Alkynes bromine addition reactions

Addition reactions form the basis for tests that distinguish alkenes and alkynes from alkanes. Bromine, Br2, has a deep reddish-brown colour. When bromine is added to an alkene or alkyne, an addition reaction takes place. As the bromine is used up, the brown colour of the bromine disappears. Since alkanes cannot undergo addition reactions, no reaction takes place when bromine is added to an alkane. 

The presence of carbon-carbon double or triple bonds in hydrocarbons markedly increases their chemical reactivity. The most characteristic reactions of alkenes and alkynes are addition reactions, in which a reactant is added to the two atoms that form the multiple bond. A simple example is the addition of elemental bromine to ethylene to produce 1,2,-dibromoethane ... 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

The classification of hydrocar bons as aliphatic or ar omatic took place in the 1860s when it was aheady apparent that there was something special about benzene, toluene, and their- derivatives. Their molecular- for-mulas (benzene is CgHg, toluene is CyKj ) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and alkynes, however, benzene proved to be inert. Benzene does react with Br-2 in the presence of iron(III) bromide as a catalyst, but even then addition isn t observed. Substitution occurs instead ... 

A DFT calculation study of the addition reaction between molecular bromine and a number of symmetrical or unsymmetrical substituted alkynes, R-C=C-R (R = R =... 

Some examples of these addition reactions are provided in the following equations. Note that each proceeds with anti addition of the two halogen atoms. In the last example, starting with an alkyne, the two bromines end up trans. 

A more subtle distinction occurred in a study of the bromination of alkynes. Bromination of benzyl alkynes in acetic acid gave the products of addition of one molecule of bromine—the 1,2-dibro-moalkenes. The reaction was successful with a variety of para substituents and there seems at first to be no special interest in the structure of the products. 

In the absence of a catalyst, alkynes react very slowly with bromine. Scheme 3.28 particularly when compared to alkenes. When a choice exists, bromine reacts preferentially with an alkene rather than an alkyne. It is possible that radical reactions play a more important role in the addition to alkynes. When the reaction of acetylene with chlorine is catalysed by iron(lll) chloride, the reaction is fast and 1,1,2,2-tetrachloroethane is formed. The uncatalysed addition of a hydrogen halide gives a tram alkenyl halide. Further addition is restricted but can give rise to dihalides. 

Benzene (CeH ) is the simplest aromatic hydrocarbon (or arene). Since its isolation by Michael Faraday from the oily residue remaining in the illuminating gas lines in London in 1825, it has been recognized as an unusual compound. Based on the calculation introduced in Section 10.2, benzene has four degrees of unsaturation, making it a highly unsaturated hydrocarbon. But, whereas unsaturated hydrocarbons such as alkenes, alkynes, and dienes readily undergo addition reactions, benzene does not. For example, bromine adds to ethylene to form a dibromide, but benzene is inert under similar conditions. 

The alk)mes contain two pi bonds, both of which are sources of electrons, and they are more reactive than the alkenes. The most common reaction of the alkynes is addition across the triple bond. The reactions with hydrogen and with bromine are typical. 

A major factor in determining the magnitude of relative rates is the solvent used. Thus, high alkene alkyne reactivity ratios are found in organic solvents of low dielectric constant (acetic acid), but the rates of reaction are comparable when water is the solvent. This is true not only for the hydration but also for bromination. Polar solvents, water in particular, are evidently capable of minimizing the energy difference between the two transition states. Whether this is accomplished by very strong solvation or by some other mechanism is not entirely clear. Addition reactions of alkenes and alkynes with trifluoroacetic acid also take place at comparable rates. ... 

Hydrogen halides also add to alkynes. The addition of HBr to alkjmes can be difficult to interpret because (as with alkenes) both ionic and free radical mechanisms may occur, and the free radical process can be difficult to suppress. Reaction of HBr with propjme (63) in the liquid phase at —78°C led to the formation of (Z)-l-bromopropene (64, equation 9.63), indicating stereoselective anti addition. When the reaction was carried out at room temperature, however, a mixture of Z (64) and (65) isomers was obtained (equation 9.64). The results suggested that addition of a bromine atom to propyne produces the vinyl radical 66, which abstracts a hydrogen from HBr to produce 64 at -78°C but which can isomerize (with Eg > 17kcal/mol) to the radical 67 at room temperature. 

Alkynes show the same kind of addition reactions with chlorine and bromine that alkenes do. 

As in alkenes, the pi electrons of alkynes are exposed to electrophilic attack (see Figure 3.17). Therefore, many addition reactions described for alkenes also occur, though usually more slowly, with alkynes. For example, bromine adds as follows ... 

Despite its molecular formula, benzene for the most part does not behave as if it were unsaturated. For instance, it does not decolorize bromine solutions the way alk-enes and alkynes do (Sec. 3.7a), nor is it easily oxidized by potassium permanganate (Sec. 3.17a). It does not undergo the typical addition reactions of alkenes or alkynes. Instead, benzene reacts mainly by substitution. For example, when treated with bromine (Br2) in the presence of ferric bromide as a catalyst, benzene gives bromobenzene and hydrogen bromide as products. 

Alkynes undergo electrophilic addition reactions with hydrogen halides and bromine, but these reactions have limited synthetic utility. However, one reaction of alkynes that is commonly used in organic chemistry is hydration of the carbon-carbon triple bond to give a ketone, a transformation that is catalyzed by mercuric ion in the presence of sulfuric acid (Eq. 11.10). 

As a general rule, and as shown in the fifth example in Table 6.7, bromine (Br2) and chlorine (CI2) in, for example, carbon tetrachloride (tetrachloromethane, CCI4) solution, add across the carbon-carbon triple bond to give tram- [antarafacial, and-, or ( )] dibromoalkene. Also, as a general rule (albeit with a very limited number of comparisons), these addition reactions occur more slowly with alkynes than they do with alkenes and a second equivalent of halogen can add to produce the tetra-halide (Equation 6.68). 

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