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Search Results: 1 - 10 of 4645 matches for " Halogen and Methyl Exchange "
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An Adjusted Model for Simple 1,2-Dyotropic Reactions. Ab Initio MO and VB Considerations  [PDF]
Henk M. Buck
Open Journal of Physical Chemistry (OJPC) , 2013, DOI: 10.4236/ojpc.2013.33015
Abstract:

With an adjusted model, we reconsider simple 1,2-dyotropic reactions with the introduction of a concept based on the intramolecular dynamics of a tetrahedron (van ’t Hoff modeling). In fact the dyotropic reactions are strongly related to conversions originated from neighbouring group participation or anchimeric assistance, defined as the interaction of a center with a lone pair of electrons in an atom and the electrons present in aδor π bond. The researchful 1,2-dyotropic reactions, based on the 1,2-interchange of halogens, methyl and hydrogen taking place in a concerted fashion, are in competition with the two-step reaction in which the neighbouring group participation or anchimeric assistance comes to full expression by ionic dissociation of the other exchangeable (halogen) atom. As to be expected there is an essential difference between halogen or methyl exchange regarding the number of electrons participating in the transition state. This aspect becomes evident in the geometries of the corresponding transition state geometries. In this paper we refer to ab initio MO calculations and VB considerations. We consider the 1,2-halogen exchange as a combination of two SN2 reactions each containing four electrons. The van ’t Hoff dynamics appears a useful model in order to illustrate the computations in a straightforward manner.

Halogen Exchange in Near-Critical Water  [PDF]
Abdulrahman M. Alhazmi, Pia R. Alburquerque, Thomas Junk
Green and Sustainable Chemistry (GSC) , 2011, DOI: 10.4236/gsc.2011.14021
Abstract: Bromoaromatics are ubiquitous in chemistry, and their manufacture is often wasteful. Halogen exchange under hydrothermal conditions constitutes a viable alternative for their synthesis in some cases. The prepara-tion of 1,2-dibromobenzene and 1-bromo-2-chlorobenzene from 1,2-dichlorobenzene, by treatment with hy-drobromic acid in hydrothermal media at temperatures ranging from 240?C to 320?C was investigated as a viable alternative to de novo synthesis. The effects of temperature, exchange duration and the presence of Fe3+ salts on product yields are discussed. Yields for both targeted haloarenes of up to 37% and 48%, respec-tively, were achieved, with very limited formation of 1,3- and 1,4-dihalobenzene isomers. A mechanism for halogen exchange was proposed.
Synthesis of 2,7-Bis(fluoromethyl)naphthalene
Jane Bogdanov,Przemyslaw Maslak
Molbank , 2008, DOI: 10.3390/m567
Abstract: No abstract available
Solvent and electronic effects on kinetics of cyclization of thermolabile aryllithium reagents. A comparison between 1-bromo-2-(2-bromoethyl)benzene and 4,5-dimethoxy-1-bromo-2-(2-bromoethyl)benzene
David A. Hunt
Organic Communications , 2009,
Abstract: A dramatic solvent effect on the stability and kinetics of intramolecular cyclization is described for the aryllithium species generated from 2-bromo-4,5-dimethoxy-(2-bromoethyl)benzene. The aryllithium generated by the halogen-metal exchange reaction with n-butyllithium, is stable for > 1h when generated at -95 to -100 oC in diethyl ether/hexane and can be trapped with electrophiles. However, when the reaction is conducted in a THF/hexane mixture, the intermediate undergoes instantaneous intramolecular cyclization to afford 4,5-dimethoxybenzocyclobutene. By comparison, the corresponding 1-lithio-2-(2-bromoethyl)-benzene intermediate is stable for >1h in either THF/hexane or diethyl ether/hexane at -95 to -100 oC. These results indicate that substituent effects as well as the nature of aggregation of these intermediates play key roles in determining the reaction pathway of functionalized aryllithium intermediates when quenched with electrophiles.
Synthesis of 2-Fluoromethyl-7-methylnaphthalene
Jane Bogdanov,Przemyslaw Maslak
Molbank , 2008, DOI: 10.3390/m568
Abstract: No abstract available
Intramolecular carbolithiation cascades as a route to a highly strained carbocyclic framework: competition between 5-exo-trig ring closure and proton transfer
William F. Bailey,Justin D. Fair
Beilstein Journal of Organic Chemistry , 2013, DOI: 10.3762/bjoc.9.59
Abstract: The preparation of fairly strained carbocyclic ring systems by intramolecular 5-exo-trig ring closure has been well documented, and the absence of proton transfers that would compromise such cyclizations is a hallmark of this chemistry. In an effort to explore the limitations of this approach to more highly strained systems, the preparation of a stellane (tricyclo[3.3.0.03,7]octane) framework by an intramolecular carbolithiation cascade involving three coupled 5-exo-trig cyclizations of the vinyllithium derived from 2-bromo-4-vinyl-1,6-heptadiene by lithium–bromine exchange was investigated. The cascade does not afford the stellane; rather, the cascade is terminated after two cyclizations by a proton transfer that occurs by an intermolecular process catalyzed by trace amounts of endo-5-methyl-2-methylenebicyclo[2.2.1]heptane present in reaction mixtures as a consequence of inadvertent quenching of an intermediate alkyllithium during prolonged reaction times at room temperature.
Copper and molybdenum sorption onto selective ion exchangers
Asresahegnova, Z.,Jelinek, L.
Ion Exchange Letters , 2009,
Abstract: The sorption of Cu and Mo was carried out by dynamic experiments using glass vertical columns. Two types of sorbents were used for sorption (ion exchange). Tests proceeded under following conditions: sorbent volume – 15 mL, specific flow rate – 6 BV/h, inlet molybdenum and copper concentration – 10 mg/L, three pH scales – 3.5; 4 and 4.5. The first ion exchanger – protonized Purolite D4123 (styrene-divinylbenzene matrix and 1-deoxy-1-methyl-amino-D-glucitol functional group) was not effective for copper sorption at all studied pHs (3.5; 4; 4.5). However, due to the high sorption capacity of D4123 for molybdenum (0.82 mol/L, pH=3.5), mixed bed was made from ion exchangers. It was composed of Purolite D4123 and Lewatit TP 207 (in form of free acid) in volume ratio 1:1. Lewatit TP 207 ion exchanger contains iminodiacetate (IDA) functional group. In this case, sorption was successful at all three initial pHs. The best breakthrough capacities, reached at pH of 3.5, were 0.37 mol/L and 0.37 mol/L for copper and molybdenum, respectively. The concurrent sorption of Cu and Mo on TP207 at pH 4 showed molybdenum breakthrough capacity 0.013 mol/L. Therefore isolated sorption of Mo on TP207 was important to measure. In this case, molybdenum was probably in the form of molybdenyl cation.
STUDY ON POLY (METHYL METHACRYLAMIDE )-TYPE LIGAND EXCHANGE CHROMATOGRAPHIC SUPPORTS
聚甲基丙烯酰胺型配体交换色谱载体的研究

YAN Hu-sheng,CHENG Xiao-hui,HE Bing-lin,
阎虎生
,程晓辉,何炳林

高分子学报 , 1993,
Abstract:
Risk Points of Flame Retardant Textiles by Halogen and Halogen-Free Laminating Film  [PDF]
Yoon Jeong Baek, Jeoung Hwa Shin
Materials Sciences and Applications (MSA) , 2014, DOI: 10.4236/msa.2014.511083
Abstract: This study was to develop the flame retardant (FR) protective clothing which had multifunction such anti-bacterial, UV cut, FR function with water repellent and water vapor permeable laminating textiles for industrial workers. First of all, the FR yarn and FR textiles were developed for this purpose. Second, the comparison analysis between the halogen laminating textiles and halogen-free laminating textiles were tested to figure out the eco-friendly laminating method. Third, the flame retardant ability was compared the halogen laminated textiles to halogen-free laminated textiles. LOI, UV protection ratio, antibiosis after 50 laundry test, water proof pressure, and moisture permeability of developed textiles were tested. GC-HR-TOF-MS was used for analysis of laminating film (halogen and halogen-free). 4.1 wt% TiO2 yarn showed antibacterial function (Pneumococcus & Staphylococcus aureus: 99.9%), UV Protection (UVA: 90.8, UVB: 92.1), and LOI (33.6). The chosen optimal compounding ratio for PU compound of HRF and HFFR were as followed: PU resin 58.3%, DMF (Dimethyl formamide, δ = 12.2) 8.3%, MEK (Methylethylketone) 8.3% and FR (flame retardants) 25.0%. Binder for laminating should not be included over 10% of FRs because of adhesion between textiles and FR laminating film. There were detected phosphorus compounds in the textiles treated by halogenated type flame retardants and halogenated-free type flame retardants. There were not any detected harmful compounds from all textile samples.
Effect of light-curing units in shear bond strength of metallic brackets: an in vitro study
Retamoso, Luciana Borges;Onofre, Niége Michelle Lazzari;Hann, Luciane;Marchioro, Ernani Menezes;
Journal of Applied Oral Science , 2010, DOI: 10.1590/S1678-77572010000100012
Abstract: objectives: to determine the influence of the light curing units on the shear bond strength of orthodontic brackets. material and methods: seventy-two premolars were divided into six groups (n=12): group i: brackets bonded with transbond and polymerization with halogen light; group ii: transbond and led; group iii: fuji ortho and halogen light; group iv: fuji ortho and led; group v: fuji ortho, without acid and halogen light; group vi: fuji ortho, without acid and led. the groups were tested to shear strength in a universal testing machine at a crosshead speed of 0.5 mm/min. data were analyzed statistically by anova and tukey's test. results: the composite resin presented higher shear bond strength than the resin-modified glass ionomer cement (p<0.05). the halogen light and led sources produced similar shear bond strength (p>0.05). conclusion: the shear bond strength was influenced by the material but not by the light-curing unit. the use of led reduced the experimental time by approximately 60%, with the same curing efficiency.
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