Chelates and

Zinc. Zinc, one of the most widely needed and used micronutrients, is appHed as sulfates (both basic and normal hydrates), carbonate, sulfide, phosphate, oxide, chelates, and other organic materials. Rates of Zn appHcation range from 0.2 to 22 kg/hm. ... 

Synthetic organic chelates and natural organic complexes are sometimes more effective agronomically per unit of micronuttient than inorganic forms, but the organic materials are more expensive. The chelates can be used with both orthophosphate and polyphosphate Hquids and suspensions. 

The bonding properties of (Ti02) have been used for size-reinforcing of glass fibers so that they adhere to asphalt or to a PTEE—polysulfide mixture to impart enhanced flex endurance (434—436). Poly(vinyl alcohol) (PVA) solutions mixed with sucrose can be cross-linked with the lactic acid chelate and used generally for glass-fiber sizing (437). 

Table 1 Hsts a number of chelating agents, grouped according to recognized stmctural classes. Because systematic nomenclature of chelating agents is frequently cumbersome, chelants are commonly referred to by common names and abbreviations. For the macrocyclic complexing agents, special systems of abbreviated nomenclature have been devised and are widely used. Some of the donor atoms involved ia chelation and the many forms ia which they can occur have been reviewed (5).

The calcium form of EDTA instead of free EDTA is used in many food preparations to stabilize against such deleterious effects as rancidity, loss of ascorbic acid, loss of flavor, development of cloudiness, and discoloration. The causative metal ions are sequestered by displacing calcium from the chelate, and possible problems, such as depletion of body calcium from ingestion of any excess of the free chelant, had it been used, are avoided. 

In the treatment of poisoning by lead or other metal ions, higher concentrations of chelant can be safely obtained in humans by administering Na2CaEDTA rather than Na EDTA. The metal ion is bound by displacing small amounts of Ca " that the body can tolerate. Use of Na EDTA would result in calcium chelation and thus serious depletion of calcium in the body fluids (44). Removal of iron in Cooley s anemia is accompHshed by using chelants that are relatively specific for iron (45). 

Frozen Food. The chelating and acidic properties of citric acid enable it to optimize the stabiUty of frozen food products by enhancing the action of antioxidants and inactivating naturally present enzymes which could cause undesirable browning and loss of firmness (57,58). 

The first synthesis of enterobactin, a microbial chelator and transporter of environmental iron, was accomplished by the coupling of three protected L-serine units and macrocyclization by the double activation method. 

Hexafluorotropone is readily hydrolyzed in aqueous sodium hydroxide to give the three monohydroxy isomers [6] (equation 8) The major product is 1 hydroxy-pentafluorotropone, the other two isomers are formed in about equal quantities Separation of the 2- and 3-hydroxy isomers from the mixture is based on the ability of the 2-hydroxy isomer to form a cupric chelate, and of the 3-hydroxy isomer to precipitate as an 5 benzylthiouronium salt... 

In addition to the oxide carboxylates, beryllium forms numerous chelating and bridged complexes with ligands such as the oxalate ion C204 , alkoxides, /9-diketonates and 1,3-diketonates. These almost invariably feature 4-coordinate Be... 

Fig. 8.22) features a unique tris(bidentate) sex-tuply bridging carbonato ligand as well as three bidentate piT-carbonato ligands. Other chelating and bridging coordination modes are also known.< 2 >... 

On the basis of the values of AS° derived in this way it appears that the chelate effect is usually due to more favourable entropy changes associated with ring formation. However, the objection can be made that and /3l-l as just defined have different dimensions and so are not directly comparable. It has been suggested that to surmount this objection concentrations should be expressed in the dimensionless unit mole fraction instead of the more usual mol dm. Since the concentration of pure water at 25°C is approximately 55.5 moldm , the value of concentration expressed in mole fractions = cone in moldm /55.5 Thus, while is thereby increased by the factor (55.5), /3l-l is increased by the factor (55.5) so that the derived values of AG° and AS° will be quite different. The effect of this change in units is shown in Table 19.1 for the Cd complexes of L = methylamine and L-L = ethylenediamine. It appears that the entropy advantage of the chelate, and with it the chelate effect itself, virtually disappears when mole fractions replace moldm . ... 

For the reactions of other 1,3-dipoles, the catalyst-induced control of the enantio-selectivity is achieved by other principles. Both for the metal-catalyzed reactions of azomethine ylides, carbonyl ylides and nitrile oxides the catalyst is crucial for the in situ formation of the 1,3-dipole from a precursor. After formation the 1,3-di-pole is coordinated to the catalyst because of a favored chelation and/or stabiliza-... 

UV absorbers have been found to be quite effective for stabilization of polymers and are very much in demand. They function by the absorption and harmless dissipation of the sunlight or UV-rich artificial radiation, which would have otherwise initiated degradation of a polymer material. Meyer and Geurhart reported, for the first time in 1945 [10], the use of UV absorber in a polymer. They found that the outdoor life of cellulose acetate film was greatly prolonged by adding phenyl salicylate (salol) [10]. After that, resorcinol monobenzoate, a much more effective absorber, was introduced in 1951 [11] for stabilization of PP, but salol continued to be the only important commercial stabilizer for several years. The 2,4-dihydroxybenzophenone was marketed in 1953, followed shortly by 2-hydroxy-4-methoxybenzophenone and other derivatives. Of the more commonly known UV absorbers, the 2-hydroxybenzophenones, 2-hy-droxy-phenyl-triazines, derivatives of phenol salicylates, its metal chelates, and hindered amine light stabilizers (HALS) are widely used in the polymer industry. 

In the case of inorganic solutes we are concerned largely with samples in aqueous solution so that it is necessary to produce substances, such as neutral metal chelates and ion-association complexes, which are capable of extraction into organic solvents. For organic solutes, however, the extraction system may sometimes involve two immiscible organic solvents rather than the aqueous-organic type of extraction. 

This section provides a brief review of a number of chelating and other extraction reagents, as well as some organic solvents, with special interest as to their selective extraction properties. The handbook of Cheng et al. should be consulted for a more detailed account of organic analytical reagents.11... 

Mixtures of manganese, magnesium, and zinc can be similarly analysed. The first EDTA end point gives the sum of the three ions. Fluoride ion is added and the EDTA liberated from the magnesium-EDTA complex is titrated with manganese ion as detailed above. Following the second end point cyanide ion is added to displace zinc from its EDTA chelate and to form the stable cyanozincate complex [Zn(CN)4]2- the liberated EDTA (equivalent to the zinc) is titrated with standard manganese-ion solution. 

Lindo-rv.ff oselecli ve oxacydization of polyepoxides is less common the first ster-eospedfic tandem endo-regioselective biomimetic oxacydization of polyepoxides to fused THP rings has only recently been reported [38a], The cyclization of the hydroxy-methoxymethyl-substituted triepoxide 75 (a 1,4,7-triepoxide), promoted and directed by La(OTf)3 through an intramolecular chelation and based on a procedure originally described for a monoepoxide system [38b], afforded the tri-... 

The second important group of configuralionally stable bis-protected a-amino aldehydes are the V-dibenzvl derivatives 5, easily prepared from amino acids in a three-step procedure65. These aldehydes react with various nucleophiles to normally provide the nonchelation-con-trolled adducts in high diastereoselectivity. This anti selectivity is observed when diethyl ether or telrahydrofuran is used as reaction solvent. Certain Lewis acidic nucleophiles or additives, such as tin(IV) chloride, in dichloromethane as solvent force chelation and therefore provide the. syn-adducts, once again with a high diastereoselectivity. 

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