Protons benzene and

The measurement is done in neat liquid, and a capillaiy filled with benzene-d6 is used as an external lock. The signals of protonated benzene and water are used for qualitative and quantitative calibration. The broad signals at 5.0 and 1.8 ppm are probe characteristics. 

An ion cyclotron resonance study of ion-molecule reactions in benzene has shown that the cyclopropenyl cation arises from a a-type complex between protonated benzene and benzene. The mass spectral fragmentation of cyclopropanol monoethers has been examined and subjected to theoretical study and the fragmentation of a series of underivatized aminocyclitol-aminoglycoside antibiotics is believed to involve the ion (387). The chemical ionization mass spectrum of oct-6-en-3-one has been interpreted as involving the cyclobutyl-cyclopropylcarbinyl cation rearrangement (388)- (389). ... 

This scenario is confirmed by the evolution of the benzene concentration alopg the bed recorded during the same experiment in which we used protonated benzene and deuterated n-hexane (Figure 5b). These profiles clearly show that benzene first adsorbs preferably in the bottom layers before displacing the n-hexane adsorbed in the top layers and finally the equilibrium is reached all over the sample. 

Increasing delocalization, as in the cyclohexadienyl cation, is further stabilizing. This structure is simply protonated benzene, and it serves as a model for the intermediate in electrophilic aromatic substitution (see Section 10.18). Similarly, benzyl ion is quite stable for a formally 1° ion. Aromaticity effects are clear, as in the much greater stability of the six it electron tropylium ion vs. the four tt electron cyclopentadienyl ion (see Section 2.4.1 for a discussion of aromaticity). 

Treating 5.5 g of 2-amino-4,5-dimethylthiazole HCl with 0.66 g of solid sodium hydroxide 15 min at 220°C yields 53% of 4.4. 5.5 -tetramethyT 2,2 -dithiazolylamine, whose structure w as proved by identification with the produa obtained from the reaction between dithiobiuret and 3-bromo-2-butanone (467). This result is comparable to the reaction between 2-aminopyridine and its hydrochloride to yield bis(pyridyl-2)amine (468). Gronowitz applied this reaction to 2-aminothiazole, refluxing it with its hydrochloride 4 hr in benzene and obtained the dimeric 2-aminothiazole (236). He proposed a mechanism (Scheme 143) that involves the addition of a proton to the 5-position of the ring to give 234. The carbocation formed then reacts on the 5-position of a second... 

Diphenylmethane is significantly more acidic than benzene and tnphenylmethane is more acidic than either Identify the most acidic proton in each compound and suggest a reason for the trend in acidity... 

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

The electrophilic substitution of thiophene is much easier than that of benzene thus, thiophene is protonated in aqueous sulphuric acid about 10 times more rapidly than benzene, and it is brominated by molecular bromine in acetic acid about 10 times more rapidly than benzene. Benzene in turn is between 10 and lo times more reactive than an uncharged pyridine ring to electrophilic substitution. 

The coupling constants of ortho ( Jhh = 7 Hz), meta Jhh =1-5 Hz) and para protons CJhh I Hz) in benzene and naphthalene ring systems are especially useful in structure elucidation (Table 2.5). With naphthalene and other condensed (hetero-) aromatics, a knowledge of zig zag coupling = 0.8 Hz) is helpful in deducing substitution patterns. 

Benzene and substituted benzenes reaet with electrophiles, leading to new functionality. The two-step mechanism involves initial attack by an electrophile to form an intermediate (benzenium ion), followed by elimination of a proton to generate the substituted benzene. 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

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