Compared to benzene

The overall reactivity of the 4- and 5-positions compared to benzene has been determined by competitive methods, and the results agreed with kinetic constants established by nitration of the same thiazoles in sulfuric acid at very low concentrations (242). In fact, nitration of alkylthiazoles in a mixture of nitric and sulfuric acid at 100°C for 4 hr gives nitro compounds in preparative yield, though some alkylthiazoles are oxidized. Results of competitive nitrations are summarized in Table III-43 (241, 243). For 2-alkylthiazoles, reactivities were too low to be measured accurately. 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

Having its pyrazolic 4-position substituted, electrophilic attack on indazoles takes place in the 3-position and in the homocycle (the 5- and 7-positions). The condensation of a benzene ring results in a decrease of the aromaticity of the pyrazole moiety, as in naphthalene compared to benzene, and therefore basic ring cleavage is easier in indazoles than in pyrazoles (Section 4.04.2.1.7(v)). 

The consequences of this replacement gives pyridine a reduced susceptibility to electrophilic substitution compared to benzene, while being more susceptible to... 

Pyridine (and quinoline) which in so many respects are aromatic and comparable to benzene, lose this character completely on hydrogenation to piperidine (and hydroquinoline), which are entirely of the same nature as secondary aliphatic amines. The completely hydrogenated heterocyclic bases undergo degradation reactions which have become important particularly in the investigation of the constitution of alkaloids. A. W. Hofmann s method of opening rings by means of exhaustive methylation may be illustrated with piperidine. By thermal decomposition of the quaternary ammonium base a C—N-link-age is broken and at the same time water is eliminated. 

Validation of the model. Validation of this model using empirical data was not done, and this may cast doubt on some of its predictions. Of particular interest is the prediction that hydroquinone production is greater following phenol administration as compared to benzene administration. This is in opposition to the prediction of the Medinsky et al. (1995) model, described below. 

The lowering of the aromatic stabilization energy of silabenzene, compared to benzene, leads one to expect that the energy of the antiaromatic stabilization of silacyclobutadiene (253) would be lower than in the case of cyclobutadiene. 

The production of ethylene by gas crackers, mostly from C2, C3, and some C4 feeds, amounts to about 40% of the world ethylene capacity. This results in a small coproduction of benzene compared to benzene co-produced in naphtha and gas oil crackers, which account for 60% of the world s ethylene production capacity. A typical overall benzene yield from ethane cracking is on the order of only 0.6% of the ethane feed, and the yield of benzene from propane cracking is on the order of 3% of the propane feed. In contrast, the... 

A detailed comparison of spectral data concluded that borazine has a delocalized 7T-electron system like that of benzene (112) other workers, however, have concluded from spin-coupled calculations that borazine has litde aromatic stabilization as compared to benzene (113). 

In competitive studies (73IJS295, 74AHC(16)123) using aprotic diazotization of aniline to generate phenyl radicals, it has been shown that the relative rates of phenylation for furan and thiophene compared to benzene are 11.5 and 2.6, respectively. The partial rate factors... 

SAMPLE SOLUTION (a) Benzene has six equivalent sites at which nitration can occur. Summing the individual relative rates of attack at each position in tert-butylbenzene compared to benzene, we obtain... 

In 5,7-dihydroxyflavonoids carbons 6 and 8 are much more shielded compared to benzene carbons. 

The overall electron withdrawal from the ring system results in the rate of attack of the nitronium ion being substantially retarded compared to benzene. Moreover, the representations of the canonical forms of nitrobenzene formulated above show that the ortho and para positions are subject to the greatest reduction in electron density. In addition the reaction intermediate resulting... 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for structure determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDC13), 6 = H2 7.12, H3 7.34, H4 7.34, and H5 7.12 ppm. Coupling constants occur in well-defined ranges J2 3 = 4.9-5.8 J3 4 = 3.45-4.35 J2 4 = 1.25-1.7 and J2 5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. 13C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. 13C chemical shifts for thiophene, from tetramethylsilane (TMS), are C2 127.6, C3 125.9, C4 125.9, and C5 127.6 ppm. 

The methyl group accelerates the reaction compared to benzene and directs the incoming electrophile to the ortho and para positions. Both the rate enhancement and the... 

So far, groups have been either activating and ortho/para directors or deactivating and meta directors. The halogens are exceptions to this generalization. They are slightly deactivating compared to benzene but still direct to the ortho and para positions. For example, chlorobenzene is nitrated 17 times slower than benzene and produces predominantly ortho- and /Mra-chloronitrobenzene. 

The differences in steric hindrance in five-membered rings compared to benzene have been demonstrated in acetylations of alkylthiophenes (7IT4667). 

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