Hydrogen bromide alkynes

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

Treatment of 10-bromodibenz[fe,/]oxepin with potassium rm-butoxide at 100 C in dioxane results in elimination of hydrogen bromide to give 10,11-didehydrodibenz[6,/ oxepin. This highly strained alkyne intermediate can be trapped with dienophiles, for example, with 2,5-dimethylfuran to give 4.160... 

Smirnov-Zamkov et al. have shown that sulfur dioxide and hydrogen bromide react with alkynes with phenylacetylene derivatives, such as the sulfonamide (42), a benzo[b]thiophene (43) is formed.50 The mixture of sulfur dioxide and hydrogen bromide appears to behave like thionyl bromide, cf. thionyl chloride (see Section V,B). 

Disubstituted alkynes and terminal alkynes form E-dibromoalkenes [4] when the tribromide is formed in situ in an essentially basic medium, an addition reaction followed by elimination of hydrogen bromide results in the conversion of terminal alkynes into the 1-bromoalkynes [5]. When the addition reaction is conducted in methanol, l,l-dibromo-2,2-dimethoxyalkanes are produced, in addition to the 1,2-dibromoalkenes [6], The dimethoxy compounds probably result from the initial intermediate formation of bromomethoxyalkenes. Under similar conditions, alkenes yield methoxy bromo compounds [7]. 

Hydrogenation using Raney nickel is carried out under mild conditions and gives cis alkenes from internal alkynes in yields ranging from 50 to 100% [356, 357, 358, 359, 360]. Half hydrogenation of alkynes was also achieved over nickel prepared by reduction of nickel acetate with sodium borohydride (P-2 nickel, nickel boride) [349,361,362] or by reduction with sodium hydride [49], or by reduction of nickel bromide with potassium-graphite [363]. Other catalysts are palladium on charcoal [364], on barium sulfate [365, 366], on... 

The principles of radical addition reactions of alkenes appear to apply equally to alkynes, although there are fewer documented examples of radical additions to triple bonds. Two molecules of hydrogen bromide can add to propyne first to give cis-1 -bromopropene (by antarafacial addition) and then 1,2-dibromopropane ... 

Higher alkynes can be synthesised from alkenes through a two-step process which involves the electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base(Following fig.). The second stage involves the loss of two molecules of hydrogen bromide and so two equivalents of base are required. 

When an alkyne is treated with one equivalent of HBr gives a bromoalkene [Following fig.(a)]. If two equivalents of hydrogen bromide are present the reaction can go twice to give a geminal dibromoalkane where both bromine atoms are added to the same carbon [Following fig.(b)]. 

Figure 3.109 In situ bromination of alkenes and alkynes using hydrogen bromide in the presence of hydrogen peroxide.

A number of the photoaddition reactions of alkynes are not reactions of the alkyne excited state but involve the formation of free radicals by photocleavage of the other compound. The photochemical a/i//-Markownikoff addition of hydrogen bromide to alkynes (equation 35) under conditions where the ionic addition is very slow is an example of this -. 

The Overall process of reduction of an alkene to an alkane, or of reduction of an alkyne to either an al-kene or an alkane, via hydroboration requires that the intermediate organoborane be protonolyzed (Section 3.10.1.1). However, simple trialkylboranes are remarkably resistant to hydrolysis. For example, trimethylborane gives only 69% of hydroxydimethylborane after 7 h at 180 °C with 1 equiv. of water. Similar resistance is shown to alcohols, phenols and amines, and even mineral acids do not completely protonolyze trialkylboranes with any ease. Thus, aqueous or anhydrous hydrogen bromide removes only one alkyl group from tributylborane after reflux for 1 h. ... 

Alkynes and dienes respond to characterization tests in the same way as alkenes they decolorize bromine in carbon tetrachloride without evolution of hydrogen bromide, and they decolorize cold, neutral, dilute permanganate they are not oxidized by chromic anhydride. They are, however, more unsaturated than alkenes. This property can be detected by determination of their molecular formulas (CnH2n-2) and by a quantitative hydrogenation (two moles of hydrogen are taken up per mole of hydrocarbon). 

Hydrogen halides may also add to alkynes. Write down the mechanism for the addition of one molecule of hydrogen bromide to one molecule of ethyne. 

Other noticeable hydrogenation processes are selective hydrogenation of alkynes, e.g., acetylene, to cw-alkenes, and similar partial hydrogenation processes. For partial hydrogenation, catalysts with reduced activity are required. Available catalysts include Lindlar s catalyst, a palladium catalyst partially poisoned with Pd and CaC03, and the nickel bromide catalyst, i.e., a NaBH4 reduced NiBr2. ... 

Dimethyl-3-heptyne is the only alkyne that will form the desired alkane upon hydrogenation. This alkyne can be dissected in two different ways. In one case the alkyne could be prepared from the reaction of an acetylide ion with a primary alkyl halide (isobutyl bromide) in the other case the acetylide ion would have to react with a secondary alkyl halide (isopropyl bromide). 

Bromo-l-alkenes. Hydrogen bromide adds to terminal acetylenes to form 1-bromo-l-alkenes. However, it adds to 1-trimethylsilyl-l-alkynes to form 2-bromo-l-alkenes. Although the reaction is a free radical reaction, a peroxide initiator is not required and may be deleterious. Trimethylsilyl bromide is the... 

It has recently been found that an appropriately prepared complex of tetraethyl-ammonium bromide with hydrogen bromide reacts with alkynes under anhydrous homogeneous conditions to yield bromoalkenes the addition product follows Markownikow s rule... 

Alkynes react with one molar equivalent of hydrogen chloride or hydrogen bromide to form haloalkenes, and with two molar equivalents to form geminal dihalides. 

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