Rare halides

Complex 6 is one of the rare halide probes that is applicable in neutral aqueous solution [27]. The quenching effect on 6 follows the order 1 > Br > Cl . 

A new family of catalysts has recently produced dramatically improved results for both enantioselective aUcoxy-and hydroxycarbonylation. When diphosphine ligands based on the planar chiral paracyclophane backbone are treated with two equivalents of [PdCl2(PhCN)2], they can form very rare halide-bridged dipalladium complexes (Scheme 14.29). °°... 

Rokni M and Jaoob J FI 1982 Rare-gas halide lasers Applied Atomic Collision Physics, Vol 3, Gas Lasers ed FI S W Massey, E W MoDaniel, B Bederson and W L Nighan (New York Aoademio)... 

The element may be obtained by separating neodymium salts from other rare earths by ion-exchange or solvent extraction techniques, and by reducing anhydrous halides such as NdFs with calcium metal. Other separation techniques are possible. 

The order of alkyl halide reactivity in nucleophilic substitutions is the same as their order m eliminations Iodine has the weakest bond to carbon and iodide is the best leaving group Alkyl iodides are several times more reactive than alkyl bromides and from 50 to 100 times more reactive than alkyl chlorides Fluorine has the strongest bond to car bon and fluonde is the poorest leaving group Alkyl fluorides are rarely used as sub states m nucleophilic substitution because they are several thousand times less reactive than alkyl chlorides... 

A reaction useful only with sub strates that do not undergo E2 elimi nation readily It is rarely used for the synthesis of alcohols since alkyl halides are normally prepared from alcohols... 

Tetrakis-Cp uranium complexes are readily prepared via metathesis of UCl and KCp in refluxing benzene. These complexes are a relatively rare example of a pseudotetrahedral complex with four Tj -Cp rings, (rj-ring) (25). The Cp derivative has been shown to react with CO and CO2 to give acyl and carboxylato complexes. This complex also reacts with alkyl haUdes to afford the U(IV) complex, Cp UX (X = halide). 

Allyl Complexes. Allyl complexes of uranium are known and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a uranium halide and an allyl grignard reagent. This synthetic method was utilized to obtain a rare example of a "naked" homoleptic allyl complex, U(T -C2H )4 [12701 -96-17, which decomposes at 0°C. Other examples, which are more stable than the homoleptic allyl complex have been synthesized, ie, U(allyl)2(OR)2 (R = alkyl), U(allyl)2X (X = halide), and U(allyl)(bipy)2. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Halide complexes are also well known but complexes with nitrogen-containing ligands are rare. An exception is the blue phthalocyanine complex formed by reaction of Be metal with phthalonitrile, 1,2-C6H4(CN)2, and this affords an unusual example of planar 4-coordinate Be (Fig. 5.5). The complex readily picks up two molecules of H2O to form an extremely stable dihydrate, perhaps by dislodging 2 adjacent Be-N bonds and forming 2 Be-O bonds at the preferred tetrahedral angle above and below the plane of the macrocycle. 

Grignard reagents are rapidly hydrolysed by water or acid to give the parem hydrcxatbon, RH, but this reacdon is rarely of synthedc importance. Hydrocarbons can also be syndiesized by nucleophilic displacement of halide ion from a reacdve alkyl halide, e.g. 

Chain extension by means of the reaction of alkyl halides with cyanide is frequently alluded to but rarely employed, mainly because of the long reaction times and poor yields usually encountered. The use of DMSO as a solvent has greatly simplified the procedures and improved the yields of many ionic reactions, and the conversion of alkyl chlorides to nitriles is a good example. 

Relatively little has been reported regarding the determination of the purity of the halide salts other than by standard spectroscopic measurements and microanalysis. This is largely because the halide salts are rarely used as solvents themselves, but are generally simply a source of the desired cation. Also, the only impurities likely to be present in any significant quantity are unreacted starting materials and residual reaction solvents. Thus, for most applications it is sufficient to ensure that they are free of these by use of FF NMR spectroscopy. 

Other detrimental factors which should to be taken into account in the materials selection process include temperature cycling and the presence of halide gases. Specialist alloys containing rare earth element additions such as cerium, lanthanum and yttrium have been developed for use in certain environments up to 130°C. 

In gasoline engines lead halides accumulate in the lubricant occasionally these give rise to the corrosion of aluminium-alloy pistons and very rarely to corrosion on aluminium-tin bearings. ... 

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

Halides of rare earth elements in a lower state of oxidation. G. I. Novikov and O. G. Polyachenok, Russ. Chem. Rev. (Engl. Transl.), 1964, 33,342-350 (86). 

A few studies are reported which describe the direct oxidation of sulphoxides to sulphonic acids, sulphonyl halides, thiosulphonates and sulphate. These reactions will be considered in this section but it should be noted that they are rarely of synthetic utility. 

---