Protonation of Aromatic Hydrocarbons

The more basic of the polynuclear benzenoid hydrocarbons dissolve in concentrated sulfuric acid, probably with the formation of simple carbonium ions.263 Benzene itself is only slightly soluble, but exchanges hydrogen with DaS04.866 Benzene and toluene will dissolve in a mixture of hydrogen fluoride and boron trifluoride.867 268 Pyrene in hydrogen 

Two structures are possible for the interaction of aromatic hydrocarbons with acids.270 In the a-structures a covalent bond is established between the acidic reagent and a particular carbon atom of the benzene ring. The a-structures are essentially classical carbonium ions. In the -structures a non-classical bond is established, not to any particular atom, but to the -electron cloud in general. It is quite likely that both types of structure are represented by actual examples. Thus m-xylene interacts more strongly with hydrogen chloride than does o-xylene, but the difference between the two hydrocarbons is much more pronounced when their interactions with a boron trifluoride-hydrogen fluoride mixture are compared. This is readily understandable 

In the r-complex a meta location of the two methyl groups is particularly favorable because it allows both methyl groups to interact effectively with the positive charge. In the yr-complex the location of the methyl groups makes less difference. 

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Thus if we axe interested only in the relative values of equilibrium or rate constants for closely similar reactions, we need only concern ourselves with changes in delocalization energy. This of course is a useful simplification. A good illustration is provided by the protonation of aromatic hydrocarbons. Aromatic hydrocarbons react with strong acids to form salts. These are now known4 to be arenonium ions thus benzene is converted to benzenonium ion (I) by strong acids ... 

The effect of temperature on the equilibrium ratios of isomeric arenium ions was not studied, but it cannot be strong. The entropy of protonation of aromatic hydrocarbons and their methylated derivatives in liquid hydrogen fluoride was shown to be practically independent of the structure and basicity of hydrocarbon (ASp = = —21 e.u. This seems to be also valid for other acid systems so the... 

Data on the relative basicity of a number of aromatic hydrocarbons and their alkyl derivatives were obtained by the Dutch researchers They measured the coefficients of hydrocarbon distribution between an inert solvent (n-hexane) and liquid hydrogen fluoride, containing additives of inorganic fluorides or saturated with BFj. The protonation of aromatic hydrocarbons in HF is described by the equation ... 

In certain cases, such as that of equation (4.5), the reactive centers X are fixed. In others, however, we may have a family of groups of reactions following the same general course where each group involves a fixed set of Xi but where the sets of X are different for different groups. A good example is the protonation of aromatic hydrocarbons to form arenonium ions e.g.. 

Figure 13.16 Free-energy relationships in the protonation of aromatic hydrocarbons and methoxy-benzenes (squares) and in the excited-state PT from aromatic naphthols (circles), in water.

Cations like that present in (iv) exist in solutions of aromatic hydrocarbons in trifluoroacetic acid containing boron trifluoride, and in liquid hydrogen fluoride containing boron trifluoride. Sulphuric acid is able to protonate anthracene at a mero-position to give a similar cation. ... 

The relative basicities of aromatic hydrocarbons, as represented by the equilibrium constants for their protonation in mixtures of hydrogen fluoride and boron trifluoride, have been measured. The effects of substituents upon these basicities resemble their effects upon the rates of electrophilic substitutions a linear relationship exists between the logarithms of the relative basicities and the logarithms of the relative rate constants for various substitutions, such as chlorination and... 

In the presence of a proton source, the radical anion is protonated and further reduction occurs (the Birch reduction Part B, Section 5.5.1). In general, when no proton source is present, it is relatively difficult to add a second electron. Solutions of the radical anions of aromatic hydrocarbons can be maintained for relatively long periods in the absence of oxygen or protons. 

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

Except for these studies of their protonation behavior, almost the only other aspect of the chemistry of sulfonic acids that has been investigated to any extent from a mechanistic point of view is the desulfonation of aromatic sulfonic acids or sulfonates. Since this subject has been well reviewed by Cerfontain (1968), and since the reaction is really more of interest as a type of electrophilic aromatic substitution than as sulfur chemistry, we shall not deal with it here. One should note that the mechanism of formation of aromatic sulfonic acids by sulfonation of aromatic hydrocarbons has also been intensively investigated, particularly by Cerfontain and his associates, and several... 

The reduction of organic halides in the presence of aromatic hydrocarbons, the subject of detailed kinetic studies, provide rate constants for the homogeneous ET [147-150] and the follow-up reaction [151]. The theoretical basis for this kind of experiment ( homogeneous redox catalysis ) was laid by Saveant s group in a series of papers during the years 1978-80 [152-157]. Homogeneous ET also plays an important role in the protonation of anion radicals [158]. 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

Aptotic solvents can be used for the reduction of aromatic hydrocarbons, particularly the condensed ring systems. Solvents used for the conversion of benzene to cyclohexa-1,4-diene at a mercury cathode under constant current conditions include dimethoxyethanc [45] and N-medtylpyrrolidone [46]. Each solvent contained water as a proton source and tetraethylammonium bromide as supporting electro-... 

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

Radical ion pairs also react by proton, atom, or group transfer. We illustrate proton transfer in reactions of aromatic hydrocarbons with tertiary amines. These reactions cause reduction or reductive coupling. In the reduction of naphthalene, the initial ET is followed by H" transfer from cation to anion, forming 67 paired with an aminoalkyl radical the pair combines to generate... 

Aromatic hydrocarbons gave products of protonation on dissolving in hydrofluoric acid. Oxidation in aromatic cation radicals did not take place (Kon Blois 1958). Triflu-oroacetic acid is an effective one-electron oxidant (Eberson Radner 1991). Meanwhile, sulfuric acid caused not only dissolution and protonation, but also one-electron oxidation of aromatic hydrocarbons. Sulfonation, naturally, proceeded too (Weissmann et al. 1957). 

Photoinduced carboxylation of aromatic hydrocarbons via their radical anions in the presence of tertiary amines has been reported by Tazuke and Ozawa [404], Similar photofixation of carbon dioxide on styrene has been reported Toki et al. [405], In these photoreactions, the anionic part of the radical anions are trapped by electrophiles such as proton and carbon dioxide (Scheme 119). 

A classification of solvents can be developed on the basis of the stability of the radial anion produced by reduction of aromatic hydrocarbons, such as naphthalene and anthracene. The solvent reactions of such anions have been widely studied2 and have generally been found to go by a sequence of reactions in either a protic solvent or in the presence of a proton donor in an aprotic solvent 3... 

The effect of proton donors on the reduction of aromatic hydrocarbons has been discussed earlier in this chapter. The importance of potential-pH relations to the understanding and utilization of the redox behavior of these systems has long been recognized extensive potentiometric data have been obtained for only a limited number of organic compounds.50... 

The mechanism of the reduction of aromatic hydrocarbons was actually established early by Hoijtink and his co-workers (Hoijtink, 1970) as an ECE mechanism (p. 25) (see also Given and Peover, 1960 Santhanam and Bard, 1966). The two one-electron waves due to formation of anion radical and dianion in an aprotic medium change in a characteristic way upon addition of incremental amounts of a proton donor the height of the first wave increases at the expense of the second one until at sufficiently high concentration only a single two-electron wave is obtained. This behaviour in combination with the HMO calculations referred to above clearly show that the radical anion is protonated to a neutral radioed which is reducible at a less negative potential than the substrate [reaction... 

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