Chlorides s. a. Halides Replacement

Characterization of compounds s. subentry derivatives Chelates s. Complex salts, inner Chloranil as reagent 16, 524 Chlorides s. a. Halides, Replacement... 

Chlorides s. a. Halides, Hydrochlorides, Replacement Chlorination s. a. Halogenation, Replacement of hydrogen by halogen... 

The replacement of an alcoholic hydroxyl group by a halogen atom is one of the most common reactions carried out in organic chemistry. The usual reagents for effecting the transformation include halogen acids, thionyl chloride, and phosphorus halides. The reaction is of particular theoretical interest since experiments with optically active alcohols suggest still another substitution process. It has been called an internal nucleophilic substitution reaction (S i).20... 

I would like to bring up a couple of points here because I think Basolo and Baddley s experiment of blocking the fifth and sixth positions is a very good one. I would like to ask them if they would do the experiment, if they have not done it, in which chloride, bromide, and iodide are replaced as leaving groups to see if the order reverses to give the octahedral halide leaving order, rather than the planar one in which iodide is slower than chloride. This would help firm up this case. 

Table II (which contains a comparison not only of the relative stabilities of the O-acetylglycosyl halides but also of O-benzoylglycosyl halides, O-acylated sugars, and O-acylated glycosides). However, it is apparent that the theory oversimplifies the situation. For example, Hassel and Ottar84 have pointed out that the formation of 1,5-m-O-acetylglycosyl halide may be achieved in S vl reaction mechanisms or in cases where the natures of the old and new substituents are sufficiently different. Tetra-0-acetyl-/3-D-glucopyranosyl chloride is such a substance, and would exist in the Cl conformation (L) since, according to the theory, the 1C conformation LI would be highly unstable. However, for replacement of...

Crystallography revealed a bound chloride ion at the amino terminus of the helix containing the proximal His336 which can be replaced with bromide. Crystal structures of the human MPO-cyanide, MPO-cyanide-bromide, and MPO-cyanide-thiocyanate have also been determined by Fenna and coworkers to 1.9 A. These results support a model for a single common binding site for halides and thiocyanate as substrates or as inhibitors near the S-meso carbon of the porphyrin ring in myeloperoxidase. 

Studies on the alkaline hydrolysis of various phosphonic and phosphinic esters have provided information on the electronic or steric effects of substituents and the effects of changes in reaction conditions amongst the substrates so extensively examined are the O-aryl esters of dimethylphosphinothioic acid (552) esters of diphenylphosphinic acid and O-aryfand S -aryf esters of diphenylphosphinothioic acid (553 Z, Y = O or S). Other studies have concentrated on aryl esters of diaryIphosphinic acids (554) , and the effects of the stepwise replacement of P-Me by P-Ph in esters of dimethyl-, methylphenyl- and diphenyl-phosphinic acids The esters 555 (R = EtO, R = Me or Ph, R = R = Ph X = SEt or hydrolyse under alkaline conditions faster than do the comparable 5 -(4-substituted-butyl) esters. Comparable steric and electronic influences on the hydrolyses of phosphonic and phosphinic fluorides phosphinic chlo-rides the phosphonothioic chlorides 556 and other phosphonic and phosphinic esters have been noted. Phosphonic and phosphinic halides are prone to undergo halo-gen-exchange reactions, a process which, in general, is faster for derivatives of phosphonic than for those of phosphonothioic and phosphonoselenoic acids, and to be particularly important for acid fluorides . ... 

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