Haloaromatic compounds

The first step of the procedure illustrates a general way of preparing aryl iert-butyl ethers.416 The second step is the best way to prepare 2-hydroxythiophene, inasmuch as the yield is good and ter/-butyl perbenzoate is a readily available perester that is relatively stable. The same procedure has been used to convert several other haloaromatic compounds to hydroxyaromatic compounds in good yield4 and is probably quite general. 

In saline sediments aliphatic and polyaromatic hydrocarbons carbohydrates haloaromatic compounds chlorophenols basic nitrogen compounds various organosulphur compounds mixtures of organic compounds total sulphur arsenic and organic compounds of lead, mercury and tin. ... 

This technique has been applied to the determination of aromatic hydrocarbons, alcohols, aldehydes, ketones, chloroaliphatic compounds, haloaromatic compounds, acrylonitrile, acetonitrile, mixtures of organic compounds and tetrahydrothiophene in soils, chloroaliphatic and haloaromatic compounds and organotin compounds in non-saline sediments, and organotin compounds in saline sediments. 

For more volatile compounds in soils, such as aromatic hydrocarbons, alcohols, aldehydes, ketones, chloroaliphatic hydrocarbons, haloaromatic hydrocarbons, acetonitrile, acrylonitrile and mixtures of organic compounds a combination of gas chromatography with purge and trap analysis is extremely useful. Pyrolysis gas chromatography has also found several applications, heteroaromatic hydrocarbons, polyaromatic hydrocarbons, polymers and haloaromatic compounds and this technique has been coupled with mass spectrometry, (aliphatic and aromatic hydrocarbons and mixtures of organic compounds). 

Suflita.J.M., Horowitz, A., Shelton, D.R., andTiedie, J.M. Dehalogenation a novel pathway for the anaerobic biodegradation of haloaromatic compounds. Science (Washington, DC), 218 1115-1117,1982. 

Aryltriazenes can also be decomposed by hydrogen fluoride in organic solution after extraction from their aqueous mother phase. In this case, hydrogen fluoride can be used in small excess but the nature of the solvent is crucial for example, tetrahydrofuran gives complex mixtures, dichloromethane promotes radical reactions (dimerizations, reductions) and acetic acid favors triazene decomposition before fluorination. Aromatic and haloaromatic compounds seem to be the best solvents.283 Such a technique, especially suited for the rapid introduction of an 18F atom, has been employed to produce [ 8F]haloperidol (3), the specific receptors of which have been localized in the brain by positron emission transaxial tomography.298... 

Molecular Volume-Kow Relationships Relationships between Kow and different volume parameters have been reported. Leo et al. [41] compare correlations with Bondi and with CPK volume for two classes of apolar molecules (1) alkanes and alkylsilanes, and (2) perhalogenated alkanes and aromatic and haloaromatic compounds. Further, these authors discuss analogous correlations for alkanols and alkylphenols. 

There has been a review of die effects of high pressure on the substitution reactions of amines witii haloaromatic compounds, including polyhalobenzenes.17 Nucleophilic substiditions by amines often proceed readily hi dimethyl sulfoxide (DMSO). The pKa values, hi DMSO, have been reported for some ammonium ions derived from amines widely used as nucleophiles in 5nAt reactions.18 Correlations have been established19 between die oxidation potentials and the basicities of some arylamhie and diarylamine anions and die rate constants for dieir reactions with aiyl halides in DMSO. 

Polychlorobiphenyls (PCBs) have been studied under the UV/Ti02 photo-catalytic process to assess the use of photocatalytic oxidation in the treatment of PCBs. These organic microcontaminants are biologically refractory therefore, AOPs were studied for the remediation of PCBs. Haloaliphatic and haloaromatic compounds have been found to be mineralized to C02 and HC1 by UV/Ti02 process (Mathews, 1988). 

Aromatic compounds Nucleophilic displacement of halide ion from haloaromatic compounds containing other electronegative substituents is well known this process has been widely exploited for the synthesis of fluorinated aromatic compounds (Table 2.1) and is discussed later (see Chapter 9, Section 11). 

The radical nucleophilic substitution is perfectly suited for tandem reactions [180]. Recent examples have been reported by the Rossi group (Scheme 66). Dihydrobenzofuranes and dihydroindoles substituted at the 3-position were prepared from ortho-functionalized haloaromatic compounds in high yields [181]. The nucleophiles involved in the initial electron transfer and subsequent coupling are varied. In particular, starting form naphthyl derivative 210, phosphinyl anions lead to tricyclic phosphine oxide 211 (after oxidation) in 98% yield. 

Radical anions of haloaromatic compounds are proposed to be intermediates in different type of reactions. Their fragmentation rates, determined electrochemically [300] or by pulse radiolysis [301] range from lO " s for phenyl halides to 10 s for some halonitrobenzenes. The rate of the reaction for some aryl hahde radical anions is too high to be measured electrochemically, the fragmentation of more stable radical anions such as those of 1-bromo- and 1-iodoanthraquinone [302], p-[303] and m-bromo- [304] and p- [303] and w-chloronitrobenzenes [304] occurs at considerably lower rates and the reaction is favored from their photoexcited state. Aryl halide radical anions may present a-n orbital isomerism depending on the orbital symmetry of their singly occupied molecular orbital [305], a proposal derived from theoretical and experimental evidences [306]. The isomerism is possible... 

A process has now been discovered from the conversion -d ai omatic compounds to hydroxy aromatic compounds which does not isolate an intermediate haloaromatic compound and which permits... 

Haloaromatic compounds. Aryl chlorides are obtained by heating aryl chloroformates with AICI3 at 200°. For the preparation of the fluorides the chloroformates are treated with excess anhydrous HF (liquid-phase fluorination-decarboxylation). 

It had been well known since the pioneering works of Wittig5 and Roberts6 that nucleophilic substitutions of haloaromatic compounds, nonactivated by electro withdrawing group, are feasible by means of elimination-additions. 

Picosecond spectroscopy enables one to observe ultrafast events in great detail as a reaction evolves. Most picosecond laser systems currently rely on optical multichannel detectors (OMCDs) as a means by which spectra of transient species and states are recorded and their formation and decay kinetics measured. In this paper, we describe some early optical detection methods used to obtain picosecond spectroscopic data. Also we present examples of the application of picosecond absorption and emission spectroscopy to such mechanistic problems as the photodissociation of haloaromatic compounds, the visual transduction process, and inter-molecular photoinitiated electron transfer. 

Photodissociation of Haloaromatic Compounds. Picosecond emission spectroscopy was recently employed (27,28) to study the photodissociation of haloaromatics and the subsequent fluorescence of the formed radicals. An earlier paper by Huppert et. al. (29) describes the predissociative pathways of the haloaromatics following excitation at 266 and 355 nm. What makes the more recent work different Is that, after the initial excitation by a 266-nm 25-ps FWHM pulse, a second 355-nm 25-ps pulse irradiates the same area of the sample. With this two-color fluorescence technique they were able to observe clearly the reaction dynamics of the radicals which were formed by the initial light pulse. 

When excited at 266 nm, all four compounds exhibited a broad blue fluorescence centered at a400 nm with lifetimes ranging from 100 to 3500 ps. Excitation with a second 355-nm laser pulse which was delayed relative to the first 266-nm pulse produced a fluorescence centered at V600 nm with a lifetime of vlO ns. Since the emission spectra (Figure 7) obtained by excitation of l-(chloromethyl)naphthalene and l-(bromomethyl)-naphthalene are superimposable in the region from 550 to 750 nm, this red fluorescence was attributed to the 1-naphthylmethyl radical. In a similar manner, the red fluorescence resulting from two-color excitation of 2-(chloromethyl)naphthalene and 2-(bromomethyl)naphthalene was attributed to the 2-naphthylmethyl radical. This two-color fluorescence technique used in conjunction with OMCDs has proven to be a powerful tool in the study of photodissociation of haloaromatic compounds. 

The reaction between haloaromatic compounds (R<)>X and ( )X R = -F, -Cl, -CH3, -CF3 X = -Cl, -Br, -I) was studied in gas phase (400 C, atmospheric pressure) in a flow microreactor in the presence of a 2 wt% Cu-HZSM-5 catalyst. In most cases a highly selective ipso exchange of halogen atoms was obtained except with fluorine which was completely unreactive. The kinetic study of the reaction between bromobenzene and 3-chlorofluorobenzene showed that the bromo compounds are more strongly adsorbed than chloro compounds and inhibit the reaction. The reactivity sequence of the halogen leaving-... 

This reaction could be of interest for substituting chlorine for bromine or for iodine in certain aromatic compounds. Indeed, in the halogenation by aromatic electrophilic substitution, the chlorinated reagents are more effective [2] and cheaper than brominated and iodinated reagents. However, bromo or iodo compounds are generally more interesting if the haloaromatic compounds are intended to be used in subsequent reactions because they are generally more reactive [2],... 

This possibility is indicated in the study of the ablation of PMMA and PS doped with haloaromatic compounds (iodonaphthalene (NapI), iodo-phenanthrene (PhenI)). Experimentally these dopants offer the advantage that they exhibit negligible fluorescence, whereas their products are moderate or strong emitters. Thus, product formation can be conveniently monitored via laser-induced fluorescence (LIF). It is noted that the study has focused on the characterization of the species/products that remain in the substrate upon ablation. Thus, the experiment is of the pump-probe type, in which pump pulses at a particular wavelength (2=308, 248 or 193 nm) are used to irradiate the films, and laser pulses of very low fluence are employed to induce photoproduct fluorescence (FLaser<3 mj cm-2, probing effected always at 2=248 nm). Detailed information on the experimental setup and procedures is provided elsewhere [83-85]. 

Fig. 13. Arynes from haloaromatic compounds in the presence of NaNH2-...

Adrian NR, Lowder A. (1999). Biodegradation of RDX and HMX by a methanogenic enrichment culture. In Bioremediation of Nitroaromatic and Haloaromatic Compounds (ed. BC Alleman). Columbus, OH Battelle, pp. 1-6. 

---