Reactivity aromatic hydrocarbons

The rate of this exchange process was identical with the rate of nitration of several reactive aromatic hydrocarbons. Discuss how this result is consistent with mechanism B on p. 196, but not with mechanisms A or C. 

Perhaps the best-known method of preparing aromatic azo compounds involves the coupling of diazonium salts with sufficiently reactive aromatic compounds such as phenols, aromatic amines, phenyl ethers, the related naphthalene compounds, and even sufficiently reactive aromatic hydrocarbons. Generally, the coupling must be carried out in media which are neutral or slightly basic or which are buffered in the appropriate pH range. The reaction may also be carried out in nonaqueous media. While some primary and secondary aromatic amines initially form an A-azoamine, which may rearrange to the more usual amino-C-azo compound, tertiary amines couple in a normal manner. 

Certain reactive aromatic hydrocarbons are formylated by dimethylforma-mide in the presence of phosphorus oxychloride (the Vilsmeier reaction, e.g. 9-formylanthracene, Expt 6.114). This method can also be used with advantage for the formylation of re-excessive heteroaromatic systems (e.g. 2-formylthiophene, cognate preparation in Expt 6.114). 

Suitably reactive aromatic hydrocarbons also form crystalline 1 1 7c-complexes ( styphnates ) with styphnic add (2,4,6-trinitroresorrinol). These derivatives do not crystallise quite so well as the corresponding picrates, but are frequently of great value. Benzene and its simple homologues do not give stable derivatives. 

Other 7r-complexing reagents are 1,3,5-trinitrobenzene and 2,4,7-trinitro-9-fluorenone (1). The crystalline adducts are usually formed in ethanol, glacial acetic add, or toluene solution and recrystallised from similar solvents. They are useful both for the characterisation and also for the isolation of appropriately reactive aromatic hydrocarbons. 

DMSO in the presence of potassium t-butoxide (potassium methylsulfinylmethide is formed) methylates diolefins and reactive aromatic hydrocarbons, for example "... 

The reagent forms ir-complexes useful for the isolation and characterization of suitably reactive aromatic hydrocarbons."... 

The effect of water addition on the nitration of toluene and some more reactive aromatic hydrocarbons in homogeneous solution in acetonitrile or acetic acid was studied at 20 C. Anhydrous nitric acid was used at first, no sulphuric acid being present, and then experiments were repeated with progressively higher initial concentrations of water. Nitric acid was always in large excess over the aromatic substrate concentration. For some experiments, filming nitric acid was used. 

In the sulfonation of the less reactive aromatic hydrocarbons or aryl halides, it is desirable to use 5-20% oleum, which brings about reaction at a convenient rate at moderate temperatures (0-50°) furthermore, if oleum is used, less sulfuric acid remains at the completion of the reaction to interfere with the isolation of the product. In the preparation of salts, however, this factor is of less significance. Although sulfur tiioxide is even more active than oleum, it can be used to advantage only occasionally because it favors the formation of a sulfone. The formation of by-products is decreased by use of a solvent a suitable solution can be prepared by passing the gaseous material, obtained by warming 60% oleum, into cold ethylene dichloride. 

Sulfuric acid is satisfactory for the sulfonation of the more reactive aromatic hydrocarbons. However, a large excess of reagent is required to give a good yield since the reaction is reversible. 

Sulphonyl Peroxides.—Continuing interest in this class of compound is a consequence of their value in aromatic sulphonoxylation and in their addition to alkenes to give sulphonate esters. A useful electrochemical preparation of bis(methanesulphonyl) peroxide from sodium methanesulphonate makes available a reagent for the synthesis of methanesulphonates of less reactive aromatic hydrocarbons. 

Forms 7r-complexes with reactive aromatic hydrocarbons which are used for characterisation. Pale-yellow needles (AcOH or C H ). Mp 176°. 

More precisely, the rate of ozone formation depends closely on the chemical nature of the hydrocarbons present in the atmosphere. A reactivity scale has been proposed by Lowi and Carter (1990) and is largely utilized today in ozone prediction models. Thus the values indicated in Table 5.26 express the potential ozone formation as O3 formed per gram of organic material initially present. The most reactive compounds are light olefins, cycloparaffins, substituted aromatic hydrocarbons notably the xylenes, formaldehyde and acetaldehyde. Inversely, normal or substituted paraffins. 

Reactivity numbers of the most reactive positions have been used to correlate the reactivities in nitration (see below) and other substitutions of a series of polycyclic aromatic hydrocarbons, and they give somewhat better correlations than any of the other commonly used indices of reactivity. The relationship shown below, which was discussed earlier ( 7.1.1),... 

An important property of aromatic hydrocarbons is that they are much more stable and less reactive than other unsaturated compounds Ben zene for example does not react with many of the reagents that react rapidly with alkenes When reaction does take place substitution rather than addition is observed The Kekule formulas for benzene seem mcon sistent with its low reactivity and with the fact that all of the C—C bonds m benzene are the same length (140 pm)... 

Polycyclic aromatic hydrocarbons undergo electrophilic aromatic substitution when treated with the same reagents that react with benzene In general polycyclic aromatic hydrocarbons are more reactive than benzene Most lack the symmetry of benzene how ever and mixtures of products may be formed even on monosubstitution Among poly cyclic aromatic hydrocarbons we will discuss only naphthalene and that only briefly Two sites are available for substitution m naphthalene C 1 and C 2 C 1 being normally the preferred site of electrophilic attack... 

The methyl and ethyl esters of cyanoacetic acid are slightly soluble ia water but are completely miscible ia most common organic solvents including aromatic hydrocarbons. The esters, like the parent acid, are highly reactive, particularly ia reactions involving the central carbon atom however, the esters tend not to decarboxylate. They are prepared by esterification of cyanoacetic acid and are used principally as chemical iatermediates. 

Dyes, Dye Intermediates, and Naphthalene. Several thousand different synthetic dyes are known, having a total worldwide consumption of 298 million kg/yr (see Dyes AND dye intermediates). Many dyes contain some form of sulfonate as —SO H, —SO Na, or —SO2NH2. Acid dyes, solvent dyes, basic dyes, disperse dyes, fiber-reactive dyes, and vat dyes can have one or more sulfonic acid groups incorporated into their molecular stmcture. The raw materials used for the manufacture of dyes are mainly aromatic hydrocarbons (67—74) and include ben2ene, toluene, naphthalene, anthracene, pyrene, phenol (qv), pyridine, and carba2ole. Anthraquinone sulfonic acid is an important dye intermediate and is prepared by sulfonation of anthraquinone using sulfur trioxide and sulfuric acid. 

Reactions other than those of the nucleophilic reactivity of alkyl sulfates iavolve reactions with hydrocarbons, thermal degradation, sulfonation, halogenation of the alkyl groups, and reduction of the sulfate groups. Aromatic hydrocarbons, eg, benzene and naphthalene, react with alkyl sulfates when cataly2ed by aluminum chloride to give Fhedel-Crafts-type alkylation product mixtures (59). Isobutane is readily alkylated by a dipropyl sulfate mixture from the reaction of propylene ia propane with sulfuric acid (60). 

Impurities can sometimes be removed by conversion to derivatives under conditions where the major component does not react or reacts much more slowly. For example, normal (straight-chain) paraffins can be freed from unsaturated and branched-chain components by taking advantage of the greater reactivity of the latter with chlorosulfonic acid or bromine. Similarly, the preferential nitration of aromatic hydrocarbons can be used to remove e.g. benzene or toluene from cyclohexane by shaking for several hours with a mixture of concentrated nitric acid (25%), sulfuric acid (58%), and water (17%). 

Both the chemical solubility and the electrical properties are consistent with those expected of a lightly polar polymer, whilst reactivity is consistent with that of a polymer containing hydrolysable carbonate ester linkages partially protected by aromatic hydrocarbon groupings. The influence of these factors on specific properties is amplified in subsequent sections. 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Comparison of localization energies has frequently been applied to prediction of the relative positional reactivity in polycyclic aromatic hydrocarbons. Simple HMO calculations have only marginal success. CNDO/2 and SCF calculations give results which show good correlation with experimental data on the rate of proton exchange. ... 

The polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenan-threne undergo electrophilic aromatic substitution and are generally more reactive than benzene. One reason is that the activation energy for formation of the c-complex is lower than for benzene because more of the initial resonance stabilization is retained in intermediates that have a fused benzene ring. 

Table 12.10. Relative Reactivities of Some Aromatic Hydrocarbons toward Oxygen"...

In the presence of certain ethers such as Me20, Me0CH2CH20Me or tetrahydrofuran, Na forms deep-green highly reactive paramagnetic adducts with polynuclear aromatic hydrocarbons such as naphthalene, phenanthrene, anthracene, etc. ... 

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