Chloro-bromo-benzenes

First-order nitrations. The kinetics of nitrations in solutions of acetyl nitrate in acetic anhydride were first investigated by Wibaut. He obtained evidence for a second-order rate law, but this was subsequently disproved. A more detailed study was made using benzene, toluene, chloro- and bromo-benzene. The rate of nitration of benzene was found to be of the first order in the concentration of aromatic and third order in the concentration of acetyl nitrate the latter conclusion disagrees with later work (see below). Nitration in solutions containing similar concentrations of acetyl nitrate in acetic acid was too slow to measure, but was accelerated slightly by the addition of more acetic anhydride. Similar solutions in carbon tetrachloride nitrated benzene too quickly, and the concentration of acetyl nitrate had to be reduced from 0-7 to o-i mol 1 to permit the observation of a rate similar to that which the more concentrated solution yields in acetic anhydride. 

Metalation. Benzene reacts with alkaH metal derivatives such as methyl or ethyUithium ia hydrocarbon solvents to produce phenyUithium [591 -51 -5], CgH Li, and methane or ethane. Chloro-, bromo-, or iodobenzene will react with magnesium metal ia ethereal solvents to produce phenyHnagnesium chloride [100-59-4], C H MgCl, bromide, oriodide (Grignard reagents) (32). 

Iodine can, however, be displaced cleanly with bromine<150) and with chlorine if iodine monochloride or sulfuryl chloride and benzoyl peroxide are used as chlorinating agents.reactivity ratios for the displacement of bromine by chlorine from substituted bromo-benzenes have been found to be in the order p-phenyl > o-methoxy > p-chloro > un-... 

The fluorination of other activated aromatic compounds, such as anisole and phenol, undergo monofluorination mainly in the ortho and para positions, whereas the fluorination of deactivated aromatics, such as nitrobenzene, trifluoromethylbenzene and benzoic acid, give predominantly the corresponding meta fluoro-derivatives which is consistent with a typical electrophilic substitution process. Also, fluoro-, chloro- and bromo-benzenes are deactivated with respect to benzene itself but are fluorinated preferentially in the ortho and para positions [139]. At higher temperatures, polychlorobenzenes undergo substitution and addition of fluorine to give chlorofluorocyclohexanes [136]. 

Reaction LH. (a) Action of Copper Powder on 2- and 4-mono-nitro-and 2 4-di-nitro-chloro- and -bromo-benzenes and their Homologues... 

The role of molecular dimensions is well demonstrated by complex formation with halogen-ated benzenes. 1 1 complexes may be prepared from chloro-, bromo-, and iodobenzenes but from chlorobenzene only witto-CD, from bromobenzene witb- andp-CDs, and from iodobenzene with P- andy-CDs. 

Ingold and Bird [2] investigated the rates of nitration of fluoro-, chloro-, bromo-and iodo-benzenes with acetyl nitrate Mid found that they could be arranged in the following order 1 0.15 0.033 0.030 ca. 0.18 (taking the relative rate of nitration... 

The percentage of the ortho product increases from fluorobenzene to iodo-benzene. We might have expected the amount to decrease as the size of the halide increases because of increased steric hindrance at the ortho position but this is clearly not the case. The series can be explained by the greater inductive effect of the more electronegative atoms (F, Cl) withdrawing electron density mostly from the ortho positions The relative rates follow a U-shaped sequence fluorobenzene nitrates most quickly (but not as fast as benzene), followed by iodo-, then chloro-, and then bromo-benzenes. This is a result of two opposing effects electron donation by conjugation and electron withdrawal by inductive effect... 

There is one very special feature of the benzyne mechanism. The triple bond could be attacked by nucleophiles at either end. This is of no consequence when we are dealing with bromo benzene as the products would be the same, but we can make the ends of the triple bond different and then we see something interesting, ortho-Chloro aryl ethers are easy to prepare by chlorination of the ether (Chapter 22). When these compounds are treated with NaNH2 in liquid ammonia, a single amine is formed in good yield. 

As seen from Fig. 5-11, although the three halobenzenes (points nos. 8, 10, and 11) have similar values of fir, when used as solvents they lead to different reaction rates, lodobenzene (no. 8), with the lowest value, gives the largest rate. This observation strongly suggests that the polarizability of the solvent is an important factor in stabilizing the dipolar activated complex of this reaction. This was confirmed by Reinheimer el al. [57], who studied some Menschutkin reactions in benzene and its chloro, bromo, and iodo derivatives. They showed that the rate of the reaction increases with increasing polarizability of the solvent. 

The stabilization of hydroxycyclohexadienyl radicals by -CHj and halogen groups is apparent from the bathochromic shifts in the spectra of OH adducts of chloro- and bromo-benzenes (325 nm) and toluene (317 nm) relative to benzene (313 nm). This is in accordance with the work of Chen and Schuler where the phenyl substituent leading to a more intense and a strong red shift in the transient absorption spectra was seen in the reaction of OH radical with biphenyl. 

Jaworski et al. [186] found that the reaction constants from the Hammett equation, in the case of the irreversible electroreduction of substituted chloro- and bromo-benzenes in a number of organic solvents, are linearly dependent on the logarithms of the solvent longitudinal relaxation times. 

Other radiolysis studies on chlorine and bromine compounds have included ethyl chloride , 1,3-dichloropropane , allyl halides , trichloroethylene and hexachlorobutadiene , hexachlorocyclopentadiene and other unsaturated chlorinated hydrocarbons - , hexachlorocyclohexane , chloro- and bromo-benzenes - - , phenyl ethyl bromides , polyvinyl chloride - - and... 

Benzene Toluene Ethyl- benzene Chloro- benzene Bromo- benzene... 

T,°C Benzene V9 Toluene V9 Ethyl- benzene V9 Chloro- benzene V9 Bromo- benzene V9... 

Besides MS detection, identification of unknown peaks in GC routine analysis of environmental samples can be aided by the use of correlations between physicochemical parameters and structure of the analytes to predict the retention times. The correlation between the boiling points and the retention times of chloro- and bromo-benzenes and of some chloro- and nitro-substituted phenols was investigated for nonpolar capillary columns and allowed tentative identification of many compounds belonging to these analogous series. ... 

Another example that has been studied in detail is that of the halogenated phenols. Chloro-, bromo- and iodo-phenols, like their benzene analogues, react rapidly with solvated electrons to undergo dehalogenation and produce hydroxyphenyl radicals (equation 5). 

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