Iodination direct methods

The preparation of anhydrous aluminum iodide by methods described previously1 2 involves direct union under conditions that invariably yield products contaminated with elemental iodine. Attempts to purify these products by sublimation under a variety of conditions (including sublimation in vacuo or in an atmosphere of carbon dioxide or helium) result in at least partial decomposition of the iodide and a final product that is colored, owing to the presence of iodine. The method described below also provides for direct union of the elements, but under conditions that eliminate contamination with elemental iodine and yield an initial product of exceptionally high purity. 

In contrast to diazomethane, a-diazo- S-carbonyl and - S-phosphoryl compounds cannot be halogenated by the direct methods mentioned above, but the corresponding a-mercury-bis(a-diazo-yff-carbonyl) (9.4) or (a-diazo-a-silver-)ff-phosphoryl) derivatives (9.6) must be synthesized first. The metallated compounds 9.4 and 9.6 react with molecular bromine and iodine or with other halogenation reagents, like sulfuryl chloride or cyanogen chloride. The compounds 9.5 and 9.7 are obtained in a yield of 30-90% (Schollkopf et al., 1968, for ethyl diazoacetate Regitz et al., 1979c, for diazophosphoryl compounds). 

Another direct method of converting (all- )-retinoic acid (3) to the two isomeric 4-oxoretinoic acids (188) and (193) consisted in irradiating (3) in the presence of iodine. It was possible to separate (188) and (193) by HPLC (McKenzie et al., 1978b). 

Considerable interest has been expressed in the development of direct methods for the synthesis of a-hydroxy ketones using nontoxic hypervalent iodine reagents and which involve the foUowing methods reaction of a ketone with iodobenzene diacetate in the presence of potassium hydroxide in methanol and then hydrolysis of the dimethylacetals oxidation of the enol sUyl ether of acetophenone using the s) tem iodosobenzene/boron trifluoride efherate/water in methylene chloride at—40°C and reaction of ketones with [fcis(trifluoroacetoxy)]iodobenzene and trifluoroacetic acid in acetonitrile-water under acidic conditions. "... 

The separation of ( )-propranolol was achieved by direct method using silica gel plates impregnated with optically pure L-lysine (0.5%) and L-arginine (0.5%) as the chiral selectors. Different combinations of acetonitrile-methanol were used as the mobile phase. Spots were detected using iodine vapor. The detection limit for ( )-propranolol was 2.6 pig [19]. The established determination conditions are described in Section 14.2.3.3. 

A complete set of trihalides for arsenic, antimony and bismuth can be prepared by the direct combination of the elements although other methods of preparation can sometimes be used. The vigour of the direct combination reaction for a given metal decreases from fluorine to iodine (except in the case of bismuth which does not react readily with fluorine) and for a given halogen, from arsenic to bismuth. 

As in the case of the thiazoles, a variation on the Hantzsch s method has been used. This consists of using a nonhalogenated carbonyl derivative directly in the presence of iodine in the reaction with selenourea (Scheme 7) (20). However, in this case the reaction with selenourea is slower than with thiourea, and normally an excess of carbonyl compound is used. 

A versatile method for the direct C-21 halogenation of a 20-ketopregnane was discovered by Ringold and Stork it consists of reaction with iodine-calcium oxide in tetrahydrofuran-methanol, and the resulting 21-iodo compound is displaced by acetate in the usual fashion ... 

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)... 

The three fluorides CIF5, Brp5 and IF5 are the only known hexa-atomic interhalogens, and IF7 is the sole representative of the octa-atomic class. The first to be made (1871) was IF5 which is the most readily formed of the iodine fluorides, whereas the more vigorous conditions required for the others delayed the synthesis of BrFs and IF7 until 1930/1 and CIF5 until 1962. The preferred method of preparing all four compounds on a large scale is by direct fluorination of the element or a lower fluoride ... 

Procedures have been worked out which increased the yield of 2-bromothiophene to 78% on direct bromination in acetic acid-ether mixtures and to 67% in carbon tetraehlorided With the mild brominating agent, dioxane dibromide, quantitative yields of 2-bromothiophene are obtained. A very convenient procedure for the iodination of thiophenes consists of the acid-catalyzed (HzSOi) reaction with iodine and HIO3, giving 2-iodothiophene in 75% yieldd In contrast to the HgO method, all the iodine is utilized. 

Ion chromatography has been successfully applied to the quantitative analysis of ions in many diverse types of industrial and environmental samples. The technique has also been valuable for microelemental analysis, e.g. for the determination of sulphur, chlorine, bromine, phosphorus and iodine as heteroatoms in solid samples. Combustion in a Schoniger oxygen flask (Section 3.31 )is a widely used method of degrading such samples, the products of combustion being absorbed in solution as anionic or cationic forms, and the solution then directly injected into the ion chromatograph. 

The direct iodometric titration method (sometimes termed iodimetry) refers to titrations with a standard solution of iodine. The indirect iodometric titration method (sometimes termed iodometry) deals with the titration of iodine liberated in chemical reactions. The normal reduction potential of the reversible system ... 

A similar procedure may also be used for the determination of antimony(V), whilst antimony (III) may be determined like arsenic(III) by direct titration with standard iodine solution (Section 10.113), but in the antimony titration it is necessary to include some tartaric acid in the solution this acts as complexing agent and prevents precipitation of antimony as hydroxide or as basic salt in alkaline solution. On the whole, however, the most satisfactory method for determining antimony is by titration with potassium bromate (Section 10.133). 

Apparent indicator constant 264, 267 Apparent stability constant 59 Aqua regia 111 Arc alternating current, 764 direct current, 763, 771 sensitivities of elements, (T), 766 Aromatic hydrocarbons analysis of binary mixtures, 715 Arsenates, D. of (ti) 357 Arsenic, D. of as silver arsenate, (ti) 357 as trisulphide, (g) 448 by iodine, (am) 634, (ti) 397 by molybdenum blue method, (s) 681 by potassium bromate, (ti) 406 by potassium iodate, (ti) 401 in presence of antimony, (s) 724 Arsenic(III) oxide as primary standard, 261... 

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