Replacement of halogen by hydrogen

In the series F Cl Br I, iodine is the most easily replaced directly by hydrogen. This reaction is usually easier when the halogen is attached to tertiary than to secondary or primary carbon. Also exchange of the halogen is facilitated by neighboring substituents that, by electron-donor effect on the carbon atom, weaken the carbon-halogen bond. 

The reducing agent used most often is the combination non-noble metal/acid (glacial acetic adic or hydrochloric acid). Catalytic processes are, however, also widely used, e.g., those with Raney nickel, palladium, or platinum oxide. 

Routes involving intermediates such as Grignard reagents are often of considerable preparative importance and reduction of halides by complex metal hydrides such as LiAlH4 and lithium tri-terr-butoxyhydridoaluminate also give good results. 

Several methods are available for reduction of acid chlorides to the corresponding aldehydes, the Rosenmund-Saytzeff and the Reissert procedure being those most commonly used. 

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For the replacement of halogen by hydrogen - hydrogenolysis of the carbon-halogen bond - many reduction methods are available. Since breaking of carbon-halogen bonds is involved, the ease of hydrogenolysis decreases in... 

These substrates show restricted rotation about the amide bond With no ortho-substituent present, the svn-form predominates with adjacent phenyl rings. Ortho-substitution increases the proportion at equlibrium of the rotamer with anft -phenyl rings. Reduction of the anti-mUmm can lead only to replacement of halogen by hydrogen. For the chlorobcnzanilides, the formation of a product with the chlorine... 

Replacement of halogen by hydrogen or a metal (including transmetallation), or by coupling... 

In the synthesis of biologically important molecules, the replacement of halogen by hydrogen in four-26,48-51, five-27,52 and six-membered32,38,40,53-61 rings has seen much interest. 

A selective reducing agent for replacement of halogens by hydrogen, triphenylstannane (triphenyltin hydride), reduces aryl bromides to arenes in high yields (Table 4). Triethylsilane (triethylsilicon hydride) converts bromobenzene to benzene in the presence of 10% palladium on carbon in 79% yield (Table 4). ... 

Direct photoreactions of halogenated aromatic compounds in water have received considerably more attention. The products of direct photoreactions include photonucleophilic replacement of halogen by OH, reductive dehalo-genation (i.e., replacement of halogen by hydrogen), photooxidation, and photoisomerization. Several such reactions are exemplified by the photoreactions of DDE in natural waters (eq 12) (38). 

Lithium aluminum hydride Replacement of halogen by hydrogen... 

Replacement of halogen by hydrogen Mono- and 1,1-di- from 1,1,1-tri-chlorides... 

C-alkyl migration 21, 923 replacement of halogen by hydrogen 21, 965 —, dehaiogenative 21, 28 —, preferential 2, 767 Aroxyls... 

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