Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Fluoride deficiency

Several studies have shown that the fluoride balance can be negative [8]. This occurs when the chronic intake is reduced to the extent that concentrations of fluoride in plasma fall and mobilization of fluoride from calcified tissues proceeds. However, there is no evidence so far from human studies that clinical signs of fluoride deficiency exist no specific clinical or biochemical diagnostic parameters have been related to fluoride deficiency [8,39]. In the present state of knowledge, therefore, negative fluoride balance cannot be used for establishing an Al of fluoride. [Pg.536]

In some areas where fluoride is absent from drinking water, the school water supply has been fluoridated as much as 4.5 times the level usually recommended for community water levels. These studies, conducted for 12 years in fluoride-deficient areas, revealed a 40% reduction in DMF surfaces. Essentially, no undesirable fluorosis resulted from this procedure. However, it should be noted that ingestion occurred only during part of each day and only on school days. This level (4.5 ppm) from birth (during major formation of... [Pg.891]

Fluoride is a monovalent anion. It appears not to be an essential nutrient, as raising animals on a fluoride-deficient diet over three generations has been found not to impair growth and reproduction. Fluoride has proven to be of value in reducing the rate of dental caries (tooth decay). There has been much interest in the possibility that fluoride can prevent and reverse osteoporosis, though such effects have not been proven. [Pg.778]

Another mineral which is considered to be supplemented in drinking water is fluoride. Fluoride is added to drinking water to prevent the incidence of dental caries, especially seen in children with a fluoride-deficient diet. The subject is, however, a controversial one since excess fluoride is believed to have adverse effects such as dental fluorosis, which is the yellow staining of the teeth. [Pg.14]

Although fluoride has beneficial effects, there is no evidence that it is a dietary essential. Fluoride prevents dental decay, but it is probably not correct to call dental decay a fluoride deficiency disease. [Pg.415]

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. [Pg.98]

Cryolite. Cryohte constitutes an important raw material for aluminum manufacturing. The natural mineral is accurately depicted as 3NaF AIF., but synthetic cryohte is often deficient in sodium fluoride. Physical properties are given in Table 4. [Pg.142]

Unless a deficiency of xenon fluoride was used in the preparation at 0°C or below, the title product exploded violently on warming to ambient temperature. [Pg.145]

Clinical syndromes are associated with deficiencies of the following trace elements zinc, copper, manganese, selenium, chromium, iodine, fluoride, molybdenum, and iron. [Pg.664]

There is only one simple anion commonly found in soil, and that is chloride (Cl ). Chloride is an essential nutrient for plants but is typically present in sufficiently high concentrations that deficiencies are never observed. If other halogens are present, they will also be present as simple anions. Most soils do contain small amounts of bromide as the second most common simple anion. In some cases, significant levels of fluoride and iodide may be present, although this is rare. These anions are generally soluble in water and tend to exist as the simple anion. However, they can combine with other components and exist as other species. For instance, halogens are present in organic compounds such as solvents, insecticides, and herbicides, which can be soil contaminants. There are also other nonionic species of these elements that may be present [20],... [Pg.140]

The base-catalysed addition of thiols to Jt-electron-deficient alkenes is an important aspect of synthetic organic chemistry. Particular use of Triton-B, in place of inorganic bases, has been made in the reaction of both aryl and alkyl thiols with 1-acyloxy-l-cyanoethene, which behaves as a formyl anion equivalent in the reaction [1], Tetra-n-butylammonium and benzyltriethylammonium fluoride also catalyse the Michael-type addition of thiols to a,P-unsaturated carbonyl compounds [2], The reaction is usually conducted under homogeneous conditions in telrahydrofuran, 1,2-dimethoxyethane, acetone, or acetonitrile, to produce the thioethers in almost quantitative yields (Table 4.22). Use has also been made of polymer-supported qua-... [Pg.144]

Examples of the Michael-type addition of carbanions, derived from activated methylene compounds, with electron-deficient alkenes under phase-transfer catalytic conditions have been reported [e.g. 1-17] (Table 6.16). Although the basic conditions are normally provided by sodium hydroxide or potassium carbonate, fluoride and cyanide salts have also been used [e.g. 1, 12-14]. Soliddiquid two-phase systems, with or without added organic solvent [e.g. 15-18] and polymer-supported catalysts [11] have been employed, as well as normal liquiddiquid conditions. The micellar ammonium catalysts have also been used, e.g. for the condensation of p-dicarbonyl compounds with but-3-en-2-one [19], and they are reported to be superior to tetra-n-butylammonium bromide at low base concentrations. [Pg.274]

Nitroalkanes react with Jt-deficient alkenes, for example, p-nitro ketones are produced from a,P-unsaturated ketones [41], whereas allylic nitro compounds have been prepared via the Michael-type addition of nitroalkanes with electron-deficient alkynes (Table 6.19). The reaction in either dimethylsulphoxide [42] or dimethyl-formamide [43] is catalysed by potassium fluoride in the presence of benzyltriethyl-ammonium chloride the reaction with dimethyl acetylenedicarboxylate is only successful in dimethylsulphoxide [42], Primary nitroalkanes produce double Michael adducts [42,44], A-Protected a-aminoacetonitriles react with alkynes under catalysed solidiliquid conditions to produce the Michael adducts [45] which, upon treatment with aqueous copper(Il) sulphate, are converted into a,p-unsaturated ketones. [Pg.281]

Cyloheptatrienylidene carbene is generated when trimethylsilyltropylium tetra-fluonoborate is treated with a stoichiometric excess of tetra-n-butylammonium fluoride in dichloromethane [50], Although the carbene dimerizes readily, it will react with electron-deficient alkenes (see Section 7.3). Tetra-n-butylammonium fluoride in a stoichiometric amount promotes the formation of adamantylidenevinylidene from 2-bromo-2-(trimethylsilylethynyl)adamantane [51 ]. [Pg.311]


See other pages where Fluoride deficiency is mentioned: [Pg.495]    [Pg.495]    [Pg.325]    [Pg.287]    [Pg.318]    [Pg.320]    [Pg.100]    [Pg.406]    [Pg.295]    [Pg.495]    [Pg.495]    [Pg.325]    [Pg.287]    [Pg.318]    [Pg.320]    [Pg.100]    [Pg.406]    [Pg.295]    [Pg.68]    [Pg.516]    [Pg.189]    [Pg.1104]    [Pg.991]    [Pg.184]    [Pg.259]    [Pg.767]    [Pg.151]    [Pg.783]    [Pg.76]    [Pg.246]    [Pg.1547]    [Pg.321]    [Pg.226]    [Pg.86]    [Pg.530]    [Pg.309]    [Pg.138]    [Pg.111]    [Pg.269]    [Pg.221]    [Pg.259]    [Pg.18]    [Pg.179]    [Pg.111]   
See also in sourсe #XX -- [ Pg.2565 ]




SEARCH



© 2024 chempedia.info