Electron donor substituents

Different behavior is expected for X = N-R Figs. 3 and 4) the equilibrium should be shifted toward 1 when an electron donor substituent (a = -l) is present (in any position) and toward form 2 when electron acceptor substituents are present. Ii has been shown chat a simultaneous... 

In azole chemistry the total effect of the several heteroatoms in one ring approximates the superposition of their separate effects. It is found that pyrazole, imidazole and isoxazole undergo nitration and sulfonation about as readily as nitrobenzene thiazole and isothiazole react less readily ica. equal to m-dinitrobenzene), and oxadiazoles, thiadiazoles, triazoles, etc. with great difficulty. In each case, halogenation is easier than the corresponding nitration or sulfonation. Strong electron-donor substituents help the reaction. 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

Given in Table 4.5 in addition to the Hammett equation are ct and substituent constant sets which reflect a recognition that the extent of resonance participation can vary for different reactions. The values are used for reactions in which there is direct resonance interaction between an electron-donor substituent and a cationic reaction center, hereas the a set pertains to reactions in which there is a direct resonance interaction between the substitutent and an electron-rich reaction site. These are cases in which the resonance conqionent of the... 

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

Among the reagents that are classified as weak electrophiles, the best studied are the aromatic diazonium ions, which reagents react only with aromatic substrates having strong electron-donor substituents. The products are azo compounds. The aryl diazonium ions are usually generated by diazotization of aromatic amines. The mechanism of diazonium ion formation is discussed more completely in Section 11.2.1 of Part B. 

Draw a Lewis structure (or a series of Lewis structures) for nitrobenzenium ion. Where is the positive charge Examine the electrostatic potential map for nitrobenzenium ion. Where would you expect electron-donor substituents to have the greatest stabilizing effect (consider meta and para positions only) Which is the more stable, meta or para-nitrotoluenium ion (intermediates in nitration of toluene) Compare electrostatic potential maps to that for nitrobenzenium ion. Does your result suggest that methyl acts as an electron donor ... 

Electron-donor substituents are known to accelerate the rate of electrophilic substitution on benzene, while electron-withdrawing groups are known to retard the reaction. One explanation is that electron donors stabilize the positive charge in the benzenium ion intermediate while electron-withdrawing substituents destabilize the positive charge. 

This activation is more pronounced on the introduction of stronger electron-donor substituents, but this point is not yet sufficiently studied. The isoxazole ring is unsymmetrical, and the activating effect of the substituent depends on its position. The available evidence shows that a substituent at C-5 activates the nucleus (or rather the 4-position) more strongly than does a substituent at C-3. 

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

In conclusion, with regard to the structure of benzenediazonium compounds with electron donor substituents in the 2- or 4-position, the most recent experimental data, mainly X-ray analyses and 13C and 15N NMR data, are consistent with 4.4 as the dominant mesomeric structure of quinone diazides, as proposed by Lowe-Ma et al. (1988). For benzenediazonium salts with a tertiary amino group in the 4-position the data are consistent with the quinonoid structure 4.20 as the dominant mesomeric form. 

Six-membered A-heteroaromatic compounds without electron-donor substituents (e. g., pyridine) were classified earlier by Albert (1968) as 7c-electron deficient. Therefore they are not reactive enough to be used as coupling components. 

After the compounds with one heteroatom in the ring, the second group of heteroaromatics are the heteroaromatic compounds with two or more heteroatoms in the same ring, but without electron donor substituents. 

The third group of heteroaromatic coupling components includes ring systems with electron-donor substituents. In principle they correspond to their (carbo)aro-matic analogues, the phenols and anilines. 

Other five-membered heteroaromatic compounds with electron-donor substituents, which have become interesting for industrial application during the last two decades, are the aminopyrazoles 12.62 and aminothiazoles 12.63 (see Schwander, 1982). 

Comproportionation equilibrium constants for Equation 9.3 between dications and neutral molecules of carotenoids were determined from the SEEPR measurements. It was confirmed that the oxidation of the carotenoids produced n-radical cations (Equations 9.1 and 9.3), dications (Equation 9.2), cations (Equation 9.4), and neutral ir-radicals (Equations 9.5 and 9.6) upon reduction of the cations. It was found that carotenoids with strong electron acceptor substituents like canthaxanthin exhibit large values of Kcom, on the order of 103, while carotenoids with electron donor substituents like (J-carotene exhibit Kcom, on the order of 1. Thus, upon oxidation 96% radical cations are formed for canthaxanthin, while 99.7% dications are formed for P-carotene. This is the reason that strong EPR signals in solution are observed during the electrochemical oxidation of canthaxanthin. 

Obviously, in the ground state of the radical cations generated, the positive charge delocalizes energetically favorably into the half-shells Si(SiR3)3, which due to the low effective nuclear charge of their Si centers [2,5a,8,9] are excellent electron donor substituents. 

The use of nitriles as dipolarophiles in 1,3-dipolar cycloaddition reactions is scarce because of their relative inertness in such reactions. Indeed, nitriles with electron-donor substituents do not react with nitrones even under harsh conditions. Hence, an additional activation of the reactants is required. This can be achieved, either by activating the nitrile (dipolarophile) or the nitrone (dipole), or both of them. For example, the reaction of electron-difficient nitriles such as... 

Yellow to brown compounds are obtained when one or more nitro groups are conjugated with electron-donor substituents such as hydroxy or, of more importance, amino groups. Such dyes are relatively cheap. Picric acid, 2,4,6-trinitrophenol, can be regarded as the oldest synthetic dye it was produced in 1771 by Woulfe by treating indigo with nitric acid. 

In contrast, there is little to no electron transfer of a it electron donor substituent to the it system of an alkene. Therefore a it electron donor substituent stabilizes more the carbene side of equation 5 and diminishes the endothermicity of the reaction. Calculations confirm the key role of G substituents of increasedit donating ability to diminish the AE of equation 5 ( G = F +54.2 kcal.mol 1, G = OMe +41.6 kcal.mol"1) [8],... 

What about jr-electron donor substituents on nitrosoarenes other than dimethylamino Pedley gives us the enthalpies of formation for three hydroxy derivatives the isomeric 4-nitroso-l-naphthol, 2-nitroso-l-naphthol and l-nitroso-2-naphthol, species 45-47 respectively. Of the three species, only the first cannot have an intramolecular hydrogen bond. By analogy to nitrophenols75 — there being no thermochemical data for the more related and hence relevant nitronaphthols—we expect that species 46 would be less stable than 45. After all, gaseous 2-nitrophenol is ca 20 kJ mol 1 less stable than its 4-isomer. We recall from the discussion of the isomeric naphthylamines that 1- and 2-naphthol are of almost identical stability. This suggests that species 46 and 47 should be of comparable stability. Both expectations are sorely violated by the literature results the enthalpies of formation of species 45, 46 and 47 increase in the order —20.3 4.9, —5.4 6.2 and 36.1 4.7 kJmol-1 respectively. If there is experimental error, where does the error lie ... 

Let us now consider the formation of aryl iodides from aryl diazonium salts and potassium iodide in methanol (Singh and Kumar 1972a, 1972b). Electron-donor substituents decelerate the process as compared with benzene diazonium (the substituent is hydrogen), whereas electron acceptor substituents accelerate it. Oxygen inhibits the reaction, and photoirradiation speeds it up. As the authors pointed out, in the case of 4-nitrobenzene diazonium, the reaction leads not only to 4-iodonitrobenzene but also to nitrobenzene, elemental iodine, and formaldehyde. All of these facts support the following sequence of events ... 

Aromatic compounds with electron-donor substituents, such as — N(CHs)2 or —OH, also react with CFsSCl via direct condensation... 

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