Bromination alkyne

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. 

Alkene and alkyne Bromination, acetic acid Chlorination, acetic acid Acid-catalyzed hydration, water... 

Alkynes Bromine test Permanganate test Silver nitrate Sodium metal addition Sulfuric acid Positive for all alkynes Positive for all alkynes Positive for all terminal alkynes only Positive for all terminal alkynes only Positive for all alkynes... 

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. 

Giacomelli and Lardicci have treated bromoalkynes with trialkylaluminum in the presence of bis(A/-methylsalicylaldimine)nickel and obtained an alkyl-substituted alkyne (Scheme 33). This reaction probably involves (i) insertion of the nickel into die alkyne-bromine bond (ii) exchange of alkyl for bromine and (iii) formation of the alkyne-alkyl bond. TTius, although the process appears to be an electrophilic substitution, it is actually nucleophilic in character. 

COPPER (7440-50-8) Cu The powder forms the friction-, heat-, or shock-sensitive explosive detonator, copper acetylide, with acetylene gas acetylenic compounds and ethylene oxides. The powder forms explosive materials with azides (e.g., sodium azide forms potentially explosive copper azide). Finely divided material forms friction-, heat-, or shock-sensitive explosive with powdered divided bromates, chlorates, and iodates of barimn, calcimn, magnesium, potassium, sodium, or zinc. Violent reaction, possibly explosive, when finely dispersed powder comes in contact with strong oxidizers ammonium nitrate alkynes, bromine vapor, calcium carbide, chlorine, ethylene oxide, hydrazine mononitrate, hydrogen peroxide, hydrogen sulfide, finely divided bromates, iodine, lead azide, potassium peroxide, sodium peroxide (incandescence), sulfuric acid. Incompatible with acids, anhydrous ammonia chemically active metals such as potassium, sodium, magnesium, and zinc, zirconium, strong bases. 

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

Because vicinal dihalides are prepared by addition of chlorine or bromine to alkenes (Section 6 14) alkenes especially terminal alkenes can serve as starting mate rials for the preparation of alkynes as shown m the following example... 

Alkynes react with chlorine and bromine to yield tetrahaloalkanes Two molecules of the halogen add to the triple bond... 

Halogenation (Section 9 13) Addition of 1 mole of chlorine or bromine to an alkyne yields a trans dihaloalkene Atetrahalide is formed on addition of a second equivalent of the halogen... 

The classification of hydrocarbons as aliphatic or aromatic took place m the 1860s when It was already apparent that there was something special about benzene toluene and their derivatives Their molecular formulas (benzene is CgHg toluene is C7Hj ) indicate that like alkenes and alkynes they are unsaturated and should undergo addition reac tions Under conditions m which bromine for example reacts rapidly with alkenes and alkynes however benzene proved to be inert Benzene does react with Bi2 m the pres ence of iron(III) bromide as a catalyst but even then addition isn t observed Substitu tion occurs instead ... 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

The rates of bromination of a number of alkynes have been measured under conditions that permit comparison with the corresponding alkenes. The rate of bromina-hon of styrene exceeds that of phenylacetylene by about For internal alkyne-... 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

The rates of bromination of dialkylacetylenes are roughly 100 times greater than for the corresponding monosubstituted alkynes. For hydration, however, the rates of reaction are less than 10 times greater for disubstituted derivatives. Account for this observation by comparison of the mechanisms for bromination and hydration. 

Thermodynamics and kinetics need not go hand in hand. Consider all possible products resulting from addition of one equivalent of bromine to phenylacetylene (phenylacetylene+Br2) and to styrene (styrene+Br2). Calculate the heat of reaction for each addition. (The energy of Br2 is given at right.) Is addition to the alkyne or to the alkene more favorable ... 

Bromine and chlorine also add to alkynes to give addition products, and trans stereochemistry again results. 

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