1,1.1-Trihalides 1,1-dihalides

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

Unlike the di-f dihalides, such compounds differ little in energy from both the equivalent quantity of metal and trihalide, and from other combinations with a similar distribution of metal-metal and metal-halide bonding. So the reduced halide chemistry of the five elements shows considerable variety, and thermodynamics is ill-equipped to account for it. All four elements form di-iodides with strong metal-metal interaction, Prl2 occurring in five different crystalline forms. Lanthanum yields Lai, and for La, Ce and Pr there are hahdes M2X5 where X=Br or I. The rich variety of the chemistry of these tri-f compounds is greatly increased by the incorporahon of other elements that occupy interstitial positions in the lanthanide metal clusters [3 b, 21, 22]. 

The reaction of cobaltocene with organoboron dihalides RBX2 (R = Me, Ph and X = Cl, Br mainly) and boron trihalides (BC13, BBr3) leads essentially to three types of (boratabenzene) cobalt complexes, 19,20, and 21 (7,57). CoCp2 plays a dual role in part it acts as a reductant, in part it... 

This material was used to show that tributyltin halides decompose into dibutyltin dihalides and butyltin trihalides in the environment. 

Three main structural sub-groups can be recognised alkylaluminium dihalides, dialkylaluminium halides, and trialkyldialuminium trihalides (equimolar complexes of a trialkylaluminium and an aluminium trihalide). While this is generally a very reactive group of compounds, similar in reactivity to trialkylaluminium compounds, increase in size of the alkyl groups present and in the degree of halogen substitution tends to reduce pyrophoricity. 

Butadiene, isoprene, chloroprene and 2,3-dimethyl-l,3-butadiene add phosphorus trihalides to form 3-phospholene 1,1,1-dihalides, e.g. 297 from isoprene and phosphorus... 

Direct treatment of organic geminal dihalides or trihalides with strongly nucleophilic transition metal complexes can also lead to the formation of carbene complexes, presumably via intermediate a-haloalkyl complexes [484-489]. Examples of such reactions are sketched in Figure 3.17. 

The reactivity of aryltellurium trihalides decreases on going from the chlorides to the iodides, the same trend occurring for hydrolysis. Aryltellurium trichlorides are very sensitive to water and moisture and are easily hydrolysed, the tribromides being more stable, while the triiodides are unaffected by cold water and can be prepared even by aqueous procedures. Diaryltellurium dihalides are stable in water, and ionic exchange reactions allow the conversion of dichlorides into dibromides and diiodides. 

Tellurium tetrachloride and aryltellurium trichloride, as well as tellurium tetrabro-mide" and aryltellurium tribromides add to acetylenes to produce, respectively, 2-halovinyl tellurium trihalides and dihalides, which can be submitted to further manipulations. 

Alkyl and Related Compounds. The alkylzirconium trihalides, RZrXj (R = Me, Et, or Pr X = Cl or Br) have been prepared by treating ZrX4 with R2Zn in toluene at 0°C the dialkylzirconium dihalides, R2ZrX2 (R = Me or Et X = Cl or Br) were obtained from these reactants in pyridine at 0°C. R2Zr.X2,bipy adducts were also obtained. ... 

Trigeminal trihalides are completely reduced by catalytic hydrogenation over palladium [62] and Raney nickel [63], and partially reduced to dihalides or monohalides by electrolysis using mercury cathode [57 ], by aluminum... 

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , > TiCL > TiCLj P-TiCl3 [Natta et al., 1957b,c],... 

Metallic samarium is obtained by heating the oxide, Sm203 with lanthanum turnings or cerium in slight excess amounts in a tantalum crucible under high vacuum. The metal is recovered by condensation of its vapors at 300 to 400°C. The metal cannot be obtained by reduction of its halides, SmFs or SmCls, or by heating with calcium or barium. In such reduction, trihalides are reduced to dihalides, but not to the metal. 

Boron subhalides are binary compounds of boron and the halogens, where the atomic ratio of halogen to boron is less than 3. The boron monohalides, BC1, [20583-55-5], bromoborane(l) [19961-29-6], BBr, and iodoborane(l) [13842-56-3], BI, are unstable species that have been observed spectroscopically when the respective trihalides were subjected to a discharge (5). Boron dihalide radicals have been studied, and structural and thermochemical data for these species ( BX2) have been deduced (5). 

The dihalides of titanium, formed by reduction of the tetrahalides, are vigorous reducing agents and unstable T1CI2 is inflammable in air. The trihalides, though more stable than the dihalides, are effective reducing agents. Ti(TTT) occurs in aqueous solutions as Ti(1120)( 3, ... 

The trihalides of zirconium, like the dihalides of titanium, are extremely strong reducing agents, reacting even with H20. 

We now turn to the 3d series elements. The dihalides and trihalides can be treated as ionic solids, although the chlorides, bromides and iodides adopt layer structures which might be better viewed as polymeric covalent crystals. In Fig. 5.2 the third ionisation energies of the 3d atoms are plotted alongside those of the lanthanides. These all involve the removal of an electron from a 3d orbital from Fe onwards, the orbital concerned is doubly occupied so that spin-pairing energy assists the ionisation. This accounts for the break between Mn and Fe, as previously discussed (Section 4.3). The increase from Sc to Mn, and from Fe to Zn, is much sharper than the corresponding increases in the lanthanide series. However, the break at the half-filled shell is less abrupt for the 3d series. This explains why the II oxidation state - which is... 

Chlorofluoro compounds were electroreductivelv added to aldehydes to give the corresponding fluorinated alcohols (231) (equation 124)214. In this process platinum or lead are employed as the cathode, under constant-current conditions. The starting dihalides or trihalides (halogen = F, Cl or Br) can be prepared on an industrial scale by the reaction of the corresponding tri- or tetrahalide at a sacrificial anode (Zn, A1 or Mg)220. 

The possibility of preparing aryl tellurium halides from equimolar amounts of diaryl ditellurium compounds and aryl tellurium trihalides has hardly been explored. Only phenyl tellurium iodide and 2-biphenylyl tellurium bromide could be obtained by this route. The other aryl tellurium halides (including 3,4-dimethoxyphenyl tellurium chloride) decomposed under the reaction conditions to give diaryl tellurium dihalides and tellurium5. 

Aryl tellurium trihalides and equimolar amounts of diaryl ditcllurium compounds in refluxing dichloromethane produce diaryl tellurium dihalides and elemental tellurium9,10. 

Diorgano tellurium dihalides are often the primary products of reactions producing compounds with two tellurium-carbon bonds. Such reactions arc the condensation of tellurium tetrachloride with aromatic compounds (p. 527), the addition of tellurium tetrachloride or organo tellurium trichlorides to carbon-carbon multiple bonds (p. 530, 544), and the alkylation or arylation of organo tellurium trihalides (p. 549). The symmetrical and unsymmetrical diorgano tellurium dihalides are convenient starting materials for the preparation of diorgano tellurium derivatives. 

Diaryl ditellurium compounds and aryl tellurium trihalides react in refluxing 1,2-dichloroethane to form diaryl tellurium dihalides and tellurium. The reactions are postulated to proceed via aryl tellurium halides that disproportionate into tellurium and diaryl tellurium dihalides. All reactions of this type thus far carried out on a preparative scale used pairs of reagents with the same aryl group2. 

Phenyl tellurium trihalides react with linear olefins and cycloalkenes in methanol or other alcohols. In contrast to reactions in inert organic solvents, in alcoholic media the intermediate adduct between the olefin and the positive phenyldihalotelluro group is attacked by the alcohol and not by the halide ion. Therefore, the product is a 2-alkoxyalkyl phenyl tellurium dihalide. The reactions are regiospecific (tellurium bonding to the less hindered carbon atom) and highly stereospecific (anti-addition). 

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