Halides geminal dihalides

In the presence of excess hydrogen halide, geminal dihalides aie fonned by sequential addition of two molecules of hydrogen halide to the caibon-caibon triple bond. 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Hydrogen halides add to alkynes in accordance with Markovnikov s rule to give alkenyl halides In the presence of 2 moles of hydrogen halide a second addition occurs to give a geminal dihalide... 

Addition of hydrogen halides to simple allenes initially gives the vinyl halide, and if the second double bond reacts, a geminal dihalide is formed. " ... 

Double dehydrohalogenation of geminal dihalides (Section 9.7) An E2 elimination reaction of a geminal dihalide yields an alkenyl halide. If a strong enough base is used, sodium amide, for example, a second elimination step follows the first and the alkenyl halide is converted to an alkyne. 

Alkynes react with HC1 and HBr to form haloalkenes or geminal dihalides depending on whether one or two molar equivalents of the hydrogen halide are used. 

Doyle, M. P., Siegfried, B., and Hammond, J. J. (1976). Oxidative deamination of primary amines by copper halide nitrosyls. The formation of geminal dihalides. J. Am. Chem. Soc. 98, 1627-1629. 

Alkynes. Because of their less nucleophilic character, alkynes react less readily with hydrogen halides than do alkenes and often require the use a metal halide catalyst. Vinyl halides are formed in the reaction with one equivalent of HHlg. They may react further in an excess of the reagent to yield geminal dihalides. High yields of these compounds can be achieved. The addition of HC1 to acetylene was studied in detail because of the practical importance of the product vinyl chloride (see Section 6.2.4). 

Geminal dihalide Sodium amide Alkyne Ammonia Sodium halide... 

Vinylic halides are less reactive than simple alkenes, but generally afford clean addition products. The addition of HBr to vinyl chloride125 or vinyl bromide61,122,126,127 in the presence of iron(III) chloride or bromide affords primarily the geminal dihalides (equation 78). 

A related approach is the treatment of geminal dihalides with metal reagents to give an intermediate a-haloalkylmetal species, from which halide may in principle be lost in a net overall a-elimination of the two halide substituents (see Section 4.6.4.2). 

When 2 moles of a hydrogen halide add to an alkyne, the second mole usually adds with the same orientation as the first. This consistent orientation leads to a geminal dihalide. For example, a double Markovnikov addition of HBr to pent-l-yne gives 2,2-dibromopentane. 

Two equivalents of HX are usually used addition of one mole forms a vinyl halide, which then reacts with a second mole of HX to form a geminal dihalide. 

This mechanism occurs particularly with aryl halides, phosphates, and ammonium salts, and with geminal dihalides. The reactions can be stimulated by light, or inhibited by p-dinitrobcnzcnc, and aryl halides can react exclusively by the SrnI mechanism or by Sn2 on halogen, depending on the structures of R3S11 and of ArX, and on the presence or... 

The mechanism of these reductions is a bimolecular nucleophilic substitution for the reaction of LAH with most primary and secondary halides [BK5, PCI]. A single-electron transfer (SET) has been proposed in the reduction of sterically hindered primary iodides [AD3, AGl, AWl], although some doubts have been cast [PCI] on this mechanism with bromocyclopropanes [HW2] and aromatic or vinyl halides [Cl], especially in the presence of CeClj [G02]. In this case, some rearrangements may be observed. SET does take place in the reduction of geminal dihalides by LAH [AIM] as well as in the reduction of bromocyclopropanes in the strict absence of molecular oxygen [PNl]. In the presence of oxygen, the C—Br bond of 2,2-diphenyl-l-bromocyclopropanecarboxylic acid is left unchanged [PN1 ]. 

Adding a halide acid such as HCl or HBr to an alkyne can create a geminal dihalide via a... 

Addition of a hydrogen halide to an internal alkyne forms two geminal dihalides because the initial addition of the proton can occur with equal ease to either of the sp carbons. 

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