Removal of halogen

Introduction of an ethylenic linkage by removal of two halogen atoms from a vicinal dihalide has little synthetic importance since the requisite dihalides are almost always obtained by addition of halogen to the double bond that is to be formed. However, the process is valuable because the halogen adducts can be useful for isolation or separation of a product. 

The dehalogenating agent may be a metal such as copper, magnesium, zinc, aluminum, iron, or an alkali metal, etc., or may be an alkali hydroxide or alkali iodide in ethanol or acetone. The removal of halogen generally liberates a considerable amount of heat, so that presence of a solvent is advisable.65 

Zinc dust in alcohol is usually applicable for removing two halogen atoms from vicinal carbon atoms, as is illustrated in the preparation of 4,4,4-tri-fluoro- 1-butene 6 6 

CF3CH2CHICH2C1 CF3CH2CH=CH2 Isolated halogen is not attacked by zinc under these conditions.67 

There is no isomerization or rearrangement during such dehalogenations. However, cyclization cannot be entirely avoided on dehalogenation of 1,3-dihalo compounds68 or some polyhalo compounds by, e.g., zinc or aluminum. Roedig and his co-workers69 used alcoholic potassium hydroxide in acetone 

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Ullman reaction The synthesis of diaryls by the condensation of aromatic halides with themselves or other aromatic halides, with the concomitant removal of halogens by a metal, e.g. copper powder thus bromobenzene gives diphenyl. The reaction may be extended to the preparation of diaryl ethers and diaryl thio-ethers by coupling a metal phenolate with an aryl halide. 

After the complete removal of halogen and metallic ions, the solution is concentrated to a volume of about 100 cc., and 300 cc. of absolute alcohol is added. Then the amino acid is precipitated by slowly adding 500 cc. of ether with stirring and cooling. 

Grignard reagents do not add directly to enamines, but their reactions with the corresponding imonium salts readily furnish tertiary amines (225,526). The reductive removal of halogen has been observed in the addition of Grignard reagents to a-bromoimonium salts (527). 

The ease of removal of halogen decreases with increasing electronegativity, 1 > Br > Cl > F 39,66) fluorine is usually quite difficult to remove unless highly activated (19,22,41,43,45). A synthesis of 2-amino-2 -fluorobenzophenone provides an example of a facile selective removal of... 

In contrast to aromatic halonitro compounds, selective removal of halogen in aliphatic halonitro compounds presents little problem. The reaction can be done by hydrogenation over palladium-on-carbon in the presence of a hydrohalide acceptor 46,73). 

Removal of halogen from aromatic rings can also be accomplished by various... 

Henson JM, Yates MV, Cochran JW, et al. 1988. Microbial removal of halogenated methanes, ethanes, and ethylenes in an aerobic soil exposed to methane. Published in FEMS Microbiology Ecology. Prepared in cooperation with Northrop Services, Inc. Ada, OK and Oklahoma Univ., Norman Dept, of Botany and Microbiology. EPA/600/J-88/066. 

The reductive removal of halogen can be accomplished with lithium or sodium. Tetrahydrofuran containing /-butanol is a useful reaction medium. Good results have also been achieved with polyhalogenated compounds by using sodium in ethanol. 

Reductive removal of halogen substituents has been of value in the synthesis of pyrimidines and purines since the time of Fisher (1899). Natural purines were de-oxygenated in a sequence of reactions involving the replacement of hydroxyl by chlorine through the reaction with phosphorus pentachloride and the reduction using zinc dust and water [152], 2-Chloropurines 45 are not reduced under these conditions. The 2-iodopurines are however reduced by zinc and water [152]. The elec-... 

DDT p,p -dichloro-diphenyl-trichloroethane. deacetylation removal of acetyl group, dealkylation removal of alkyl group, deaminate removal of amine group, dechlorination removal of chlorine group, de-ethylation removal of ethyl group, dehalogenation removal of halogen atom(s). 

A number of preparatively useful reductions with removal of halogen was discussed in Part I. Often a stepwise removal of the halogens takes place in a polyhalogenated compound. 4-Methyl-2,6-dichloropyrimidine (288) thus gives three peaks in DMF, the third peak being the reduction of the nucleus. Preparative reduction at the potential of the first peak affords 4-methyl-2-chloropyrimidine436 [Eq. (148)]. 

Ethyl ethylenetetracarboxylate has been prepared from monochloro- and monobromomalonic ester through removal of halogen acid with sodium,1 sodium ethylate,2 potassium acetate,3 potassium carbonate,4 sodium urethane, sodium formanilide and sodium acetanilide.6 It has also been prepared by treating the disodium derivative of ethyl ethane-1,1,2,2-tetracarboxylate with bromine,7 or iodine 8 by treating dibromomalonic ester with sodium,9 or sodium ethylate 10 and by treating the disodium derivative of malonic ester with iodine.11... 

The dehalogenation of naphthyridines with hydrogen over palladium on calcium carbonate in a weakly basic alcoholic solution gives excellent yields (90-95%) of reduced compounds.38,45,134,137,138 This method for removal of halogens has been extensively used and generally surpasses the classic hydrazine-copper sulfate reduction method. 

Both these metals bring about condensation by the removal of halogens, for which they havo considerable affinity, and so function in this respect as sodium, but the reactions are not so vigorous,... 

In the electroreductive treatments of halogenated organic pollutants the goal is either the complete dehalogenation (mostly hydrodehalogenation) of the substrates to ease their further treatment (e.g. improve their biodegradability), or their transformation into value-added compounds (e.g. selective removal of halogens, carboxyla-tion/carbonylation). 

Tertiary stibines are volatile liqnids or solids. The aryl derivatives are air-stable bnt the trialkyl stibines are air-sensitive compounds, with at least the lower members being spontaneously flammable in air. They are strong redncing agents and can be used for the removal of halogen atoms from many substrates. 

Halogen exchange at less activated sites requires a Lewis acid catalyst and an important part of the function of the catalyst, usually a metal fluoride or a chromium species, is to assist the removal of halogen as halide ion. Therefore, these reactions could be considered to involve carbocationic intermediates (Figure 2.4). 

In the second phase of the parametric study the removal of halogens from the reaction products was examined. Two different halogen removal methods were studied. The first method used a calcium oxide fixed bed placed between the reactor and condenser to remove organochloride vapours. However, it was found that the calcium oxide bed would plug up very rapidly if PET was present in the feed mixture and would become ineffective. This is because the PET would depolymerize into terephthalic acid and CO2 and would react with calcium oxide and cause it to plug up. The second method that was used for removal of chlorine was to add calcium oxide or hydroxide directly into the reactor with the plastic feed. It was found that this approach is far more effective than the previous method. Through trial and error it was found that calcium hydroxide feed of 10 wt% would remove the highest amount of chlorine (Table 19.7) [9]. 

Transition metal carbonyl derivatives of magnesium can be prepared readily by the reductive cleavage of the metal-metal bond in numerous dimeric transition metal carbonyl complexes or by the removal of halogen from transition metal carbonyl halide complexes with an excess of 1 % magnesium amalgam in the presence of a Lewis base. These preparations may be represented by the general equations... 

Halogen substituents in a "-haloketones are easily removed even by zinc dust, because of the greater tendency of the halogen to ionisation. The reduction may be represented by a mechanism like that for acetoxy ketones, the ease of removal of halogens following their usual order of reactivity F < Cl < Br < I. In one case the intermediacy of an enolate anion was revealed by direct isolation of an enol acetate aa-chloro-Sa-cholestan 3 One was dechlorinated with zinc in acetic anhydride to give 3-acetoxycholest-2 ene [ 112]. 

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