96-38-8Relevant academic research and scientific papers
Isoprene synthesis from formaldehyde and isobutene over Keggin-type heteropolyacids supported on silica
Sushkevich,Ordomsky,Ivanova
, p. 6354 - 6364 (2016/08/18)
Gas phase Prins condensation of isobutene with formaldehyde has been studied over different Keggin-type heteropolyacids supported on amorphous silica. The catalysts were characterized by elemental analysis, X-ray diffraction, low temperature nitrogen adsorption, TPD of ammonia, FTIR of adsorbed pyridine and NMR spectroscopy. The activity of the supported heteropoly compounds was found to increase in the following order: H4SiMo12O40 3PMo12O40 4SiW12O40 ≈ H3PW12O40. The lower activity of the supported molybdenum heteropolyacids was attributed to their low thermal stability and partial decomposition during catalyst activation, which resulted in lower acidity. The variation of HPA content from 5 to 33 wt% was also shown to increase catalyst activity. Based on the relationship between the content of weak Br?nsted sites, the amount and type of carbonaceous deposits and the catalytic activity, it was concluded that the generation of working active sites over HPA catalysts involves the formation of unsaturated branched surface species over weak Br?nsted sites. These active carbonaceous species are responsible for selective isoprene synthesis. The best catalyst performance is observed over the catalyst with 20 wt% of H3PW12O40, which shows an isoprene yield of 48% with a selectivity of 63%.
Oxygen-Carbon bond dissociation enthalpies of benzyl phenyl ethers and anisoles. An example of temperature dependent substituent effects
Pratt,De Heer,Mulder,Ingold
, p. 5518 - 5526 (2007/10/03)
For some time it has been assumed that the direction and magnitude of the effects of Y-substituents on the Z-X bond dissociation enthalpies (BDE's) in compounds of the general formula 4-YC6H4Z-X could be correlated with the polarity of the Z-X bond undergoing homolysis. Recently we have shown by DFT calculations on 4-YC6H4CH2-X (X = H, F, Cl, Br) that the effects of Y on CH2-X BDE's are small and roughly equal for each X, despite large changes in C-X bond polarity. We then proposed that when Y have significant effects on Z-X BDE's it is due to their stabilization or destabilization of the radical. This proposal has been examined by studying 4-YC6H4O-X BDE's for X = H, CH3, and CH2C6H5 both by theory and experiment. The magnitudes of the effects of Y on O-X BDE's were quantified by Hammett type plots of ΔBDE's vs σ+ (Y). Calculations reveal that changes in O-X BDE's induced by changing Y are large and essentially identical (ρ+ = 6.7-6.9 kcal mol-1) for these three classes of compounds. The calculated ρ+ values are close to those obtained experimentally for X = H at ca. 300 K and for X = CH2C6H5 at ca. 550 K. However, early literature reports of the effects of Y on O-X BDE's for X = CH3 with measurements made at ca. 1000 K gave ρ+ ≈ 3 kcal mol-1. We have confirmed some of these earlier, high-temperature O-CH3 BDE's and propose that at 1000 K, conjugating groups such as -OCH3 are essentially free rotors, and no longer lie mainly in the plane of the aromatic ring. As a consequence, the 298 K DFT-calculated ΔBDE for 4-OCH3-anisole of -6.1 kcal mol-1 decreases to -3.8 kcal mol-1 for free rotation, in agreement with the ca. 1000 K experimental value. In contrast, high-temperature O-CH3 ΔBDE's for three anisoles with strongly hindered substituent rotation are essentially identical to those that would be observed at ambient temperatures. We conclude that substituent effects measured at elevated temperatures may differ substantially from those appropriate for 298 K.
Kinetics of the Thermal Decomposition of Methoxybenzene (Anisole)
Mackie, J. C.,Doolan, K. R.,Nelson, P. F.
, p. 664 - 670 (2007/10/02)
The thermal decomposition of anisole vapor dilute in argon has been studied in a perfectly stirred reactor over the temperature range 850-1000 K and at total pressures of (16-120)E-3 atm.Decomposition of anisole takes place principally by the reaction C6H
On the Thermal Cycloisomerization of Long-Chain Alkylacetylenes in the Gas Phase
Ondruschka, Bernd,Zimmermann, Gerhard,Remmler, Matthias,Ziegler, Ulrich,Kopinke, Frank-Dieter,et.al.
, p. 715 - 720 (2007/10/02)
The thermal cycloisomerization of some alkylacetylenes was investigated in a tubular quartz reactor.At 570 deg C 1-hexyne (1) rearranges to 3-methyl-1-cyclopentene (5) with a selectivity of about 27 by a reaction sequence including an acetylene-vinylidene rearrangement and 1,5-C,H insertion of the intermediately formed alkylidenecarbene species. 5-methyl-1-hexyne (2) behaves analogously forming 3,3-dimethyl-1-cyclopentene (6), while 2-hexyne (3) provides 1-methyl-1-cyclopentene (7) indicating that the acetylene-vinylidene rearrangement is obviuosly not restricted to 1,2-H shifts.The mechanism of the cycloisomerization of alkylacetylenes is investigated by means of D-labeled parent alkynes.The results show that the unimolecular cycloisomerization via alkylidenecarbenes obviously can be an important channel despite the dominance of a radical chain course. - Keywords: Alkyne/ Carbene/ Cycloisomerization/ Pyrolysis
