinones and their reaction characteristics are discussed in some detail in Section II, E. Because of their dual electrophilic-nucleophilic nature, the azinones may be bifunctional catalysts in their own formation (cf. discussion of autocatalysis below) or act as catalysts for the desired reaction from which they arise as byproducts. The uniquely effective catalysis of nucleophilic substitution of azines has been noted for 2-pyridone. 

Tile CH carbon shift ranges between S 62.4 and 95.1 ppm and is typical of carbons carrying two electronegative substituents. Tliis could be a hint for the observed overall ease of substitution reaction at this center. However, electronic and structural properties of these substituents cause no characteristic differences in the S values. 

As a result, we could open the door to a new frontier in indole chemistry. Various 1-hydroxyindoles (4a), l-hydroxytryptophans(la), 1-hydroxytryptamines (lb), and their derivatives have been given birth for the first time. As predicted, 1-hydroxytryptophan and 1-hydroxytryptamine derivatives are found to undergo previously unknown nucleophilic substitution reactions. In addition, we have been uncovering many interesting reactivities characteristic of 1-hydroxyindole structures. From the synthetic point of view, useful building blocks for indole alkaloids, hither to inaccessible by the well-known electrophilic reactions in indole chemistry, have now become readily available. Many biologically interesting compounds have been prepared as well. 

The nucleophilic substitution reactions are still more limited in scope owing to the instability of the isoxazole ring toward nucleophilic reagents. Homolytic reactions appear to be unknown though some of the reactions being studied are possibly of this type. Besides those reactions which are characteristic of the reactivity of the isoxazole nucleus itself, we shall consider in this section some substitution reactions in the side chain organomagnesium synthesis in the isoxazole series, condensation reactions of the methyl groups of methyl-isoxazoles, and finally some miscellaneous reactions. 

Reactions of isoxazole derivatives that take place with the cleavage of the heterocyclic nucleus are as characteristic in this series as the substitution reactions. They are due to the cleavage of the bond... 

It may not be appropriate to compare the thermal stability characteristics of VC/VAc copolymer to that of a VC homopolymer (PVC). The copolymerization would involve different kinetics and mechanism as compared to homopolymerization resulting structurally in quite different polymers. Hence, copolymerization of VC with VAc cannot be regarded as a substitution of chlorines in PVC by acetate groups. To eliminate the possibility of these differences Naqvi [45] substituted chlorines in PVC by acetate groups, using crown ethers (18-crown-6) to solubilize potassium acetate in organic solvents, and studied the thermal stability of the modified PVC. Following is the mechanism of the substitution reaction ... 

As a general rule, nucleophilic addition reactions are characteristic only of aldehydes and ketones, not of carboxylic acid derivatives. The reason for the difference is structural. As discussed previously in A Preview of Carbonyl Compounds and shown in Figure 19.14, the tetrahedral intermediate produced by addition of a nucleophile to a carboxylic acid derivative can eliminate a leaving group, leading to a net nucleophilic acyl substitution reaction. The tetrahedral intermediate... 

Aromaticity (Chapter 15 introduction) The special characteristics of cyclic conjugated molecules. These characteristics include unusual stability, the presence of a ring current in the 1H NMR spectrum, and a tendency to undergo substitution reactions rather than addition reactions on treatment with electrophiles. Aromatic molecules are planar, cyclic, conjugated species that have An + 2 7T electrons. 

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

Reactions of the type shown in (ii) and (34) are called substitution reactions. The substitution reaction is the characteristic reaction of benzene and its derivatives and is the way in which a multitude of compounds are prepared by the organic chemist. By this means he is able to introduce functional groups, which can then be... 

When reacting two phases that are not very soluble in each other, for example when carrying out nucleophilic substitution reactions, phase transfer catalysts should be considered when scaling down from equipment with poor mixing characteristic, rather than buying new equipment. 

These procedures were further extended to tris(acetylacetonato)technetium(III) [24]. In an acetonitrile solution of Tc(acac)3, the absorbances at the characteristic absorption maxima at 348,375,505 and 535 nm decreased with time, while an increase in the absorbance at 272 nm corresponded to an increase of free acetylacetone liberated during the substitution reaction. The final absorption spectra of the reaction mixture exhibited absorption maxima at 271,325 and 387 nm. The first order rate constant k for decomposition was found to be k = (8.86 + 0.08) x 10 4s 1at [H+] = 2.0 M at 30°C. 

Displacements such as this show all the usual characteristics of electrophilic aromatic substitution (substituent effects, etc., see below), but they are normally of much less preparative significance than the examples we have already considered. In face of all the foregoing discussion of polar intermediates it is pertinent to point out that homolytic aromatic substitution reactions, i.e. by radicals, are also known (p. 331) as too is attack by nucleophiles (p. 167). 

The effects predicted are qualitative at best. There are other factors that must be taken into account when predicting how various characteristics of the metal and ligand affect substitution reactions. For example, increasing the size of the metal ion is predicted to assist the formation of the transition state in SN1,... 

Substitution reactions of metal carbonyls frequently indicate differences in bonding characteristics of... 

Photoexcited aromatic compounds undergo substitution reactions with (non-excited) nucleophiles. The rules governing these reactions are characteristically different and often opposite to those prevailing in aromatic ground state chemistry 501a,b>, in contrast to the well known ortho/para activation in thermal aromatic substitutions, nitro groups activate the meta position in the photochemical substitution, as shown in (5.1) 502). 

The characteristic reaction of the relatively unreactive alkanes is the substitution reaction which involves the replacement of one a bonded atom for another and requires heat or light. 

The use of transition metals for the facilitation of substitution reactions on vinylic carbon has proven to be quite successful. For example, vinylic chlorides in the presence of nickel(II) chloride react with trialkyl phosphites to substitute phosphorus for the halide (Figure 6.17j.71-72 While reminiscent of a direct Michaelis-Arbuzov reaction, including final dealkylation by a chloride ion, the reaction actually involves an addition-elimination process. It appears that chloride provides a more facile reaction than bromide, a characteristic noted in several reaction systems. 

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