Methyl-substituted Benzenes Research Articles

ChemInform Abstract: CYCLOPENTADIENYL‐ OR PENTAMETHYLCYCLOPENTADIENYL(ARENE)COBALT(III) COMPLEXES: ARENE = INDOLE, BENZENE, MESITYLENE, HEXAMETHYLBENZENE, 1,4‐DIHYDROXY‐ AND 1‐HYDROXY‐4‐METHOXYTETRAMETHYLBENZENE

AbstractIndol (II) substituiert in dem Co‐Komplex (I) die Acetonliganden und bildet das Derivat (III), in dem das Co‐Atom mit dem 6‐gliedrigen Ring koordiniert ist.

Ultimate corrosion-resistant optoacoustic cell for spectroscopy of liquids

We describe a new optoacoustic technique for liquids that utilizes a quartz cuvette to contain the sample and an externally coupled piezoelectric transducer for gated detection of the optoacoustic signal produced by a pulsed dye laser. Such a technique is characterized by ultimate corrosion resistance, absence of contamination of the sample, and ease of sample change. With this technique, we measure quantitatively the weak absorption profiles that are due to the sixth harmonic of the C—H stretch in a series of halogen- and methyl-substituted benzenes. Also, optoacoustic spectra of ions dissolved in a strong acid (H2SO4) are observed for the first time.

The radical ions of 4,5,7,8-tetramethyl[2.2]paracyclophane

By the use of strong oxidising and reducing agents, 4,5,7,8-tetramethyl[2.2]paracyclophane (II) has been converted to its radical cation (II+˙) and to its radical anion (II–˙). These radical ions, as well as those generated from the 12,13,15,16-tetradeuterio ([2H4]II) and the 1,1,10,10,12,13,15,16-octadeuterio-derivative ([2H8]II), have been studied by e.s.r. and, in part, by e.n.d.o.r. spectroscopy. In (II+˙), the bulk of the spin population is located on the methyl-substituted benzene ring, whereas, in (II–˙), it is found on the ring bare of methyl groups. This result corresponds to the effect of the alkyl substitution which enhances the donor and weakens the acceptor character of a π-system. Comparison of the hyperfine data for (II˙+) with those of the dimeric radical cation (II2+˙) reveals considerable deviations from the ratio of ca. 0.5, expected for the coupling constants of the protons in such two species. In the e.s.r. spectra of (II–˙) the large splittings from the nuclei of the counterions (39K 0.102 and 133Cs 0.596 mT) are notable; they exceed by one order of magnitude the corresponding values for the radical anion of [2.2]paracyclophane (I).

Cyclopentadienyl- or pentamethylcyclopentadienyl-(arene)cobalt(III) complexes: arene = indole, benzene, mesitylene, hexamethylbenzene, 1,4-dihydroxy- and 1-hydroxy-4-methoxytetramethylbenzene

Indole displaces acetone from the complex [Co(C5Me5)(OCMe2)3][PF6]2 to give indole(pentamethylcyclopentadienyl)cobalt(III) hexafluorophosphate in which the cobalt is complexed to the six-membered ring of the indole; attempts to prepare other arene complexes by this procedure have failed. The benzene complex [Co(C5Me5)(η-C6H6)][BF4]2 has been prepared by treatment of [Co(C5Me5)(η4-C6H8)] with [CPh3][BF4]. The related complex [Co(C5H5)(η-C6H6)][BF4]2 undergoes arene exchange with mesitylene and hexamethylbenzene to give [Co(C5H5)(η-C6MenH6–n)][BF4]2(n= 3 or 6) and with suitable donors (e.g. methyl-substituted benzenes) forms charge-transfer complexes in solution. 2,3,5,6-Tetramethyl-1,4-benzoquinone, C6Me4O2, reacts with [Co(C5Me4R)(CO)2] to give [Co(C5Me4R)(η4-C6Me4O2)](R = Me or Et). Protonation and methylation of the pentamethylcyclopentadienyl complex gives the 4-hydroxy- and 4-methoxy-2,3,5,6-tetramethyl-1-oxocyclohexadienyl complexes [Co(C5Me5){η6-C6Me4(OR)O}][BF4](R = H or Me). The latter are converted into the corresponding substituted-benzene complexes [Co(C5Me5){η6-C6Me4(OR)(OH)}]2+ by acid. Infrared and 1H and 13C n.m.r. spectra are reported and the relative stabilities of the complexes are discussed.

Calculated properties of arene oxides of biological interest. 1. Molecular orbital examination of simple models.

The INDO calculational formalism is shown to account for the relative ease of carbonium ion formation and for the variation in isomerization rates of some simple arene oxides. Transition-state energy differences provide a reasonable correlation with observed isomerization rates for benzene oxide, methyl-substituted benzene oxides, naphthalene 1,2-oxide, and phenanthrene 9,10-oxide.

Anodic functionalisation in synthesis. Part 1. Methoxylation of methyl-substituted benzene and anisole derivatives, and the synthesis of aromatic aldehydes by anodic oxidation

The anodic oxidation in methanol of a series of alkylanisoles and hydroquinone ethers, and of p-xylene has been studied. It was established that the initial step in the product formation always involves direct anodic oxidation of the aromatic compound to the corresponding cation radical. If the electrolysis is conducted under conditions where formation of methoxyl radicals also takes place (platinum anode, supporting electrolyte NaOMe) nuclear methoxylation dominates. Substitution in the nucleus of the aromatic compound presumably occurs either by nucleophilic attack of a methoxide ion on the anodically generated cation radical or by a coupling reaction between the latter and an anodically generated methoxyl radical. If the electrolysis is conducted under conditions where formation of methoxyl radicals does not take place (carbon or platinum anode with supporting electrolyte LiBF4) side-chain oxidation of p-alkylanisoles and p-xylene, with the formation of benzyl ethers, benzaldehyde dimethyl acetals, or ortho-esters of benzoic acids becomes important. This side-chain oxidation occurs via nucleophilic attack of methanol on a benzyl cation formed by deprotonation–anodic oxidation of the initially formed cation radical. Phenol ether cation radicals without p-alkyl substituents under these conditions either dimerise or undergo nuclear methoxylation via nucleophilic attack of methanol on the cation radical.

Reaction product identification from O(3P)+benzene, toluene, and 1,3,5-trimethylbenzene collisions in crossed molecular beams

Although absolute rate coefficients have been measured for a number of reactions of aromatic molecules with O(3P) atoms, very little is known about the mechanisms of these reactions. In order to identify the products of O atom–aromatic molecule reactions, the products of single reactive collisions between oxygen atoms and benzene, toluene, and 1,3,5,-trimethylbenzene have been observed in crossed molecular beam experiments. The products were detected with a quadrupole mass spectrometer. Two product paths were observed for O+benzene: (1) the O atom–benzene adduct, which is most likely phenol, and (2) carbon monoxide and a C5H6 hydrocarbon which is probably 3-penten-l-yne. An additional path involving the formation of (methyl-substituted) benzaldehyde and H2 was observed for the reaction with methyl-substituted benzenes. Only the adducts, the benzaldehydes, and the olefins were observed directly. The identities of the corresponding products were inferred from the difference between the mass-to-charge ratio of the observed product and that of the C6H6O, C7H8O, and C9H12O adducts. Arguments based on electronic state symmetry indicate that the carbon monoxide-olefin path involves the decomposition of vibrationally excited phenol in its ground electronic state. This state is reached by a spin-forbidden transition from the triplet reactant state. The benzaldehyde products can be explained by a mechanism analogous to one of the reaction paths of O(3P)+ethylene.

A.C. polarographic investigation of the adsorption of benzene and methylsubstituted benzenes at the DME from propylene carbonate

A.C. polarographic investigation of the adsorption of benzene and methylsubstituted benzenes at the DME from propylene carbonate

A.C. polarographic investigation of the adsorption of benzene and methylsubstituted benzenes at the DME from propylene carbonate

A.C. polarographic investigation of the adsorption of benzene and methylsubstituted benzenes at the DME from propylene carbonate

Effect of methyl and halogen substituents on the decay of singlet excited benzenes

Radiative and nonradiative decay constants for singlet excited states of halogen- and methyl-substituted benzenes were determined in solution. The effect of halogen substitution on the nonradiative decay is much greater than that of methyl substitution, which is obscured by effects of the solvent.

Molekülstruktur und retentionsverhalten : V. Berechnung der adsorptionsenergie aromatischer kohlenwasserstoffe an graphitiertem thermischen russ

Molecular structure and retention behaviour. V. Calculation of adsorption energies for aromatic hydrocarbons on graphitized carbon black The energies of adsorption are calculated for some aromatic hydrocarbons and methyl-substituted benzenes by use of empirical potential functions. Suggestions about the orientation of adsorbed molecules on the graphite surface are made. It is necessary for explanation of the partly small differences in adsorption energy for aromatic and conjugated systems to include both changes of the energy of non-bonding interactions and the electronic structure. The participation of charge-transfer interactions in adsorption is discussed for compounds of this type.

ChemInform Abstract: A PHOTO‐SELECTION STUDY OF THE CHARGE‐TRANSFER PHOSPHORESCENCE

The polarization of the charge-transfer phosphorescence has been studied by the method of photo-selection. It has been measured relative to the electric vector of the excitation light corresponding to the charge-transfer band in wavelength for the complexes of tetracyanobenzene, o-, m-, and p-dicyanobenzenes, tetrachlorophthalic anhydride, and phthalic anhydride as electron acceptors, and methyl-substituted benzenes as electron donors, in organic glass at 77 K. It has been shown that the degree of polarization through excitation into the charge-transfer band is almost constant in the phosphorescence region, while it gradually decreases as the ionization potential of the donor increases for any of the acceptors. These observations can reasonably be interpreted in terms of a model in which the charge-transfer phosphorescence is polarized normal to the molecular plane, irrespective of the complex, and in which the change in the polarization degree of the charge-transfer phosphorescence is responsible for the...

A Photo-selection Study of the Charge-transfer Phosphorescence

Abstract The polarization of the charge-transfer phosphorescence has been studied by the method of photo-selection. It has been measured relative to the electric vector of the excitation light corresponding to the charge-transfer band in wavelength for the complexes of tetracyanobenzene, o-, m-, and p-dicyanobenzenes, tetrachlorophthalic anhydride, and phthalic anhydride as electron acceptors, and methyl-substituted benzenes as electron donors, in organic glass at 77 K. It has been shown that the degree of polarization through excitation into the charge-transfer band is almost constant in the phosphorescence region, while it gradually decreases as the ionization potential of the donor increases for any of the acceptors. These observations can reasonably be interpreted in terms of a model in which the charge-transfer phosphorescence is polarized normal to the molecular plane, irrespective of the complex, and in which the change in the polarization degree of the charge-transfer phosphorescence is responsible for the change in the direction of the transition moment of the charge-transfer absorption band.

Reactions of lead(IV). Part XXVIII. Oxidative coupling of methylsubstituted benzenoid compounds: formation of biaryls and diarylmethanes in trifluoroacetic acid

Methyl-substituted benzenes are oxidised readily at low temperatures by lead tetra-acetate in the presence of trifluoroacetic acid; the products are mainly biaryls and diarylmethanes, in proportions which vary markedly with the structure of the aromatic compound. Evidence has been adduced that the first step is the formation of an aromatic radical cation. This undergoes competitive reactions; it can react with another aromatic molecule to give ultimately a biaryl, and in this respect behaves as an electrophilic radical rather than as a cation, or it can undergo further oxidation to give a benzylic cation from which diarylmethanes are derived.

Oxidation of some aromatic hydrocarbons by cerium(IV) trifluoroacetate

The preparation of cerium(IV) trifluoroacetate, and its reactions with some benzenoid hydrocarbons in trifluoroacetic acid containing lithium trifluoroacetate, are described. With electron-rich compounds, oxidation is rapid at room temperature; the products are biaryls, diarylmethanes, and trifluoroacetate esters, the proportions of which depend on the structure of the aromatic compound. Evidence is presented that the first step is the transfer of one electron to the oxidant to form an aromatic radical cation; with methyl-substituted benzenes, this is followed either by reaction with another aromatic molecule or by deprotonation, subsequent reactions then yielding an ester or a diarylmethane.

Low molecular weight aromatic hydrocarbons in gas condensates from Alberta, Canada

Forty one gas condensates from Alberta, Canada, from stratigraphie units ranging in age from U. Cretaceous to M. Devonian were gas chromatographically analysed for low molecular weight aromatic hydrocarbons. Aromatic hydrocarbons range from 0.5 to 45.6 per cent with benzene, toluene and the xylenes averaging 80 per cent of the aromatic content. Trimethylbenzenes (of which 1,2,4-trimethylbenzene is the most important), ethylbenzene, and naphthalene comprise the major portion of the remainder. Other alkyl substituted benzenes and naphthalenes are relatively insignificant. Factor analysis demonstrated relations amount the low molecular weight aromatic hydrocarbons, n-paraffins and sulphur. These relations are interpreted in terms of the derivation of toluene and xylenes from precursors such as the carotenes, with subsequent temperaturecontrolled disproportionation of both toluene and xylenes to yield benzene and trimethylbenzenes. Although the relative positions of benzene, toluene, xylenes and trimethylbenzenes on one hyperplane are in the same relative proportions as their free energies of formation, they are not in thermodynamic equilibrium. These positions may be interpreted also as proportional to their solubilities. Study of the distribution of the xylene and trimethylbenzene isomers suggests the dominance of specific precursors. Ethylbenzene abundance is independent of the abundance of other low molecular weight aromatic hydrocarbons, and probably arises from phenylalanine or tyrosine by decarboxylation, deamination, and conversion of vinylbenzene to ethylbenzene. Naphthalene may have a source distinct from that of benzene and the methylsubstituted benzenes.

The electronic spectra of s-tetracyanobenzene complexes in the phosphorescent state

The T n -T 1 spectra of the s-tetracyanobenzene (TCNB)-naphthalene complex are similar to the T n -T 1 spectra of the donor, while the spectra of TCNB complexes with methylsubstituted benzenes give many bands in the visible and near infrared regions. The difference between the T- n -T 1 and S n -S 1 spectra of the TCNB-hexamethylbenzene complex was attributed theoretically to the different geometry and different charge-transfer degree between the phosphorescent and fluorescent states.

Energy Transfer Concerning the Lowest Triplet State of Tetracyanobenzene with Methyl-Substituted Benzenes

Abstract The quantum-yield ratios of phosphorescence to fluorescence and the phosphorescence lifetimes were measured for the charge-transfer complexes of tetracyanobenzene with methyl-substituted benzenes. By combining the observed results with the fluorescence quantum yields and lifetimes reported previously, the upper and lower limits of the rate constants for the intersystem crossing and for the radiative and non-radiative energy-transfer processes from the lowest triplet state to the ground state were obtained with the complexes. The intersystemcrossing rate constant of tetracyanobenzene was found to be decreased by the complex formation. This tendency was explained by the mixing of the charge-transfer configuration with the appropriate singlet and triplet states of the component molecules. The phosphorescence radiative rate constant was found to be determined by the contribution of the higher excited singlet configurations of each complex to the phosphorescent state. The temperature dependence of the phosphorescence-quenching rate constant was determined, and the activation energy of the process was interpreted as being due to intramolecular out-of-plane bending vibrations.

Enthalpies of mixing of pentafluorobenzene with some hydrocarbons

The excess enthalpies of pentafluorobenzene with cyclohexane, methylcyclohexane, and a series of methyl-substituted benzenes have been measured at temperatures in the range 308 to 328 K over a wide composition range. The results obtained for the alicyclic compounds are positive and very similar to values for the corresponding mixtures with hexafluorobenzene. The mixtures formed with the aromatic hydrocarbons have negative excess enthalpies which decrease almost linearly with the number of methyl groups substituted into the benzene ring.

Radiative and Nonradiative Transitions from the First Excited Singlet State in Methyl Substituted Benzenes

Radiative and Nonradiative Transitions from the First Excited Singlet State in Methyl Substituted Benzenes