Aromatic compounds, reactions

The technique commercially used for this process is a rapid agitation of two liquid phases, one essentially hydrogen fluoride and one essentially hydrocarbon. The reaction apparently takes place rapidly at the liquid-liquid interface. Reaction also takes place in either of the liquid phases but at a much slower rate. With aromatic compounds reaction takes place rapidly and homogeneously in either a hydrocarbon liquid phase or a hydrogen fluoride liquid phase. 

Dichloro-l-fluoropyridinium triflate (lm)55 has been used for the direct fluorination of biologically important aromatic compounds. Reaction of estra-1.3,5(10)-triene-3,17/ -diol and... 

Therefore, one-electron oxidation of naphthalene by NO+ is the rate-determining stage at low naphthalene concentrations (<=> means equilibrium of this oxidation). At high naphthalene concentrations, the rate of the process no longer depends on the rate of accumulation of the cation radical species. In this case the rate depends on recombination of the species with N02 radical. The authors point out that for many of the more reactive aromatic compounds, reaction paths involving electron transfer in nitration will become more important as the concentration of the aromatic compound is increased, irrespective of the concentration of the species accepting the electron (Leis et al. 1988). 

Numerous synthetic, technical and natural processes involve formation of aromatic radical cations as reaction intermediates. [1,2] In general, aromatic radical cations are formed upon ionization, e.g., by direct photolysis or radiolysis, or one-electron oxidation of aromatic compounds (reaction 1). 

The electron withdrawing effect of the Cr(CO)3 unit 7r-bonded to an aromatic ring has a dramatic effect on the reaction pathway of aromatic compounds. Reaction... 

Azulene is an aromatic compound and undergoes substitution reactions in the 1-position. At 270 C it is transformed into naphthalene. 

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

It is a typically aromatic compound and gives addition and substitution reactions more readily than benzene. Can be reduced to a series of compounds containing 2-10 additional hydrogen atoms (e.g. tetralin, decalin), which are liquids of value as solvents. Exhaustive chlorination gives rise to wax-like compounds. It gives rise to two series of monosubstitution products depending upon... 

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

Nitration at a rate independent of the concentration of the compound being nitrated had previously been observed in reactions in organic solvents ( 3.2.1). Such kinetics would be observed if the bulk reactivity of the aromatic towards the nitrating species exceeded that of water, and the measured rate would then be the rate of production of the nitrating species. The identification of the slow reaction with the formation of the nitronium ion followed from the fact that the initial rate under zeroth-order conditions was the same, to within experimental error, as the rate of 0-exchange in a similar solution. It was inferred that the exchange of oxygen occurred via heterolysis to the nitronium ion, and that it was the rate of this heterolysis which limited the rates of nitration of reactive aromatic compounds. 

Many aromatic compounds are sufficiently basic to be appreciably protonated in concentrated sulphuric acid. If nitration occurs substantially through the free base, then the reactivity of the conjugate acid will be negligible. Therefore, increasii the acidity of the medium will, by depleting the concentration of the free base, reduce the rateof reaction. This probably accounts for the particularly marked fall in rate which occurs in the nitration of anthraquinone, benzoic acid, benzenesulphonic acid, and some nitroanilines (see table 2.4). 

When large concentrations of water are added to the solutions, nitration according to a zeroth-order law is no longer observed. Under these circumstances, water competes successfully with the aromatic for the nitronium ions, and the necessary condition for zeroth-order reaction, namely that all the nitronium ions should react with the aromatic as quickly as they are formed, no longer holds. In these strongly aqueous solutions the rates depend on the concentrations and reactivities of the aromatic compound. This situation is reminiscent of nitration in aqueous nitric acid in which partial zeroth-order kinetics could be observed only in the reactions of some extremely reactive compounds, capable of being introduced into the solution in high concentrations ( 2.2.4). 

A similar circumstance is detectable for nitrations in organic solvents, and has been established for sulpholan, nitromethane, 7-5 % aqueous sulpholan, and 15 % aqueous nitromethane. Nitrations in the two organic solvents are, in some instances, zeroth order in the concentration of the aromatic compound (table 3.2). In these circumstances comparisons with benzene can only be made by the competitive method. In the aqueous organic solvents the reactions are first order in the concentration of the aromatic ( 3.2.3) and comparisons could be made either competitively or by directly measuring the second-order rate constants. Data are given in table 3.6, and compared there with data for nitration in perchloric and sulphuric acids (see table 2.6). Nitration at the encounter rate has been demonstrated in carbon tetrachloride, but less fully explored. ... 

It has already been noted that, as well as alkylbenzenes, a wide range of other aromatic compounds has been nitrated with nitronium salts. In particular the case of nitrobenzene has been examined kinetically. Results are collected in table 4.4. The reaction was kinetically of the first order in the concentration of the aromatic and of the nitronium salt. There is agreement between the results for those cases in which the solvent induces the ionization of nitric acid to nitronium ion, and the corresponding results for solutions of preformed nitronium salts in the same solvent. 

It has long been known that, amongst organic solvents, acetic anhydride is particularly potent in nitration, and that reaction can be brought about under relatively mild conditions. For these reasons, and because aromatic compounds are easily soluble in mixtures of nitric acid and the solvent, these media have achieved considerable importance in quantitative studies of nitration. 

Recent experiments have shown that the concentration of aromatic compound needed to maintain zeroth-order kinetics (see below) was much greater than for nitrations with solutions of nitric acid in some inert organic solvents reactions which were first order in the concentration of the aromatic were obtained when [ArH] < c. 2 x io mol 1 . ... 

This qualification must be applied to the results of Dewar and his co-workers relating to the reaction of a series of polynuclear aromatic compounds with solutions of nitric acid in acetic anhydride at o °C... 

Expt. ig. The aromatic compound was added to a freshly prepared solution of nitric acid in acetic anhydride. The reaction was very fast ( < i min.) About 2 % of an acetoxy-lated product was formed (table 5.4). 

The behaviour of benzene is the datum from which any discussion of aromatic compoimds must start the reactivity of an aromatic compound is its rate of reaction relative to that of benzene when both are taking part in reactions occurring under the same conditions and proceeding by the same mechanism. 

The above definition implies that the reactivity of an aromatic compound depends upon the reaction which is used to measure it, for the rate of reaction of an aromatic compound relative to that for benzene varies from reaction to reaction (table 7.1). However, whilst a compoimd s reactivity can be given no unique value, different substitution reactions do generally set aromatic compoimds in the same sequence of relative reactivities. 

The significance of establishing a limiting rate of reaction upon encounter for mechanistic studies has been pointed out ( 2.5). In studies of reactivity, as well as settii an absolute limit to the significance of reactivity in particular circumstances, the experimental observation of the limit has another dependent importance if further structural modification of the aromatic compound leads ultimately to the onset of reaction at a rate exceeding the observed encounter rate then a new electrophile must have become operative, and reactivities established above the encounter rate cannot properly be compared with those measured below it. 

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