Protonated benzene carbons

In addition to electrophilic attack on the pyrrole ring in indole, there is the possibility for additions to the fused benzene ring. First examine the highest-occupied molecular orbital (HOMO) of indole. Which atoms contribute the most What should be the favored position for electrophilic attack Next, compare the energies of the various protonated forms of indole (C protonated only). These serve as models for adducts formed upon electrophilic addition. Which carbon on the pyrrole ring (C2 or C3) is favored for protonation Is this the same as the preference in pyrrole itself (see Chapter 15, Problem 2)1 If not, try to explain why not. Which of the carbons on the benzene ring is most susceptible to protonation Rationalize your result based on what you know about the reactivity of substituted benzenes toward electrophiles. Are any of the benzene carbons as reactive as the most reactive pyrrole carbon Explain. 

Thus for these reactions m is necessarily less than unity, a result that has now been widely observed in practice,117,118,120,161,180,181 and thus the m1 value offers a clear distinction between the A1 and A-SE2 mechanisms, which is not the case with the H0 correlations discussed above. A number of different excess acidity plots according to equation (56), covering a wide reactivity range, are shown in Fig. 9. These are for the hydration of oc-methylstyrene,120 equation (58), and the mechanistically similar hydration of phenylacetylene 118 for the isomerization of m-stilbene 120 and for the detritiation of tritiated benzene, equation (28) above."7 As can be seen, all four plots are good straight lines the references cited may be consulted for the details. The slopes look steep, but m values for carbon protonation approximate 1.8,36 and the nfi values are all calculated to be... 

As discussed by Wayner et al. [76], acetonitrile and ethyl acetate are strong Lewis bases, acting as proton acceptors from phenol. The hydrogen bond between PhOH and the solvent makes Aso v//° (PhOH) more negative than ASO V/7°(PhO). The remaining solvents included in figure 5.2 (benzene, carbon tetrachloride, and isooctane) are weaker Lewis bases and their interactions with PhOH and PhO are more similar. 

Table 3.10. One-, Two-, Three-, and Four-Bond Carbon-Proton Coupling Constants (Hz) of Benzene (a), Pyridine (b), Pyrimidine (c), Monosubstituted Benzenes [128] (d) and Five-Membered Heteroaromatics [130] (e).

The benzenonium ion, obtained by dissolving benzene in magic acid , behaves correspondingly Only one signal with <5C = 144.5 ppm is recorded at — 80 °C due to rapid proton migration. An averaged one-bond carbon-proton coupling of 26.5 Hz is expected... 

The absence of rapid isotropic motion of the benzene molecules allows efficient transfer of proton magnetization, not only to the directly bonded benzene carbons, but also to the C60 carbons. For this reason, the CP/MAS spectrum consists of a single narrow line at 143.7 ppm surrounded by a set of spinning sidebands (Fig. 27). 

A particular example is the protonation of l,3,5-tris(iV,iV-dialkylamino)benzenes (35), which are powerful nucleophilic reagents. The reaction between 35 and the proton may occur at a nitrogen atom to afford 36A, or at a carbon atom to afford the a cationic complex 36B (Scheme 6). A particular situation for 36 is that the 1H NMR spectrum of the all-benzene ring protons shows a singlet. This feature may be explained by the formation of a n complex81 as reported in 37, or by a dynamic process where the proton quickly shifts from one amino group to another amino group. 

P. Hobza, V. Spirko, H. L. Selzle, and E. W. Schlag, Anti-hydrogen bond in the benzene dimer and other carbon proton donor complexes, J. Phys. Chem. A 102, 2501-2504 (1998). 

Hobza, P., and Spirko, V., Anti-hydrogen bond in the benzene dimer and other carbon proton... 

In NMR, alkene and benzene carbons came in the same region of the spectrum, but in the NMR spectrum the H atoms attached to arene C and alkene C atoms sort themselves into two groups. To illustrate this point, look at the i Cand H chemical shifts of cyclohexene and benzene, shown in the margin. The two carbon signals are almost the same (1.3 ppm difference, < 1% of the total 200 ppm scale) but the proton signals are very different (1.6 ppm difference = 16% of the 10 ppm scale). There must be a fundamental reason for this. 

For the benzene molecule, we could answer questions like What is the mean value of the carbon-carbon, carbon-proton, proton-proton, electron-electron, electron-proton, and electron-carbon distances in the benzene molecule in its ground and excited states Note that because all identical particles are indistinguishable, the carbon-proton distance pertains to any carbon and any proton, and so on. To discover that the benzene molecule is essentially a planar hexagonal object would be very difficult. What could we say about a protein A pile of paper with such numbers would give us the trae (though non-relativistic) picture of the benzene molecule, but it would be useless, just as a map of the world with 1 1 scale would be useless for a tourist. It is just too exact. If we relied on this, progress in the investigation of the molecular world... 

Abstract - The temperature dependence of the proton nmr spectra of dithiocarbamato iron(III) complexes is markedly solvent dependent. A study is made of the temperature dependence of the nmr shifts for the N-CH2 protons in tris(N,N-dibutyldithiocar-bamato) iron(III) in acetone, benzene, carbon disulfide, chloroform, dimethyIformamide, pyridine and some mixed solvents. This contribution shall outline first how the nmr shifts may be interpreted in terms of the Fermi contact interaction and the dipolar term in the multipole expansion of the interaction of the electron orbital angular momentum and the electron spin dipol-nuclear spin angular momentum. This analysis yields a direct measure of the effect of the solvent system on the environment of the transition metal ion. The results are analysed in terms of the crystal field environment of the transition metal ion with contributions from (a) the dithiocarbamate ligand (b) the solvent molecules and (c) the interaction of the effective dipole moment of the polar solvent molecule with the transition metal ion complex. The model yields not only an explanation for the unusual nmr results but gives an insight into the solvent-solute interactions in such systems. 

The 60 MHz proton nmr shift, AB, for the N - CH2 protons in tris(N,N-diethyldithiocarbamato)iron(III) was determined in a number of solvents and in some solvent mixtures over a wide temperature range. (The diamagnetic contribution to the nmr shift was obtained by measuring the nmr shift of the N - CH protons in the diamagnetic cobalt analogue). In this paper six different solvents were used acetone, benzene, carbon disulfide, chloroform, dimethyl-formamide and pyridine. Also three chloroform-dimethylformamide solvent mixtures were used 0.75, 0.49 and 0.24 mole fraction of chloroform. 

Laboratory studies have now demonstrated the feasibility of the formation of benzene with gas-phase Ion chemistry involving acyclic 3-carbon units. The final step in this formation would Involve the neutralization of the benzenlum ion (protonated benzene) by proton transfer or recombination with electrons. Formation of the benzenlum Ion In Ionized allene and Ionized propyne has been shown to be possible with the following blmolecular reactions (31) ... 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

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