1076-88-6Relevant academic research and scientific papers
Unexpected interactions between sol-gel silica glass and guest molecules. Extraction of aromatic hydrocarbons into polar silica from hydrophobie solvents
Badjic, Jovica D.,Kostic, Nenad M.
, p. 11081 - 11087 (2000)
Properties of a solute may differ greatly between a free solution and that solution confined in pores of a sol-gel glass. We studied the entry of various aromatic organic compounds from solution into the monolith of sol-gel silica immersed in this solution. Partitioning of the solute is quantified by the uptake coefficient, the ratio of its concentrations in the glass and in the surrounding solution at equilibrium. The dependence of this coefficient on the solvent gives insight into possible interactions between the solute and the silica matrix. We report the uptake of 31 compounds altogether: 18 halogen derivatives of benzene; 5 condensed (fused) aromatics; and stilbene and three substituted derivatives of it, each in both cis and trans configurations. When the solvent is hexane, the uptake coefficients are as follows: 1.0-1.9 for the halobenzenes; 3.0-4.6 for the hydrocarbons; and 3.3-4.9 for the stilbenes. When the solvent is carbon tetrachloride or dichloromethane, the uptake coefficients become 0.82-1.39 for the hydrocarbons and 0.90-1.25 for the stilbenes. The excessive uptake of organic compounds from hexane is unexpected, for it amounts to extraction of nonpolar or slightly polar solutes from a nonpolar solvent into a polar interior of silica glass. The solute-silica interactions responsible for this extraction are not of the van der Waals type. Our findings are consistent with hydrogen bonding between the aromatic n system in the solutes and the hydroxyl groups on the silica surface. Hexane cannot interact with this surface but dichloromethane and carbon tetrachloride can: they shield the hydroxyl groups from the organic solvents and thus suppress the hydrogen bonding. This explanation is supported by the emission spectra of the aromatic compound pyrene when it is dissolved in acetonitrile, dichloromethane, cyclohexyl chloride, and hexane and when it is taken up by monoliths of sol-gel silica from these four solutions. The relative intensities of the emission bands designated III and I change greatly when pyrene is taken up from hexane but remain unchanged when it is taken up from the other three solvents. Evidently, hexane does not, whereas the other three solvents do, line the silica surface and shield it from approach by pyrene molecules. Even though solute molecules are much smaller than the pores in the sol-gel glass,.diffusion of these molecules into the monolith may result in an uneven partitioning at equilibrium. This fact must be taken into consideration in the design of biosensors, immobilized catalysts, and other composite materials because their function depends on the entry of analytes, substrates, and other chemicals into the glass matrix.
Method for estimating SN1 rate constants: Solvolytic reactivity of benzoates
Matic, Mirela,Denegri, Bernard,Kronja, Olga
supporting information, p. 8986 - 8998,13 (2012/12/12)
Nucleofugalities of pentafluorobenzoate (PFB) and 2,4,6-trifluorobenzoate (TFB) leaving groups have been derived from the solvolysis rate constants of X,Y-substituted benzhydryl PFBs and TFBs measured in a series of aqueous solvents, by applying the LFER equation: log k = sf(Ef + Nf). The heterolysis rate constants of dianisylmethyl PFB and TFB, and those determined for 10 more dianisylmethyl benzoates in aqueous ethanol, constitute a set of reference benzoates whose experimental ΔG ? have been correlated with the ΔH? (calculated by PCM quantum-chemical method) of the model epoxy ring formation. Because of the excellent correlation (r = 0.997), the method for calculating the nucleofugalities of substituted benzoate LGs have been established, ultimately providing a method for determination of the SN1 reactivity for any benzoate in a given solvent. Using the ΔG? vs ΔH? correlation, and taking sf based on similarity, the nucleofugality parameters for about 70 benzoates have been determined in 90%, 80%, and 70% aqueous ethanol. The calculated intrinsic barriers for substituted benzoate leaving groups show that substrates producing more stabilized LGs proceed over lower intrinsic barriers. Substituents on the phenyl ring affect the solvolysis rate of benzhydryl benzoates by both field and inductive effects.
Room temperature observation of p-xylylenes by 1H NMR and evidence for diradical intermediates in their oligomerization
Trahanovsky, Walter S.,Lorimor, Steven P.
, p. 1784 - 1794 (2007/10/03)
p-Quinodimethanes (p-QDMs) are reactive molecules that have been invoked as transient intermediates in a number of reactions. Dilute solutions of benzene-based p-QDMs, p-xylylene (1), α-methyl-p-xylylene (10), and 2,5-dimethyl-p-xylylene (11) can be prepared by fluoride-induced elimination of trimethylsilyl acetate from the appropriate precursor. It has been found that these solutions are stable enough to allow these reactive p-QDMs to be observed by 1H NMR spectroscopy at room temperature. For the first time, the 13C NMR spectrum of p-QDM 1 was observed. After several hours at room temperature, these p-QDMs form dimers, trimers, and insoluble oligomers. Formation of trimers provides evidence that p-QDMs 1, 10, and 11 dimerize by a stepwise mechanism involving dimeric diradicals as intermediates.
Photochemical nitration by tetranitromethane. Part XXVI. Adduct formation in the photochemical reaction of 1,2,3-trimethylbenzene: The formation of 'double' adducts including nitronic esters
Butts, Craig P.,Eberson, Lennart,Hartshorn, Michael P.,Robinson, Ward T.,Timmerman-Vaughan, David J.,Young, Dawson A. W.
, p. 29 - 47 (2007/10/03)
The photolysis of the charge-transfer complex of 1,2,3-trimethylbenzene and tetranitromethane gives a complex mixture of products, most of which arise by initial attack of trinitromethanide ion on the unsubstituted ring positions at C4(C6) and C5 of the radical cation of 1,2,3-trimethylbenzene. The products 7-19 are adducts resulting directly or indirectly from the addition of the elements of tetranitromethane to 1,2,3-trimethylbenzene, and the trinitromethyl aromatic compounds 22-25 are formed by eliminations from intermediate adducts. Six adducts are simple 'single' adducts, nitro-trinitromethyl adducts 7, 8, 10-12, while nitro cycloadduct 9 is formed by cycloaddition of nitro-trinitromethyl adduct 8. The remaining addition products are 'double' adducts, formed by secondary addition reactions initiated by attack of nitrogen dioxide on the buta-1,3-diene system of 'single' adducts, and include trinitro-trinitromethyl compounds 13 and 15, the hydroxy-dinitro-trinitromethyl compound 14, and a group of four nitronic esters 16-19 formed by nitro-denitrocyclization of initially formed hydroxy-trinitromethyl and nitro-trinitromethyl 'single' adducts. Minor amounts of other products are formed including two nitrodienones 21 and 22, and the rearrangement product, 4,5,6-trimethyl-2-nitrophenol (28), and the 2,3,4-trimethyl- and 3,4,5-trimethylnitrobenzenes 26 and 27. The modes of formation of the above products are discussed, and X-ray crystal structure determinations are reported for compounds 9, 13, 14, 18, 19, 22 and 29. Acta Chemica Scandinavica 1996.
Nitration of 2,3,4,6-Tetramethylphenol and 1,2,3,5-Tetramethylbenzene
Hartshorn, Michael P.,Readman, Jennifer M.,Robinson, Ward T.,Vaughan, John
, p. 587 - 603 (2007/10/02)
Nitration of 1,2,3,5-tetramethylbenzene (2a) with fuming nitric acid gives the tetramethylnitrobenzene (22), products of side-chain modification (23)-(27), the rearranged 6,6-dimethylcyclohexenones (8), (28), (29) and (30), and 2,3,4,6-tetramethyl ketone
THE PHENYLCARBENE REARRANGEMENT REVISITED
Gaspar, Peter P.,Hsu, Jong-Pyng,Chari, Sarangan,Jones, Maitland Jr.
, p. 1479 - 1508 (2007/10/02)
The evolution of mechanistic ideas about the phenylcarbene rearrangement has been reviewed, and three closely linked problems have been identified toward whose solution this research has been aimed: 1.Why do the ratios of the stable end products from the rearrangements of o-, m- and p-tolylmethylene differ when all three reactions have been thought to pass through a common intermediate? 2.Why does the rearrangement of 2-methylcycloheptatrienylidene lead to exclusive formation of styrene? 3.What is the mechanism of styrene formation from o-tolylmethylene? New mechanisms have been proposed in which m- and p-tolylmethylene can rearrange to styrene without necessarily being converted to o-tolylmethylene.The formation of a small amount of 2,6-dimethylstyrene from the rearrangement of 3,4,5-trimethylphenylmethylene is viewed as evidence for such a mechanism, and a set of interconverting norcaradienylidenes are believed to be the crucial intermediates.Other alternatives are considered and rejected on the basis of the rearrangement products of 3,5-dimethyl- and 3,4,5-trimethylphenylmethylene.
