Molecular chlorination

Addition of up to a tenfold molar excess of hydrogen chloride did not appreciably alter the reaction rate. Orton and Bradfield227 obtained the same kinetic form for the chlorination of formanilide, acetanilide, benzanilide, and benzene-sulphonanilide in 99 % aqueous acetic acid at 20 °C reaction rates were higher than previously obtained with the less aqueous medium, and this medium effect has been subsequently found to be general. 

SECOND-ORDER RATE COEFFICIENTS (I04/c2) FOR REACTION OF ArH WITHClj IN VARIOUSSOLVENTS235 

ArH Hexane Chloroform Dichloroethane Nitrobenzene 99% aq. acetic acid 

Dewar and Mole236 derived second-order rate coefficients for chlorination at 25 °C of benzene (6xl0-7), diphenyl (6.9 xlO-4), naphthalene (6.3 xlO-2), phenanthrene (2.9xl0 1) and triphenylene (2.2xlO-2) in Analar acetic acid and of diphenyl (9 x 10-7), naphthalene (1.9 x 10-4), phenanthrene (1.3 x 10-3), 

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Chang J P, Arnold J C, Zau G C H, Shin H-S and Sawin H H 1997 Kinetic study of low energy ion-enhanced plasma etching of polysilicon with atomic/molecular chlorine J. Vac. Sc/. Technol. A 15 1853-63... 

His data suggested values for y of — 12 and — 6 kcal mol for molecular chlorination and nitration respectively, indicating that the transition states in nitration resemble the reactants more than do the transition states in chlorination. 

Step 3 Reaction of methyl radical with molecular chlorine... 

Hydrogen peroxide—hydrochloric acid reagent converts 2-aminoben2otrifluoride to 2-amino-5-chloroben2otrifluoride [121 -50-6], a dye intermediate (Cl A2oic Dia2o Component 17), without contamination by the 3-chloro isomer such as is observed with molecular chlorine (CI2) (302). 

The acid addition shifts the chlotine solution equihbtium to favor molecular chlorine. The hypochlorous acid—chlorine equihbria is... 

In the case of phenazine, substitution in the hetero ring is clearly not possible without complete disruption of the aromatic character of the molecule. Like pyrazine and quinoxa-line, phenazine is very resistant towards the usual electrophilic reagents employed in aromatic substitution reactions and substituted phenazines are generally prepared by a modification of one of the synthetic routes employed in their construction from monocyclic precursors. However, a limited range of substitution reactions has been reported. Thus, phenazine has been chlorinated in acid solution with molecular chlorine to yield the 1-chloro, 1,4-dichloro, 1,4,6-trichloro and 1,4,6,9-tetrachloro derivatives, whose gross structures have been proven by independent synthesis (53G327). 

In the photolysis of difiuorodiazirine (218) a singlet carbene was also observed (65JA758). Reactions of the difiuorocarbene were especially studied with partners which are too reactive to be used in the presence of conventional carbene precursors, such as molecular chlorine and iodine, dinitrogen tetroxide, nitryl chloride, carboxylic acids and sulfonic acids. Thus chlorine, trifiuoroacetic acid and trifiuoromethanesulfonic acid reacted with difiuorodiazirine under the conditions of its photolysis to form compounds (237)-(239) (64JHC233). 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

Molecular chlorine is believed to be the active electrophile in uncatalyzed chlorination of aromatic compounds. Simple second-order kinetics are observed in acetic acid. The reaction is much slower in nonpolar solvents such as dichloromethane and carbon tetrachloride. Chlorination in nonpolar solvents is catalyzed by added acid. The catalysis by acids is probably the result of assistance by proton transfer during the cleavage of the Cl-Cl bond. ... 

The reactivity pattern of some organogold derivatives is of interest. Thus, complex 90 oxidatively adds molecular chlorine, bromine, or iodine to yield the gold(III) product 103 (97JOM(544)91, 98JOM(552)69). The latter is... 

Despite the relative simplicity of the kinetics of molecular chlorination, there has so far been only one measurement of the rate coefficient with a heterocyclic compound and the need for more work in this area is indicated. Marino265 found that chlorination of thiophene by chlorine in acetic acid at 25 °C gave the second-order rate coefficient of 10.0 1.5, so that thiophene is 1.7 x 109 times as reactive as benzene in this reaction and this large rate spread is clearly consistent with the neutral and hence relatively unreactive electrophile. 

Kinetic studies have been carried out using the 1 1-complex iodobenzene dichloride as a source of molecular chlorine. In acetic acid solutions, the dissociation of this complex is slower than the rate of halogenation of reactive aromatics such as mesitylene or pentamethylbenzene, consequently the rate of chlorination of these is independent of the aromatic concentration. Thus at 25.2 °C first-order chlorination rate coefficients were obtained, being approximately 0.2 x 10-3 whilst the first-order dissociation rate coefficient was 0.16 xlO-3 from measurements at 25.2 and 45.6 °C the corresponding activation energies... 

Relative rates and partial rate factors have been determined for the chlorination of some aromatics by chlorine acetate in 76 % aqueous acetic acid at 25 °C209 these are given in Table 65. The spread of rates is, therefore, smaller than is found with molecular chlorine and this is entirely consistent with the lower reactivity of the latter reagent. 

It then follows that at low bromine concentrations this latter process is less likely, consequently the kinetic order is reduced. More ionic media will facilitate equilibrium (137) without the need for intervention of equilibrium (139) and vice versa, so that the observed variation in the kinetic order with this condition then follows. The absence of high kinetic orders in molecular chlorination also becomes rationalised since the C1J ion is not as stable as BrJ. 

Evidence for molecular chlorine or bromine as the attacking species in these cases is that acids, bases, and other ions, especially chloride ion, accelerate the rate about equally, though if chlorine dissociated into Cl" and Cl , the addition of chloride should decrease the rate and the addition of acids should increase it. Compound 27 has been detected spectrally in the aqueous bromination of phenol. ... 

C05-0089. Sodium metal reacts with molecular chlorine gas to form sodium chloride. A closed container of volume 3.00 X 10 mL contains chlorine gas at 27 °C and 1.25 X 10 torr. Then 6.90 g of solid sodium is introduced, and the reaction goes to completion. What is the final pressure if the temperature rises to 47 °C ... 

Chlorine molecules must be broken apart into chlorine atoms. Table gives the bond energy BE) of molecular chlorine, 240 kJ/mol. We need — mole of CI2 to form 1 mole of NaCl, so the energy... 

The reaction of sodium metal with molecular chlorine gas to produce solid sodium chloride can be analyzed by breaking the overall process into a series of steps involving ions in the gas phase. 

As mentioned earlier, molecular chlorine has long been one of the leading industrial chemicals. Table 21-1 provides a summary of the industrial importance of chlorine. 

In addition to making organic chlorine compounds, a significant fraction of CI2 production is used to make inorganic halides. One important use, described in Chapter 20, is in the metallurgy of titanium, in which molecular chlorine is used to convert Ti02 into TiCl4, which is easy to purify by distillation. 

Ti02 + C + 2 CI2 TiCl4 + CO2 In similar fashion that is described earlier in this chapter, easily-purified SiCl4 is produced from molecular chlorine and Si02 in the presence of coke ... 

As Table 21-1 suggests, molecular chlorine is a tremendously versatile industrial chemical. This element is a leading industrial chemical because of this versatility rather than any single application, although polymers account for about one third of its uses. In recent years, however, the industrial use of chlorine has come under strong attack from many environmentally conscious groups. One major reason is that dioxins, one class of by-products of chlorine reactions, have a very detrimental effect on biosystems. The controversy over industrial chlorine is described in our Chemistry and the Environment Box on page 936. 

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