Fluoro- and chlorobenzene

FIGURE 3. Plots of normal boiling point (Tbp, °C) versus molecular weight (MW, g mol ) for some fluoro- and chlorobenzenes, taken separately... 

Datka et al. [711] applied besides benzene many of the above-listed probes (including toluene, p-xylene,fluoro- and chlorobenzene, and -hexane) and concluded from the splitting of the shifted OH bands, resulting in four submaxima in the case of, e.g., n-hexane/H, Na-ZSM-5, that the acid OH groups are markedly heterogeneous, exhibiting different acid strengths. 

Hydroxylation of benzonitrile and fluoro- and chlorobenzenes was realized with N O over ZSM-5 zeolite [101]. In this system, chlorobenzeneprodncedchlorophenol(o m=28 72) with 58% selectivity at 23% substrate conversion [101a], while benzonitrile gave hydroxybenzonitrile... 

Substrate inactivated toward photosubstitution, viz. monosubstituted arenes such as fluoro- and chlorobenzene, anisole, benzonitrile, and related compounds. 

Similar decomposition is observed in p-bromoacetophenone, o-bromo-, p-bromo, and p,p -dibromobenzophenone, and p-iodobenzophenone44 but not in the fluoro- and chloro-substituted compounds. This order of reactivity follows the bond dissociation energies for aromatic halides which are about 90 kcal/mole for chlorobenzene, 70 kcal/mole for bromobenzene, and 60 kcal/ mole for iodobenzene. The lowest-lying triplet of p-bromoacetophenone is 71.2 kcal45 while that of the substituted benzophenones is slightly lower since benzophenone itself has a lower triplet energy than acetophenone. p,p Dibromobenzophenone was the least reactive of the compounds that photoeliminated halogen atoms. 

The electrochemical fluorination (Simons cell anhydrous HF electrolyte, sometimes with the addition of NaF to improve conductivity) of benzene, fluoro- and chloro-benzene, w-dichlorobenzene, anisole, o-chloroanisole, thiophenol, p-chloro-thiophenol, iw-thiocresol, 2-chloropyridine, and 3-chloropyridine has been examined in detail (see also p. 212). Benzene and fluorobenzene give mainly dodecafluoro-cyclohexane, but a low yield of chloroundecafluorocyclohexane is obtained from chlorobenzene, and the ethers give fully saturated perfluoroethers with some cleaved products cleavage of the thiophenols leads to extensive formation of SFe. Mixtures of perfluoro-alicyclic and -aliphatic tertiary amines are obtained by similar fluorination of AW-dialkylanilines [PhNEta and PhNMeR (R = Et, Pr, or Bu )] and the three A A -dimethyltoluidines. The relatively new technique (Vol. 2, p. 353) of controlled fluorination at a platinum anode in acetonitrile con-+ -... 

While preparing benzonorbornadienes by the Diels-Alder cycloaddition of substituted benzyne intermediates with furan. Caster and coworkers discovered significantly differentregiochemistries be tween bromobenzene 121 and chlorobenzene 124 in the n-BuIi induced metallation step (Schemes 12.40 and 12.41) [72]. In the case of bromobenzene 121, benzyne intermediate 122 was generated via bromine-Uthium exchange, followed by elimination of UF trapping by furan then afforded 6-fluoro-9-oxabenzonorbornadiene (123). In contrast, aryne 125 was produced... 

Deprotonation. The o-metalation of aromatic halides is a weU-established route to arynes. In cases where the starting material contains two different halogens in 1,3-positions, the deprotonation reaction occurs at 2-position and then the elimination may lead to two different aryne derivatives depending on the halide anion that suffers the elimination reaction. In this context, the solvent of the reaction plays a crucial role. Thus, for example, in the case of 3-fluoro-l-chlorobenzene, the... 

Forlani and coworkers184 determined that the magnitude of k was found to increase linearly with nucleophile concentration for the reaction of picryl fluoride with 2-hydroxypyridine in chlorobenzene, and k E/k D = 1.5 for mono-deutero-2-hydroxypiridine was observed184. Since isotope effects are usually small in S/yAr in apolar solvents1 the authors attributed the isotope effect to the formation of a substrate-catalyst molecular complex. They obtained a value of k E/kp, D = 1-75 for the ratio of the association constants, hAd- When the substrate was picryl chloride, the slight increase of k with nucleophile concentration was interpreted in terms of Scheme 6 giving a value of K = 2.9 1 identical with that for the fluoro-substrate (3.0 1). 

The products of the electrochemical perfluorination of aromatic and heteroaromatic compounds are the corresponding perfluorinated cyclic and heterocyclic alkanes.28 and also per-fluorinated derivatives of the heteroaromatic compounds. Perfluorocyclohexane is the principal product from the electrochemical fluorination of benzene and fluorobenzene. Chloro derivatives of perfluorocyclohexane are produced from chlorobenzenes. Anisoles give fully saturated per-fluoro ethers, together with cleavage products. Extensive cleavage is observed in the fluorination of benzenethiols. Chloropyridines, fluorocarbons and sulfur hexafluoride or nitrogen trifluoride are characteristic byproducts from the above scries of reactions. 

Phenols are present as phenoxide ions in Na/NH3 solutions. Therefore, the transfer of an electron into a rr-antibonding orbital of a phenoxide ion should be slower than the transfer to benzene. Both chlorophenols and fluorophenols are dehalogenated more slowly than chlorobenzenes or fluo-robenzenes [24], Fluorophenoxide ions were more difficult to defluorinate. Thus, 4-fluoro-2-chlorophenol could be converted selectively to 4-fluorophe-nol in Na/NH3 [24], In contrast, chlorofluorobenzenes lost both Cl and F at close to diffusion-controlled rates [24], indicating that using a stoichiometric deficiency of Na would not give any selectivity to fluorobenzene. 

Derivatives of chlorobenzene are of course the most important, as they are the most readily available. For certain syntheses nitro derivatives of fluoro- or bromo-benzene are used, They may be of value sometimes, since the former are less reactive and the latter more reactive than chlorobenzene derivatives. For example, recently 2,4-dinitrofluorobenzene has been widely applied in the elucidation of the structure of peptides (Sanger [1]). 

The first evidence for the existence of a stable iodine cation was obtained by Masson (2) in 1938. He postulated the presence of Ig" and Ig in solutions of iodine and iodic acid in sulfuric acid in order to explain the stoichiometry of the reaction of such solutions with chlorobenzene to form both iodo and iodoso derivatives. Later, Symons and co-workers (6) gave conductometric evidence for I3 formed from iodic acid and iodine in 100% sulfuric acid and suggested that Ig" " may be formed on the basis of changes in the UV and visible spectra when iodine is added to la solutions. Gillespie and co-workers (7) on the basis of detailed conductometric and cryoscopic measurements confirmed that Ig is formed from HlOg and Ig in 100% sulfuric acid according to Eq. (1). The Ig cation may also be prepared in fluoro-... 

The combination of boron trifluoride etherate and tert-butyl nitrite,17 in place of the conventional conditions (NaNC>2, HC1, HBF4), followed by decomposition of the diazonium salt in chlorobenzene afforded 5-fluorobenzo[c]phenanthrene (ll).l7b Nitrosonium tetrafluoroborate has been used to diazotize 8-amino-1-tetralone (12) en route to 8-fluoro- 1-tetralone (13).18... 

Andon RJL, Counsel JF, Hales JL, Lees EB, Martin JF (1968) Thermodynamic properties of fluorine compounds. Part VII. Heat capacity and entropy of penta fluoro chlorobenzene and pentailuorophenol. J. Chem. Soc. A 2357... 

In addition to alkyl-substituted aromatics, halo-substituted substrates were investigated, such as chlorobenzene and fluorobenzene, and were observed to produce moderate yields of the mono-hydroxylated product. As expected, the halo-substituted substrates produced lower product yields than the alkyl-substituted substrates due the deactivating effect of the chloro- and fluoro-substituents. In the case of fluorobenzene, the product distribution consisted of predominately the p-hydroxylated product (>94%), whereas with chlorobenzene a larger proportion of the o-substituted and m-substituted products were formed. 

Concentrated sulfuric acid will liberate B2Hg from NaBHi in the absence of a solvent or (with advantage) in a chlorobenzene (44) however, it has been shown that partial reduction of the acid to sulfur dioxide and even to hydrogen sulfide occvirs, although this difficulty can be avoided if the sulfuric acid is replaced by meth2uiesulfonic acid (325) Alternatively it is possible to utilize concentrated phosphoric acid (40-70% yield with KBHij) (78, 236) or anhydrous fluoro-phosphoric acid (79% yield with under vacuum)... 

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