Rates of base-catalysed cleavage of pyridyl-, quinolyl-, picolyl- and (quinolylmethyl)-trimethylsilanes

Rates of cleavage of some picoyl- and (quinolylmethyl)-trimethylsilanes (RSiMe₃, where R = PyCH₂ or QnCH₂SiMe₃) have been measured in “90%” aqueous methanolic sodium methoxide at 50°C. Relative reactivities are: 2-PyCH₂, 1.0; 3-PyCH₂, 0.030; 4-PyCH₂, 8.9; 2-QnCH₂, 41; 3-QnCH₂, 0.161; 4-QnCH₂, 37. The rates correlate well with those for base-catalysed hydrogen-exchange in the parent carbon acids RH. Approximate pK_a's (based on the scale of ion-pair acidities in CsNHC₆H₁₁H₂NC₆H₁₁, with pK_a of 9-phenylfluorene = 18.6) for the carbon acids, RH, can be derived as follows: 2-PyCH₃, 29.5; 3-PyCH₃, 34; 4-PyCH₃, 27; 2-QnCH₃, 25; 3-QnCH₃, 32; 4-QnCH₃, 25.Rates of cleavage of pyridyl- and quinolyl-trimethylsilanes (PySiMe₃ and QnSiMe₃) by sodium hydroxide in 4 : 1 v/v Me₂SO/H₂O at 50°C have also been measured; and the relative reactivities are: 2-Py, 1.0; 3-Py, 2.9; 4-Py, 8.4; 2-Qn, 15.9; 3-Qn, 12.7; 4-Qn, 184. The sequence of reactivity differes from that for base-catalysed hydrogen-exchange at the relevant positions of pyridine and quinoline, indicating that the reactivities are not determined in both cases (if in either) solely by the stabilities of the corresponding carbanions.     

OXIDATIVE DEGRADATION OF LIGNIN BY PHOTOCHEMICAL AND CHEMICAL RADICAL GENERATING SYSTEMS

Oxidation of specifically radiolabeled ¹⁴C-lignins by UV/H₂0₂. Fenton's reagent, photosesitizing riboflavin. UV- and γ-irradiation was examined. In the presence of UV/H₂O₂, a hydroxyl radical (·OH) generating system. ¹⁴C-methoxy. 2-[¹⁴C-sidechain] and ¹⁴C-ring labeled lignin were rapidly and extensively degraded as measured by gel filtration of the reaction products on Sephadex LH-20. This suggested that exposure to OH leads to rapid, nonspecific lignin degradation. Rapid degradation of 14C-methoxy, 2-[¹⁴C-sidechain] and ¹⁴C-ring labeled lignin also occurred in the presence of the ·OH generating system, Fenton's reagent, confirming the primary role of ·OH in these reactions. Photosensitizing riboflavin, also capable of effecting transformation of organic compounds via Type I hydrogen radical abstractions, caused extensive oxidative degradation of ¹⁴C-methoxy labeled lignin and significant degradation of 2-(¹⁴C-sidechain] and ¹⁴C-ring labeled lignin. In addition, UV- and γ-irradiation caused slower but extensive degradation of the polymers, probably via radical type mechanisms. All of these results indicate that ·OH as well as organic radical generating systems are effective agents for the purpose of degrading this heterogeneous, optically inactive and random biopolymer.     

Thin-layer chromatography of pitch and a petroleum vacuum residue. Relation between mobility and molecular size shown by size-exclusion chromatography

A coal tar pitch and a petroleum vacuum residue have been separated by TLC using pyridine, acetonitrile, toluene and pentane to develop the chromatograms. The bands of material detected were recovered in 1-methyl-2-pyrrolidinone (NMP) solvent and examined by size-exclusion chromatography (SEC) in NMP eluent. The relation between elution time in SEC and mobility on the TLC plate indicated that molecular size increased steadily with increasing immobility on the plate. This relation was reinforced by UV fluorescence spectroscopy in that the fluorescence moved to longer wavelengths with increasing immobility. The molecular size of the material excluded from the porosity of the SEC column remains undefined; some excluded material was found in all of the fractions from both samples. The valley of zero intensity separating the retained material from the excluded material may suggest a change of structure from near-planar in the retained region to three-dimensional in the excluded region.     

Automatic analysis of mixed spectra : Generalised spectral subtraction applied to electron spin resonance spectroscopy

A new technique is described for estimating the pure component spectra from a set of linearly independent spectra. The process is one of generalised spectral subtraction in which an iterative combination of multivariate linear least-squares analysis and matrix transformation is applied to the input data to give estimates of the number of independent components in the original mixed spectra. This technique is applicable to bipolar data (e.g. from e.p.r. spectra) as well as absorption spectra determined by any spectroscopic technique, provided that the spectra may be reasonably assumed to be an additive mixture of unknown pure components. Numerical model examples are given together with an experimental application to electron spin resonance.     

Thermal analysis of phenylethynyl end-capped fluorinated imide oligomer afr-pepa-4

Thermal analysis of phenylethynyl end-capped imide oligomer AFR-PEPA-4 was performed to characterize cure reaction, thermal stabilities and semicrystalline behavior of AFR-PEPA-4 oligomer and its cured polyimide. Cured AFR-PEPA-4 polyimide showed high T _gs up to 418°C. Both AFR-PEPA-4 oligomer and polyimide exhibit excellent thermal stabilities comparable to PETI-5 polyimides. AFR-PEPA-4 imide oligomer has a T _m of 330°C and exhibits spherulite crystalline morphology in the film. The crystallinity in AFR-PEPA-4 films could not be regenerated under any annealing conditions after the initial melt.     

Oxygen rearrangement of molecular ions of 3-phenylpropionates

The oxygen rearrangement in molecular ions of 3-phenylpropionates has been investigated with the aid of mass analyzed ion kinetic energy spectra. Elimination of an allyl radical followed by expulsion of ketene from the molecular ion of allyl 3-phenylpropionate is shown to result in formation of protonated benzaldehyde. The oxygen rearrangement has been found to be inoperative in ionized methyl 3-methyl-3-phenylbutyrate. [M ? CH₃ ? CH₂CO]⁺ ions in the spectrum of the latter compound are formed by elimination of the 3-methyl substituent and subsequent methoxy migration.     

The use of bis(diphenylphosphino)amines with N-aryl functionalities in selective ethylene tri- and tetramerisation

A systematic study was conducted on the Cr catalysed tri- and tetramerisation of ethylene using bis(diphenylphosphino)amine ligands with N-aryl functionality. This study revealed that the oligomerisation reaction product selectivity is primarily dependent on the structure and size of the N-aryl groups.

Addition of sufficient steric bulk to the N-phenyl group via ortho-alkyl substitution increased the combined 1-hexene and 1-octene selectivity (overall alpha selectivity) to above 82% at an overall 1-octene selectivity of 56%. The introduction of a single carbon spacer between the N-atom and the aryl-moiety, as well as the addition of branching on this carbon, resulted in further selectivity improvements, achieving an overall 1-octene selectivity of 64% and an overall alpha selectivity of 84%. This was obtained at catalyst productivities in excess of 1,000,000 g/g Cr/h.     

Naphthalene Complexation by β-Cyclodextrin: Influence of Added Short Chain Branched and Linear Alcohols

Naphthalene forms 1 : 1 complexes with -cyclodextrin (-CD)in water. The binding constant is 377 ± 35 M^-1. Addition of linear or branched alcohols causes a reduction in the apparent strength of naphthalene binding (K_app) compared to the value in the absence of additives. For example, 1% 1-pentanol reduces K_app to 184 ± 31 M^-1. Branching does not alter K_app much for a given number of carbon atoms, e.g., it is 113 ± 9 M^-1for 2-pentanol and 116 ± 8 M^-1for 3-pentanol. The exception to this is tert-butanol for which K_app is 577 ± 40 M^-1. The variation in K_app as a function of [1-pentanol] yields values for the individual equilibrium constants contributing to K_app. This reveals that a ternary complex forms involving naphthalene, the CD and 1-pentanol. The constant for formation of the ternary complex is 99 ± 29 M^-2. NaI quenching of naphthalene fluorescence indicates that the CD cavity partially protects the naphthalene excited state fromthis water phase quencher. Interestingly, the Stern–Volmer constant is lower in the presence of 1-pentanol than in its absence, although there should be more unbound (and therefore more NaI accessible) naphthalene in the former system than in the latter. These apparently contradictory results are discussed in terms of ternary complex formation.     

On the formation of artefact peaks in linked scans of the magnet and electric sector fields in a mass spectrometer

The apperrance of peaks at non-integral masses in linked scans, where the ratio B/E is kept constant, is shown to be caused by fragmentations of metastable ions occuring in the region between the magnetic and electric sectors. A formula is given for calculating the mass of the parent metastable ion and that of its daughter ion.