600-06-6

The effect of benzene and a wide variety of substituted benzenes upon the photochlorination of 2,3-dimethylbutane (DMB) has been investigated under standard conditions (0.15 M DMB, 2.0 M arene in CCl4 at room temperature).The standard selectivities, Sstdm, are given by the product ratios / under these conditions.For arenes which are less electron rich than benzene, log Sstdm values decrease monotonically with the increase in the arenes' ionization potentials and with the decrease in the arenes' ?-basicities.For arenes which are moreelectron rich than benzene, Sstdm values are greater than for benzene only for the mono- and dialkylated benzenes and 1,3,5-tri-tert-butylbenzene.Chlorination in the presence of trimethylbenzenes, more highly methylated benzenes, and anisole afforded less selective mixtures of DMB chlorides than benzene.The reduced selectivities of the most electron-rich arenes is attributed to the low reactivities of their Cl./arene ?-complexes and the low yields of DMB chlorides, much of which are formed by hydrogen abstraction by the free Cl. atom.Ipso substitution occurs with bromo- and iodobenzene and these two arenes are more selective than would be predicted.It has also been demonstrated by laser flash photolysis that Cl./arene and Br./arene ?-complexes will appear to react with O2 only if the arene contains substituents which can donate hydrogen to the halogen atom and/or complex, viz., CH3, C(CH3)3, and OCH3 substituents for chlorine but only CH3 and OCH3 for bromine.This apparent reaction is due to interference by O2 in the process which otherwise would very rapidly "regenerate" the halogen atom/arene complex followingsuch hydrogen abstraction.

The chlorination of 2,3-dimethylbutane and cyclohexane by N-chloropiperidine and N-chloro-2,2,6,6-tetramethylpiperidine was studied.The rate of abstraction of the hydrogen atom from various positions is determined mainly by the dissociation energies of the respective C-H bonds.The higher reactivity of the R2CH2 group compared with R3CH for the last reagent is due to the significant sensitivity of the radical to steric factors.In the absence of highly reactive substrates N-chloro-2,2,6,6-tetramethylpiperidine dissociates through β-fragmentation of the radical-cation.

The free-radical chlorination of alkanes produces polychlorides even at low conversions. These are formed by reaction of chlorine atom/monochloride (or dichloride) geminate pairs. This process has been studied in detail in various solvent systems, and a kinetic scheme has been proposed. Deviations from this scheme have been rationalized as being due to competition between monochloride rotation and reaction of the chlorine atom with reactive molecules in the 'cage walls' surrounding the chlorine atom/chloride geminate pair. Analysis of the dichloride products supports the suggestion that monochloride rotation is not completely 'free' within the lifetime of the geminate pair.

The factors affecting changes in the measured selectivity for the photochlorination of 2,3-dimethylbutane (DMB) in alkane solvents and in CCl4 have been identified.At low chlorine concentrations in CCl4 as solvent, the selectivities, Sm = /, are dramatically enhanced because of CCl4 and Cl3C-radical participation in the overall chain process.In the absence of CCl4, the selectivities may also increase at very low chlorine concentrations because residual O2 and peroxyl radicals participate in the overall chain process.In the absence of either of the above phenomena, Sm was calculated to be 0.64, a value that compares well with a value of 0.62, which we have measured in the gas phase.

It is demonstrated that photohalogenation of low reactivity substrates with BrCl occurs mainly with Cl. selectivity.With tertiary or benzylic hydrogens in the substrate, mainly Br. selectivity is observed.These observations are rationalized, taking into account the relative concentrations of halogen atoms and their respective rates of hydrogen abstractions.The resultant radicals react with BrCl to make (RBr/RCl) in ratios between 1 and 15.

The concept of radical reactivity mediated by solvation has rested mainly on the alteration of Cl. properties by aromatic solvents.For this reason, the full scope of the benzene/Cl. system has been reexamined to evaluate the discription of that system based largely on a ?-complex (solvation).At the present time, the ?-complex description rests narrowly on the assignment of a 490-nm absorption, which, even if correct, could not provide an unambiguous structure assignment.Results are presenteed which described the selectivities in alkane substitutions as a function of the concentrations of both benzene and the alkane.Selectivities increase with decreasing alkane concentrations, reaching a plateau below 0.1 M alkane.The change in selectivity is the result of variable contributions of both a low- and a high-selectivity intermediate, LSI and HSI, respectively.The observed selectivity at a given and is the consequence of a unique / ratio.A range of substrates and their effect of DMB selectivity were studied, and from these results details regarding the chemistry of the HSI were extracted.Several features of the LSI/HSI equilibrating system are realized. (1) Reaction of alkyl radicals with Cl2 in benzene produces the LSI, (2) the LSI does not exhibit the characteristics of free chlorine atom, and (3) at alkane concentrations and , added reagents (T) which react with CCH, such as maleic anhydride (MA) or Cl2, bring about an increase in the LSI/HSI ratio.Low-selectivity hydrogen abstractions (LSI function) are best ascribed to a mixture of chlorine atom and chlorine atom/benzene ?-complex.The chemistry of CCH is as follows: (1) loss of the ipso H to O2 yielding PhCl and HO2., (2) reactions of Cl2 or (3) maleic anhydride with the aromatic nucleus of CCH resulting in additions to the ring, (4) the transfer of Cl to alkenes, and (5) the highly selective retardation of rates of reaction with alkanes producing alkyl radicals, HCl, and benzene.The results of a kinetic analysis, accounting for the effect of , , and CCH trapping agents (T), are presented.For CCH, the following reactivity order is established: maleic anhydride (6) > trans-dichloroethene(5) > 2,3-dimethylbutane (2) > pentane (1) > Cl2 > neopentane ( 2 (27) > 1 .These properties can be rationalized with canonical structures for CCH wherein spin density at carbon, chlorine, and the ipso hydrogen makes contributions to the hybrid.

Triplet lifetimes in deaerated cyclopentane have been measured for a variety of ring-substituted benzophenones, acetophenones, and α-α-α-trifluoroacetophenones.The ketones undergo photoreduction under these conditions to mixtures of products formed from cyclopentyl and hemipinacol radicals.That triplet lifetimes are determined by rates of hydrogen abstraction from solvent is indicated by lifetimes being three times in cyclohexane-d12 than in cyclohexane-h12.For the benzophenones, reciprocal lifetimes correlate comparably well with Hammett ? or ?+ constants, with ρ values of 0.55 and 0.43, respectively.The effect of two substituentsis best fitted to the sum of both ? values.These weak inductive effects are in accord with expectations for a reactive n,?* triplet.Rates for the acetophenones show the same substituent effects as previously observed for valerophenones; conjugating and electron-donating substituents stabilize the ?,?* triplet and sharply reduce reactivity.Substituent effects are largest for the trifluoroacetophenones, consistent with their all having ?,?* lowest triplets and reacting from weakly populated but highly reactive (k>107 M-1 s-1) n,?* states.Fluorine substitution, both on the ring and at the α-carbon, produces large rate enhancements, decafluorobenzophenone triplet being too short-lived to measure.

Trialkylboranes initiated the chlorinations of alkanes with radical chlorinating agents such as chlorine, t-butyl hypochlorite and N-chlorosuccinimide.The reaction proseeded at 20 deg C even in the dark giving monochloroalkanes.

A simple system has been devised to facilitate the first processes for the catalytic replacement of unactivated alkane C-H bonds with C-X bonds, X = nitrogen and iodine.The system also enables alkane C-H bonds to be replaced by C-X bonds, X = chlorine, bromine, and oxygen.The system is composed of two liquid phases and the oxidant iodosylbenzene (iodosobenzene).The alkane substrate, the MnIIITPPX catalyst, and the organic solvent (dichloromethane, chlorobenzene, or other aromatic hydrocarbon) constitute one phase, a saturated aqueous solution of the sodium salt of the anion to be incorporated into the alkane, NaX, X = N3(1-), NCO(1-), I(1-), Br(1-), or Cl(1-), constitutes the second phase, and the sparingly soluble oxidant iodosylbenzene constitutes a third phase.When the two liquid phases and the oxidant iodosylbenzene are stirred under an inert atmosphere, both RX and ROH products are produced catalytically based on MnTPP and in reasonable yield based on iodosylbenzene.The MnTPP moiety functions as a catalyst for C-H bond cleavage and for phase transfer of X(1-) from the aqueous phase to the organic phase where the functionalization chemistry takes place.The oxidant hypochlorite can be used in place of, but is less effective than, iodosylbenzene, and the oxidants hydrogen peroxide, periodate, and persulfate are ineffective.Product distributions obtained from the oxidation of cyclohexane, isobutane, 2,3-dimethylbutane, and tert-butylbenzene are most consistent with a product-determining step that involves transfer of X from manganese to a free alkyl radical intermediate.

The free-radical chain reactions of chlorosulfonyl isocyanate with alkanes can be initiated with either light or thermal initiators.The major products in these reactions are chlorides, with low yields of isocyanates and sulfonyl chlorides.On the basis of tertiary to primary hydrogen selectivity of about 120:1 and the relative reactivities of various substrates toward the abstracting radical from chlorosulfonyl isocyanate, the hydrogen-abstracting radical is suggested to be the NCO radical.

The free-radical chlorination of 2,3-dimethylbutane, 2,2,4-trimethylpentane, adamantane, and cyclohexane by arylchloroiodonium chlorides was investigated by the method of competing reactions.It was shown that the introduction of electron-withdrawing substituents into the arylchloroiodonium chloride leads to an increase in the sensitivity of the process to the polar effect of the substituents in the substrate and to a decrease of the selectivity toward variation in the dissociation energy of the C - H bond.The importance of steric effects in the reaction was noted.

In the cases of 2-methylbutane and 2-methylpentane the formation of tertiary sulphochlorides in the sulphochlorination of the parent hydrocarbons could be established by means of 13C-n.m.r.-spectroscopy.The relative rates of the various C-H-bonds in 2-methylbutane, 2-methylpentane and 3-methylpentane in the sulphochlorination reaction were determined.The relative rates of the tertiary C-H-bonds in the sulphochlorination were considerably lower than the corresponding values for the chlorination of the branched-chain paraffins.