Addition of Halogen Acids to Alkenes

The addition of halogen acids to alkynes is a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the halogen acid donates a proton to the alkyne p-system, which is functioning as a Lewis base. The protonated p-system has a short lifetime and can rapidly revert to starting materials, or can rearrange from a (cationic) protonated p-bond, to an sp² sigma bond adjacent to an sp² carbocation center. If the alkyne is asymmetrical, the protonated p-cloud intermediate can break down by two pathways to potentially form carbocations having differing ground-state energies. The reaction pathways leading from this intermediate to the two carbocations will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway. The resulting carbocation is formed on the carbon of the alkyne which is best able to stabilize the cationic center. In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbocations formed from alkyne protonation will be secondary > primary. Since secondary centers have no attached hydrogens and primary centers have one, there is an apparent inverse relationship between the "number of attached hydrogens" and the likelihood that the carbocation will form at that center and this is another example of Markovnikov's Rule, which was described for alkenes.

Once the carbocation is formed, the most favorable reaction will involve the addition of a nucleophile to form a vinyl halide intermediate. This alkene can now undergo a second protonation step, just like any other alkene, except that the carbocation will always be formed on the carbon bearing the halogen, since this carbocation is now stabilized by resonance with the halonium ion. The final result of the addition is that two moles of halogen halide are added, to give a 1,1-dihalide.

Addition of Halogen to Alkynes

The addition of halogen to alkynes is a stepwise process involving a "halonium" ion intermediate. The formation of this intermediate is initiated through attack of halogen on the alkyne p-system, to form the cyclic halonium ion (i.e., bromonium or chloronium ion) and expel the halogen anion (i.e., bromide or chloride). This intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system; that is, the halide anion expelled in the previous step. Attack by halide generates a vinyl halide, which is an alkene and can undergo a second addition of halogen. The final product of the reaction is therefore a 1,1,2,2-tetrahalide.

Addition of Water to Alkynes

The mercury-catalyzed addition of water to alkynes is another example of a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the mercury ion interacts with the alkyne p-system, which is functioning as a Lewis base. The chelated p>-system rearranges to form an sp² sigma bond adjacent to an sp² carbocation center. If the alkyne is asymmetrical, two carbocations are possible and the addition will proceed to form the most stable carbocation. As before, secondary centers will be favored over primary, and overall addition of water will follow the order predicted by Markovnikov's Rule. Addition of water forms a vinyl alcohol, which is termed an "enol". Enols are unstable compounds which rapidly interconvert with the corresponding carbonyl compound. Hence, the final product of the hydration reaction is the formation of an aldehyde or ketone, with the oxygen bonded to the carbon of the alkyne which would ultimately yield the most stable carbocation.

Hydroboration of Alkynes

The reaction of BH₃ with an alkyne begins with the Lewis acid chelation of the alkyne p-system by the boron. This complex then rearranges in a more or less concerted manner to produce the vinyl borane. The reaction seems to be dominated by steric effects and the boron attaches to the least hindered carbon. All three equivalents of the boron hydride can be utilized in separate reactions to give a trivinyl borane. The organoborane which is formed can be oxidized by alkaline peroxide to form the alcohol by a mechanism which involves attack of peroxide anion on the boron, followed by alkyl migration to the oxygen, with loss of hydroxide anion. The resulting borate ester is rapidly hydrolyzed by the alkaline conditions to form an "enol". Rearrangement of the enol to the corresponding carbonyl compound yields an aldehyde or ketone, with the oxygen bonded to the carbon of the alkyne which would generally yield the least stable carbocation (generally, anti-Markovnikov addition).

Reduction of Alkynes

Catalytic hydrogenation of alkynes with H₂ and a standard catalyst (Pt or Pd supported on charcoal, etc.) produces the corresponding alkane. However, partial reduction of an alkyne to an alkene is possible using a "poisoned catalyst", such as Pd or Pt on BaSO₄, or with the "Lindar Catalyst". In these reactions, addition of hydrogen is syn (cis) to yield the cis alkene. The transfer of hydrogen occurs in a strictly cis manner, probably due to the geometric constraints of the metal surface. The detailed mechanism is not trivial, and probably involves several metal-carbon bonded species.

Alkynes can also be partially reduced to trans-alkenes using a "dissolving metal reduction", in which the alkyne is formed by a radical mechanism in the presence of Li or Na metal dissolving in liquid ammonia. Please note that this differs from the base sodium amide, which is formed from sodium metal previously dissolved in liquid ammonia.

Oxidation of Alkynes

Acidic potassium permanganate is a powerful oxidant towards organic molecules and will readily cleave alkynes. Alkyne carbons are converted into carboxylic acids in this reaction.

Alkynes Anions as Nucleophiles

Terminal alkynes are slightly acidic and a powerful base such as sodium amide (sodium previously dissolved in liquid ammonia) will react with these compounds to give alkyne anions, which are powerful nucleophiles. The most common reaction in which these nucleophiles are utilized involves reaction with alkyl halides to displace the halogen and form a new alkyne with a longer carbon chain. The mechanism of this reaction, an S_N2 reaction, will be discussed in detail in the chapter under alkyl halides.