Increasing Alkyl Substitution Research Articles

Kinetics and mechanism of oxidation of aromatic hydrocarbons by Ru(trpy)(bpy)O2+

The kinetics of oxidation of p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH(CH/sub 3/)/sub 2/,p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH/sub 2/CH/sub 3/,and p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH/sub 3/ by Ru(trpy)(bpy)O/sup 2 +/ (trpy is 2,2',2''-terpyridine; bpy is 2,2'-bipyridine) to the corresponding ..cap alpha.. alcohols in water and of C/sub 6/H/sub 5/CH(CH/sub 3/)/sub 2/ and C/sub 6/H/sub 5/CH/sub 3/ by Ru(bpy)/sub 2/(py)O/sup 2 +/ in acetonitrile have been studied. The following conclusions are drawn from kinetics data obtained spectrophotometrically: (1) Rate constants increase with increasing alkyl substitution; for the carboxylates in water at 24.3/sup 0/C, k = 12.2 +/- 1.2, 3.4 +/- 0.3, and 0.43 +/- 0.04 M/sup -1/ s/sup -1/ in the order shown above. (2) Rate constants decrease dramatically for the reactions in acetonitrile; k(24.3/sup 0/C) = 0.026 +/- 0.003 M/sup -1/ s/sup -1/ for isopropylbenzene. (3) In water, rate constants are independent of added O/sub 2/ or of changes in ionic strength. (4) In acetonitrile the added nucleophiles water, tert-butyl alcohol, or bromide ion enter the rate law directly in terms first order in added nucleophile. From the temperature dependence of k for the oxidation of p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH(CH/sub 3/)/sub 2/, ..delta..H/sup + +/ = 7 +/- 1 kcal/mol andmore » ..delta..S/sup + +/ = -32 +/- 4 eu. It is concluded that the redox step for the reactions involves a two-electron, hydride ion transfer step. The reactions occur by a template mechanism in that oxo group transfer from Ru to the substrate does not occur and the added oxygen atom must come from the solvent, p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH(CH/sub 3/)/sub 2/ (-H:/sup -/; +H/sub 2/O) ..-->.. p-/sup -/O/sub 2/CC/sub 6/H/sub 4/C(OH)(CH/sub 3/)/sub 2/. The solvent or added nucleophile (in acetonitrile) is directly involved in the redox step, apparently by assisting the loss of the hydride ion by electron pair donation.« less

OBSERVATIONS OF C‐S AND S‐S BOND CLEAVAGE IN THE PHOTOLYSIS OF DISULFIDES IN SOLUTION

Abstract— A comparison of the transient absorption spectra from the photolysis of disulfides in solution suggests that C‐S bond breakage is a common primary photolytic process. This process becomes more important as the resulting carbon centered radical is stabilized by increasing alkyl substitution or resonance interaction with an aromatic system. The perthiyl radical product is characterized by λmax?380 nm,ε380?1700 M−1 cm−1 and decays by second order kinetics with k2?3.7×108M−1 s−1 in water.In the presence of O2, the photolysis of disulfides which produce the thiyl radical give transient absorptions in the 500–600 nm region. Possible identities of these transients are discussed.

A multinuclear ( 1H, 13C, 113Cd) nuclear magnetic resonance and magnetic circular dichroism spectroscopic study of thiolate complexes of cadmium

Appropriate combinations of 1H, 13C and 113Cd NMR have been used to study ca. 0.05 M solutions of a wide range of 113Cd-enriched thiolatocadmates formed from the corresponding poorly soluble cadmium thiolates in the presence of excess thiolate. In addition, MCD spectra have been recorded for dilute, ca. 10 −5 M, solutions of a representative set of thiolate complexes of cadmium. Cadmium-113 chemical shifts are given for the complexes formed by 31 different thiolates individually in water at ambient probe temperature, and slow-exchange 1H and/or 13C and/or 113Cd NMR spectra are reported for several typical complexes at reduced temperature in CD 3OD. In aqueous solution at ambient probe temperature, ligands with vicinal thiolate groups ▪ and −SCH 2CHRS − (R = H, Me, Et, CH 2OH, CH 2SO − 3 or CH 2S −)) form bis(chelate) complexes which have δ Cd ⪖ 778 ppm, less shielded than the range for tetrahedral cd(SR) 2− 4 (R = Me, Et, Pr, Me 2CH, Bu, EtMeHC, Ph, PhCH 2, CH 2CH 2X (X = OH, NH 2, or CO − 2, CH 2CH(OH)Me, CH 2CH(OH)CH 2OH, LCH 2CH(NH 2)CO − 2, or CH 2CO − 2 for which δ Cd < 666 ppm. Increasing alkyl substitution kin the α-position causes increasing shielding of the cadmium resonance for both the chelate complexes and the complexes of monodentate thiolates. Separate 113Cd and, in some cases, 1H resonances have been found for members of the series [Cd(SR) n(SR′) 4−n] 2− (R = Me, R′ = CHMe 2; R = Ph, R′ = CH 2Ph; n = 0−4) and [Cd(S(CH 2 2S) n/2(SCHMe 2) 4−n 2− (n = 0,2, or 4) at low temperature: 113Cd chemical shifts change additively or near-additively in these series. The MCD data indicate that in most of the complexes formed at the ca. 10 −5 M concentrations used for the optical studies, the local microsymmetry about the cadmium approaches tetrahedral. Specifically, the complexes [Cd(SR) 2.5] 0.5n− n (R = Me, Et, or Pr) exhibited well-resolved, symmetric A terms under the S → Cd charge transfer band in the 250 nm region. Complexes of the dithiolates −S(CH 2 nS − (n = 2,3, or 4) and −SCH 2CHMeS − gave MCD spectra which were much broader and less well-resolved. The spectra of the systems containing the vicinal dithiolates showed the formation of two complexes, as the band centre of the MCD signal blue-shifted when L/M was increased from 1 to 2; at L/M = 2, bis(dithiolate) complexes are most likely formed. With the ligand −SCH 2CHCH 2OH (BAL), the corresponding S − spectra were similar at L/M = 2 but quite different at L/M = 1. A B term centred on the 250 nm charge transfer band was observed for the solution where BAL/Cd = 1, demonstrating loss of tetrahedral symmetry about cadmium, probably due to ligation by the ligand hydroxyl group. These studies provide 113Cd NMR and MCD ‘fingerprint’ data necessary for the interpretation of the corresponding spectra of related and more complicated, biologically important cadmium complexes.

29Si and 13C NMR Studies of organofunctional di- and trisilanes

A series of di- and trisilanes of general structure Ph 3SiSiMe 2R and (Ph 3Si) 2SiR′R″ were synthesized, and the 29Si and 13C chemical shifts and one-bond siliconsilicon coupling constants ( 1 J SiSi) were measured. The coupling constants of the disilanes were found to be primarily dependent upon the inductive effect of the alkyl group, R, as measured by the Taft o ★ constant. In both series of compounds, increasing alkyl substitution at silicon led to a decrease in 1 J SiSi.

Structure-toxicity correlations of organic contaminants in aqueous coal-conversion effluents.

Five series of organic contaminants associated with aqueous coalconversion effluents were studied to determine the correlation between their known partition coefficients in the octanol-water system and their toxicity to the ciliateTetrahymena pyriformis. In all five series, toxicity increased and solubility decreased with increased alkyl substitution. Compounds containing the equivalent of two or more methyl groups are more toxic than those with one or no alkyl substitutions.

Reactions of keten. Part V. Gas-phase reactions with carboxylic acids. Effect of alkyl substituents

Keten reacts homogeneously in the gas phase with carboxylic acids between 95 and 216 °C to give anhydrides in a reaction which is first order in keten and in acid monomer. Rates of reaction increase with increasing alkyl substitution and branching in the acid. This increase does not correlate well with the pKa values of the acids but runs parallel to their gas-phase acidities.

Rhodium (II) carboxylates as new selective stationary phases in gas-liquid chromatography

A new stationary phase for gas chromatography, dimeric rhodium (II) benzoate, was found to interact rapidly and reversibly with sigma donor molecules such as ethers, ketones and esters, thereby considerably increasing their retention volumes. In addition, interaction and separation of olefins has been achieved with the same phase. The specific interaction of olefins with the rhodium (II) complex is particularly interesting since the relative retention increase is quite different than that for rhodium (I) and silver (I). Furthermore, an “inverse” steric effect is observed as the interaction of olefins increases with increasing alkyl substitution at the double bond. The mechanism of interaction involved and the analytical applications are briefly discussed.

Synthesis of aliphatic hydrazonyl bromides and kinetics of their conversion into hydrazides

The hydrolytic conversion of N-(4-nitrophenyl)-, N-(2-bromo-4-nitrophenyl)-, and N-(2,4-dinitrophenyl)-alkanohydrazonyl bromides into the corresponding alkyl hydrazides has been investigated kinetically in 80% aqueous dioxan. The rates of hydrolysis for all three series of compounds were found to decrease with increasing alkyl substitution. This was attributed to the bulk of the alkyl group retarding solvolysis, since electron-withdrawing groups strongly retard reaction. Further evidence (large solvent, common-ion, and salt effects) also supports a mechanism involving rate-determining unimolecular ionization. The reaction of benzenediazonium ion with the parent hydrazones was also investigated and showed the same dependence on the bulk of the aliphatic group.

Gas-phase basicities of amines

Relative gas-phase basicities of some aliphatic amines have been determined by ion cyclotron resonance spectroscopy. It is found that increasing alkyl substitution increases basicity if similar substituents are compared, that increases in alkyl group size also increase basicity, and that increasing degree of substitution lowers the N-H bond dissociation energy in ammonium ions. Possible origins of these defects are discussed.

UEAITR: A new computer program for analysis of NMR spectra analysis of the proton spectrum of triisopropylphosphine

A new computer program for the analysis of NMR spectra is reported. It adds iterative evaluation of input parameters to an earlier program (UEANMR II) which makes use of magnetic equivalence factoring to reduce the size of certain matrices. The present program, by iterating on line frequencies rather than energy levels, and by using magnetic equivalence factoring, offers advantages of speed and convenience over previously described computer programs for NMR analysis. The analysis of the proton spectrum of triisopropylphosphine is described as an example and the trend of 2 J H- P with increased alkyl substitution is found to be opposite to that of the trend of 2 J H- X for all other hetero-atoms, X, reported in the literature.

The graphitization of organic compounds—II. X-ray properties of carbons prepared from a series of alkylated carbazoles

The synthesis, carbonization, and graphitization of a series of carbazole derivatives, substituted at the ring carbon atoms by one or more methyl, ethyl, or butyl groups, are described. The carbons prepared from these compounds become more graphitic, as measured by X-ray crystallographic data, with increasing alkyl substitution and with increasing chain size of the alkyl groups.

The reaction of triphenylphosphine with organic disulphides and polysulphides

The rate and course of reaction of triphenylphosphine (Ph 3P) with organic sulphur compounds (R·S x ·R′; R,R′ = alkyl or alkenyl, x = 2 or 4) at 80° or 140° depend on the nature of R and R′ and on the reaction medium. Dialkyl- and diaralkyl tetrasulphides (RR′ = iso-Pr; n-Bu; T-Bu; PhCH 2) yield triphenylphosphine sulphide and dialkyl- or diaralkyl disulphides which are resistant to further attack by Ph 3P, even at 140°. Dialkenyl- and alkenyl-alkyl disulphides at 80° undergo a less facile desulphuration to give monosulphides which result from the allylic rearrangement of an alkenyl group; the yield of allylically-unrearranged monosulphide is negligible. The rates of the Ph 3P-disulphide reactions increase with increasing solvent polarity and decrease with increasing alkyl substitution at γ-carbon atoms of the alkenyl groups or the α-carbon atom of the alkyl groups. The Ph 3-P-tetrasulphide reactions are believed to involve nucleophilic substitutions at the SS bonds, initially by the reagent and then by a resultant incipient alkanethiolate or alkaneperthiolate ion. The rates and course of the Ph 3P-disulphide reactions are only consistent with nucleophilic attack of the Ph 3P on sulphur yielding an intimate ion-pair which then undergoes an almost exclusive S Ni′ process.

Intensities of Electronic Transitions in Molecular Spectra IV. Cyclic Dienes and Hyperconjugation

The spectra of cyclopentadiene, 1,3-cyclohexadiene, thiophene, furan, and pyrrole are considered. Theoretical explanations of their departures from the spectra of openchain conjugated dienes are given. The cause of these departures in the cases of cyclopentadiene and cyclohexadiene is designated hyperconjugation. This is a mild sort of conjugation, apparently not recognized as such hitherto, between saturated groups and double or triple bonds or even other saturated groups. Observed displacements of spectra toward longer wave-lengths, and corresponding effects in the refractivities (cf. VI), when alkyl groups are substituted for hydrogens in ethylene, butadiene, benzene, or other double-bonded compounds, are attributed to hyperconjugation. Further, the decreasing heat of hydrogenation observed with increasing alkyl substitution in these compounds may indicate increasing stability caused by hyperconjugation. However, it is likely according to the theory that appreciable energy effects caused by hyperconjugation are usually restricted to excited states, and so should affect mainly ultraviolet spectra and refractivities. If this is correct, heats of hydrogenation ought not to be much influenced by hyperconjugation, since they depend on the energies of molecules in their normal states. Hence, it is likely that the above-noted variations in heats of hydrogenation are caused partly or mainly by other causes. Conjugation of triple bonds is briefly discussed, and it is also pointed out that in ethane there must be hyperconjugation which is analogous to the triple-bond conjugation in cyanogen. It is shown that this hyperconjugation in ethane should not appreciably hinder free rotation. Likewise, hyperconjugation in propylene should produce no barrier to free rotation of the methyl group. It is possible, however, that hyperconjugation may help to restrict free rotation in some cases. Carbon to carbon single bond distances should be slightly shortened by hyperconjugation. The unusually small C–C distance in methylacetylene as determined spectroscopically by Herzberg and co-workers may perhaps be explained in this way, although in other cases no appreciable effects are apparent. It is helpful to classify examples of hyperconjugation according to various types, which may differ in the magnitudes of their effects. Possibly only first-order hyperconjugation (between a double or triple bond and a saturated group) is important. Second-order hyperconjugation (between two saturated groups, as, for example, in ethane) very likely produces smaller effects.