Characterisation of aromatic hydrocarbons

Unlike aliphatic hydrocarbons, aromatic hydrocarbons can be sul-phonated and nitrated they also form characteristic molecular compounds with picric acid, styphnic acid and 1 3 5-trinitrobenzene. Many of the reactions of aromatic hydrocarbons will be evident from the following discussion of crystalline derivatives suitable for their characterisation. 

Picrates, Many aromatic hydrocarbons (and other classes of organic compounds) form molecular compounds with picric acid, for example, naphthalene picrate CioHg.CgH2(N02)30H. Some picrates, e.g., anthracene picrate, are so unstable as to be decomposed by many, particularly hydroxylic, solvents they therefore cannot be easily recrystaUised. Their preparation may be accomplished in such non-hydroxylic solvents as chloroform, benzene or ether. The picrates of hydrocarbons can be readily separated into their constituents by warming with dilute ammonia solution and filtering (if the hydrocarbon is a solid) through a moist filter paper. The filtrate contains the picric acid as the ammonium salt, and the hydrocarbon is left on the filter paper. 

Picrates are usually prepared by mixing solutions of equivalent quantities of the two components in the minimum volume of rectified spirit and allowing to cool the derivative separates in a crystalline condition. It is filtered off, washed with a little ether, and pressed on a porous tUe. If the picrate is stable, it is recrystaUised from alcohol, ethyl acetate or ether. 

The foUowing are typical experimental details for the preparation of naphthalene picrate. Dissolve 0 -1 g. of naphthalene and 0-2 g. of picric acid separately in the minimum volume of hot rectified spirit (about 2 ml.), mix the solutions and allow to cool. FUter and wash with 2 ml. of alcohol. RecrystaUise from hot alcohol, ethyl acetate or ether. 

Styphnates. Aromatic hydrocarbons (and also some amines and heterocyclic bases) form 1 1-addition products with styphnic acid (2 4 6-trinitroresorcinol), 

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. 

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The compound is employed for the characterisation of aromatic hydrocarbons (compare Section IV,9), ethers and amines. 

Aroylation of an aromatic system by reaction with phthalic anhydride under Friedel-Crafts conditions yields the o-aroylbenzoic acid. These readily available compounds have characteristic melting points which make them useful as derivatives in the characterisation of aromatic hydrocarbons and of aryl halides (Section 9.6.3, p. 1238). 

In addition to inorganic radicals, which profoundly modify the properties of a paraflSn hydrocarbon residue, there is a whole series of organic groupings which are distinguished by exceptional reactivity, for example, the ethylene and acetylene groupings, and the phenyl and naphthyl radicals. Thus the characterisation of unsaturated hydrocarbons and their derivatives, e.g., the aromatic compounds, becomes possible. 

Phthalate esters C(H (COOR)2 are well-characterised, aoderately polar liquid phases [8]. As might be expected, the polarity of the phases declines as the alkyl (R) group increases in size, while their volatility decreases. Hi volatility compared to other available liquid phases has reduced their importance in recent years. Tetraunsaturated hydrocarbons and aromatic hydrocarbons (electron-donor solutes) Iqf... 

In another AT study, Terrado et al. [15] characterised pollution patterns in different parts of the Ebro catchment. In the upper part of the Ebro, pollution was found to be mainly in the form of heavy metals (Zn, Cu, Cr, Pb, Cd and Hg), polycyclic aromatic hydrocarbons (PAHs), hexachlorocyclohexanes (HCHs) and trichlorobenzenes (TCBs). Etrophic conditions were also found. Pollution was found to source mainly from industry and urbanisation. The central Ebro was characterised by nutrient pollution such as the accumulation of Ca, Na, Mg and K, which highlighted the importance of salinisation effects from intensive irrigation and soils with high salt content. In the lower Ebro, organic [DDTs, hexachlorobenzene (HCB) and hexachlorobutadiene (HCBu)] and heavy metal (Hg, Cd, Zn and As) contamination was found to derive mainly from industrial and agricultural activities. 

With stringent precautions to avoid the presence of water, polycyclic aromatic hydrocarbons show two one-electron reversible waves on cyclic voltammetry in dimethylformamide (Table 7.1). These are due to sequential one-electron additions to the lowest unoccupied molecular n-orbital [1]. Hydrocarbons with a single benzene ring are reduced at very negative potentials outside the accessible range in this solvent. Radical-anions of polycyclic aromatic hydrocarbons [2] and also alkyl benzenes [3] were first obtained by the action of alkali metals on a solution of the hydrocarbon in tetrahydrofuran. They have been well characterised by esr spectroscopy. The radical-anions form coloured solutions with absorption bands at longer wavelength than the parent hydrocarbon [4,5]. 

Characterisation of the aromatic profile in commercial guava essence and fresh fruit puree extracted with solvent yielded a total of 51 components [29]. Commercial essence was shown to be rich in components with low molecular weight, especially alcohols, esters, and aldehydes, whereas in the fresh fruit puree terpenic hydrocarbons and 3-hydroxy-2-butanone were the most abundant components. 

Haloform reaction, 297 Halogenated aromatic hydrocarbons, 533 reactions and characterisation of, 542 ... 

For characterisation, aromatic hydrocarbons can be sulphonated, chloro-sulphonated, carboxybenzoylated and nitrated. Polynuclear aromatic hydrocarbons, and many of their derivatives, yield crystalline adducts with picric acid, styphnic acid, 1,3,5-trinitrobenzene and 2,4,7-trinitrofluorenone. 

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. 

In addition to the well-characterised groups of contaminants such as PCBs and polycyclic aromatic hydrocarbons (PAHs), there are hundreds of thousands of other organic chemicals that are emitted by industrial processes and which have the potential to enter the food chain by a variety of routes. It would be... 

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