Substituent Effects in Aromatic Compounds

Table 2 lists a few of the substituent effects for aromatic compounds. It should be noted that the phenoxide ion (-0"), which is present in alkaline solutions of phenols, absorbs at a considerably longer wavelength than the parent phenol (-OH). Generally electron donating and lone-pair substituents cause a red shift and more intense absorption. More complex shifts arise when there is more than... 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

High selectivity is observed, and in aromatic compounds other substituents (C02R, OR, CN, halide) are not affected. Dinitro aromatics could be sequentially hydrogenated to nitroamines and diamines (105). [Some ruthenium and iron carbonyls were less effective (104)]. 

Boncz, M.A., et al., Substituent effects in the Advanced Oxidation of Aromatic Compounds, in Proceedings of the International Regional Conference of the IOA, Poitiers, 1998, France. 

In aromatic compounds carbon-13 shifts are largely determined by mesomeric (resonance) and inductive effects. Field effects arising from through-space polarization of the n system by the electric field of a substituent, and the influences of steric (y) effects on the ortho carbon nuclei should also be considered. Substituted carbon (C-l) shifts are further influenced by the anisotropy effect of triple bonds (alkynyl and cyano groups) and by heavy atom shielding. 

Reaction LXXL Replacement of Halogen by Hydroxyl. (B., 14, 2394 16, 2954 25, 3290 J. pr 11, 229 A. Ch., [3], 55, 400.)—When alkyl halides are refluxed with dilute caustic alkali or alkali carbonate, hydro-xylation occurs smoothly. If the halide be tertiary the replacement takes place with great ease, warming with water being sufficient a secondary halide reacts less readily, but more so than a primary. Halogen in aromatic compounds is replaced with great difficulty unless there be present negative substituents in the ortho- or para-position. The replacement, however, can be effected under pressure (U.S.P., 1996745). 

C chemical shifts in aromatic compounds are dependent on the polarity of the substituent. Appendix 3, Table A3.14 shows the substituent effects for a range of substituted benzenes. The 13C spectra of substituted benzenes can often be interpreted on the basis of these substituent parameters in association with data from off-resonance decoupled spectra. 

Hammett s equation (Hammett, 1940) correlates the rate of reactions with the electronic nature of the substituent in aromatic compounds. The Hammett a constants characterize the conjugation eifect and the inductive effect of the substituent for side-chain reactions. 

If the substituents on an aromatic compounds prevent the close approach of the cation radical to its neutral counterpart (due to steric hindrance), n-mer formation is inhibited. Such effects can clearly be seen on comparing 2,3,6,7-tetramethylnaphthalene (TMN) and its hindered analogue OMN. Spectrophotometric studies show that the tetramethylnaphthalene forms the dimeric (TMN)2+ cation radical, as characterized by a very broad absorption band at 1150 nm and the formation constant Kdimer = 490 m 1 at —10 °C. In contrast, dimer formation is not observed with the hindered cation radical OMN+. The fact that relatively few ion-radical dimers are known is associated with the opposing requirements for their formation. On the one hand, the cation radical should be relatively stable, which is usually the case with encumbered donors (for which steric hindrance prevents dose contact between the radicals), while on the other hand they should be able to approach each other in order for electronic interaction to be appreciable. 

P Tomasik, TM Krygowski, and T Chellathurai. The Hammett-Type Approach to the Substituent Effects in the UV Absorption Spectra of Aromatic Compounds. III. The Spectra of orf/io-Disubstituted Benzene Derivatives. Bull. Acad. Polon. Ser. Set Chim. 22 1065-1074, 1974. 

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