18 crown

Crown 6 potassium fluonde complex (in solution)  [c.671]

Find the molecular model of 18 crown 6 (see Figure 16 2) on Learning By Modeling and examine its electrostatic potential map View the map in vanous modes (dots contours and as a transparent surface) Does 18 crown 6 have a dipole moment Are vicinal oxygens anti or gauche to one another"d  [c.700]

Chiral crown ethers based on 18-crown-6 (Fig. 9) can form inclusion complexes with ammonium ions and protonated primary amines. Immobilisation of these chiral crown ethers on a chromatographic support provides a chiral stationary phase which can resolve most primary amino acids, amines, and amino alcohols. However, the stereogenic center must be in fairly close proximity to the primary amine for successful chiral separation (105,106). Significandy, the chiral crown ether phase is unique in that it is one of the few Hquid chromatographic chiral stationary phases that does not require the presence of an aromatic ring to achieve chiral separations. Although chiral recognition seems to be enhanced for analytes containing either bulky substituents or aromatic groups near the stereogenic center, only the presence of the primary amine is mandatory.  [c.67]

The match between crown cavity diameter and cation diameter is obvious from Table 3 showing that, eg, and 12-crown-4 (la) or, respectively and 18-crown-6 (Ic) correspond. Similar are the cryptands of gradually increasing cavity size [2.1.1], [2.2.1] and [2.2.2] for and  [c.178]

Adipic acid is a colorless, odorless, sour tasting crystalline soHd. Its fundamental chemical and physical properties are Hsted in Table 1. Further information may be obtained by referring to studies of infrared and Raman spectroscopy of adipic acid crystals (11,12), ultraviolet spectra of solutions (13), and specialized thermodynamic properties (14,4). SolubiUty and solution properties are described in Table 2. The crystal morphology is monoclinic prisms strongly influenced by impurities (21). Both process parameters (22) and additives (21) profoundly affect crystal morphology in the crystallization of adipic acid, an industrially significant process. Aqueous solutions of the acid are corrosive and their effect on various steel alloys have been tested (23). Generally, austenitic stainless steels containing nickel and molybdenum and over 18% chromium are resistant. Data on 20 metals were summarized in one survey (24). Bulk and handling properties of adipic acid are summarized in Table 3.  [c.238]

Simple and Complex Organic Molecules. Using modem direct fluorination technology, the synthesis of even the most complex perfluorocarbon stmctures from hydrocarbon precursors is now possible. For example, syntheses of the first perfluoro crown ethers, perfluoro 18-crown-6, perfluoro 15-crown-5, and perfluoro 12-crown-4 (54) have been reported. Perfluoro crown ethers (54,55) are becoming important as the molecules of choice for many F-nmr imaging appHcations (56) in humans and are particularly effective in brain and spinal diagnostics when  [c.278]

Halex rates can also be increased by phase-transfer catalysts (PTC) with widely varying stmctures quaternary ammonium salts (51—53) 18-crown-6-ether (54) pytidinium salts (55) quaternary phosphonium salts (56) and poly(ethylene glycol)s (57). Catalytic quantities of cesium duoride also enhance Halex reactions (58).  [c.319]

Many types of stainless steels are available. The type most widely used in the dairy industry is 18—8 (18% chromium, 8% nickel plus iron). Small amounts of siHcon, molybdenum, manganese, carbon, sulfur, and phosphoms maybe included to obtain characteristics desired for specific appHcations.  [c.360]

Aluminosilicate of faujasite topology, but higher Si/A1 ratio (<5), has been synthesized with the use of a crown ether, 15-crown-5, as the directing agent. The same Si/A1 ratio was obtained when 18-crown-6 was applied, but the topology, although related to faujasite, had a different stacking order of the sodahte cages, so that the stmcture has hexagonal instead of cubic symmetry (60). This product, sometimes called hexagonal faujasite, has the designation  [c.459]

The unsaturation present at the end of the polyether chain acts as a chain terminator ia the polyurethane reaction and reduces some of the desired physical properties. Much work has been done ia iadustry to reduce unsaturation while continuing to use the same reactors and hoi ding down the cost. In a study (102) usiag 18-crown-6 ether with potassium hydroxide to polymerise PO, a rate enhancement of approximately 10 was found at 110°C and slightly higher at lower temperature. The activation energy for this process was found to be 65 kj/mol (mol ratio, r = 1.5 crown ether/KOH) compared to 78 kj/mol for the KOH-catalysed polymerisation of PO. It was also feasible to prepare a PPO with 10, 000 having narrow distribution at 40°C with added crown ether (r = 1.5) (103). The polymerisation rate under these conditions is about the same as that without crown ether at 80°C.  [c.352]

The most common ferritic grade is Type 430, containing 0.12% carbon or less and 14—18% chromium. Because of its high chromium content, the corrosion resistance of Type 430 is superior to that of the martensitic grades. Furthermore, Type 430 may be drawn, formed, and, using proper techniques, welded. At one time widely used for automotive and architectural trim, it is also employed in equipment for the manufacture and handling of nitric acid (qv), to which it is resistant. Type 430 does not have high creep strength but is suitable for some types of service up to 815°C, and thus has appHcation in combustion chamber for domestic heating furnaces.  [c.399]

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules.  [c.382]

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use.  [c.393]

Alkali Metal Catalysts. The polymerization of isoprene with sodium metal was reported in 1911 (49,50). In hydrocarbon solvent or bulk, the polymerization of isoprene with alkaU metals occurs heterogeneously, whereas in highly polar solvents the polymerization is homogeneous (51—53). Of the alkah metals, only lithium in bulk or hydrocarbon solvent gives over 90% cis-1,4 microstmcture. Sodium or potassium metals in / -heptane give no cis-1,4 microstmcture, and 48—58 mol % /ram-1,4, 35—42% 3,4, and 7—10% 1,2 microstmcture (46). Alkali metals in benzene or tetrahydrofuran with crown ethers form solutions that readily polymerize isoprene however, the 1,4 content of the polyisoprene is low (54). For example, the polyisoprene formed with sodium metal and dicyclohexyl-18-crown-6 (crown ether) in benzene at 10°C contains 32% 1,4-, 44% 3,4-, and 24% 1,2-isoprene units (54).  [c.4]

Displacements forming carbon to heteroatom bonds are very important also in the synthesis of large rings. Chain length, chain flexibility and the cis or trans geometry of double bonds (if any) obviously have a strong effect on the rate of ring formation. Seven-membered rings are obtained readily (Sections but larger rings can also be obtained, such as [18]-crown-6 (Section  [c.33]

Diederich F, Jonas U, Gramlich V, Herrmann A, Ringsdorf H and Thilgen C 1993 Synthesis of a fullerene derivative of benzo[18]crown-6 by Diels-Alder reaction complexation ability, amphiphilic properties, and x-ray crystal structure of a dimethoxy-1,9-(methano[1, 2]benzomethano)fullerene[60] benzene clathrate Helv. Chim. Acta 76 2445-53  [c.2430]

The intramolecular carbopalladation (or insertion) of the triple bond in dimethyl 4-pentynylmalonate (215) with Pd—H species and malonate anion as shown by 216 proceeds in the presence of f-BuOK and 18-crown ether, affording the methylenecyclopentane derivatives 217 and 218, the amounts of which depend on the reaction conditions. The Pd—H species may be formed  [c.497]

A two-step synthesis of indoles from o-nitrobenzaldehydes proceeds by condensation with nitromcthanc followed by reductive cyclization. Like the Leim-gruber Batcho method, the principal application of the reaction is to indoles with only carbocyclic substituents. The forniation of the o,p-dinitrostyrenes is usually done under classical Henry condensation conditions but KF/18-crown-6 in propanol was found to be an advantageous reaction medium for acetoxy-substituted compounds[1]. The o,p-dinitrostyrenes can also be obtained by nitration of p-nitrostyrenes[2].  [c.11]

Crown 4 and 18 crown 6 are a cyclic tetramer and hexamer respectively of repeat mg —OCH2CH2— units they are polyethers based on ethylene glycol (HOCH2CH2OH) as the parent alcohol  [c.669]

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this  [c.669]

In media such as water and alcohols fluoride ion is strongly solvated by hydro gen bonding and is neither very basic nor very nucleophilic On the other hand the poorly solvated or naked fluoride 10ns that are present when potassium fluoride dis solves m benzene m the presence of a crown ether are better able to express their anionic reactivity Thus alkyl halides react with potassium fluoride m benzene containing 18 crown 6 thereby providing a method for the preparation of otherwise difficultly acces sible alkyl fluorides  [c.671]

Retardation and Control. This influence may manifest itself in a reduced volatility and therefore lower possible storage and handling problems of a compound when included toxic and ha2ardous substances become safer. As most inclusion compounds dissociate in a solvent or decompose under intensified conditions, it is easy to recover the guest. 18-crown-6 ((3) Fig. 3), for example, forms a solid inclusion complex with dimethyl sulfate that makes this dangerous compound easy to handle and to dose (130). The hydroquinone clathrate of krypton can be used as a way of handling radioactive krypton ( Kr) turned into a soHd, thus providing a safe and useful radioactive source (131). Moreover, odorous substances and flavoring compounds may be solidified via inclusion and reduced in their vapor pressure and volatility which makes controlled, retarded, and suppressed release possible. Cyclodextrin inclusions (Fig. 7) are most useful in this sense and aroused great value for many industries including cosmetology, food industry, laundering, pharmacy, agriculture, and others (57,132,133). For instance, cyclodextrins can be used to trap clinging and unpleasant odors (fish, garlic, cigarettes, alcohol). They can be used in the form of Hquids, powders, tablets, chewing gums, toothpastes, sprays, and mouthwashes (133). Interesting examples of prolonged release obtained with cyclodextrins concern dmgs (134), pesticides (135), and pheromones (136). An extreme form of molecular imprisonment and environmental shielding is found in carceplexes (see (17) Fig. 5) that can release guest molecules only by breaking covalent bonds of the container shell (46).  [c.75]

In 1967, DuPont chemist Charles J. Pedersen (21) discovered a class of ligands capable of complexing alkaU metal cations, a discovery which led to the Nobel Prize in Chemistry in 1987. These compounds, known as crown ethers or cryptands, allow gready enhanced solubiUty of sodium and other alkaU metals in amines and ethers. About 50 crown ethers having between 9—60 membered oligoether rings were described (22). Two such stmctures, dibenzo-18-crown-6 (1) and benzo-9-crown-3 (2), are shown.  [c.163]

The austenitic iron—chromium—nickel alloys were developed in Germany around 1910 in a search for materials for use in pyrometer tubes. Further work led to the widely used versatile 18% chromium—8% nickel steels, the socaHed 18—8.  [c.397]

See pages that mention the term 18 crown : [c.116]    [c.17]    [c.90]    [c.669]    [c.671]    [c.261]    [c.297]    [c.297]    [c.307]    [c.176]    [c.178]    [c.326]    [c.62]    [c.62]    [c.383]    [c.428]    [c.167]    [c.169]    [c.453]    [c.62]    [c.250]    [c.13]    [c.13]   
Carey organic chemistry (0) -- [ c.6 , c.665 ]