Reactions of Alkynes

Reactions of alkynes include addition of HX, addition of X2, hydration, and catalytic hydrogenation. The stoichiometry of HX and X2 addition determines whether the reagent adds once or twice.

Alkynes react with halide reagents in a similar fashion to alkenes. Alkynes react with HX or HX/HOOH to add a halide to the alkyne. If one equivalent of reagent is used, the triple turns to a double and adds the reagent once. If two equivalents of reagents are added, the triple reacts once to add one halide and form a double bond, and then the double bond reacts once with the reagent to add a second halide and turn to a single bond.

Alkyne reactions depend on the quantity of reagent in relation to the quantity of alkyne. This is based on the stoichiometry of the reaction. Stoichiometry is the relationship between the reactants and products in a balanced chemical equation.

The $\pi$ electrons from the triple bond attack the hydrogen on the hydrogen halide, creating a carbocation and releasing the halide. The negatively charged halide then bonds with the carbocation to form the first addition product. If there is another equivalent of hydrogen halide, the reaction will occur again with the $\pi$ electrons from the double bond attacking the hydrogen halide and eventually forming a single bond with the addition of two halides to the same carbon.

The addition of HX to a triple bond results in a Markovnikov addition of X to the triple bond. When the double bond reacts (in the presence of excess reagent), the addition also follows Markovnikov's rule. This results in the halide adding to the same carbon after both addition reactions. Markovnikov's rule states that in an addition of an asymmetric reagent to an alkene, the more-electronegative atom of the reagent will bond to the more-substituted carbon of the alkene.

Addition Reactions of Alkynes with a Hydrogen Halide

If one equivalent of the hydrogen halide (usually HBr) is added in the presence of hydrogen peroxide (HOOH), the reaction proceeds to give the anti-Markovnikov product similarly to the reaction with alkenes. Excess reagent will add two halogens to adjacent carbons. After the first adds to the least-substituted carbon, the next will add again to the least-substituted carbon, which will usually be the carbon without the first added bromine.

Addition Reactions of Alkynes with a Hydrogen Halide and Peroxide

Alkynes react with Br2 or Cl2 to add two halides to the alkyne in a stoichiometric reaction. When using one equivalent of reagent, the triple bond turns into a double bond and adds the dihalide reagent once, with both halides adding trans to each other. When adding two equivalents, the triple bond turns to a single bond, and the reagent adds twice. The addition of a dihalide, X2, occurs when the triple bond attacks one of the halides and breaks its bond. The halides bond to the adjacent carbons, and a double bond remains. If additional dihalides are added, they will bond with the adjacent carbons, leaving a remaining single bond.

Addition Reactions of Alkynes with a Dihalide

Hydration is the addition of a water molecule as ${-}{\rm {H}}$ and ${-}{\rm{OH}}$ to two adjacent carbon atoms. This is sometimes referred to as adding water to a $\pi$ bond of an alkene. Hydration of an alkyne is similar to hydration of an alkene except the hydration of an alkyne forms a vinyl alcohol that rearranges to a ketone or aldehyde. Vinyl alcohols are alcohol functional groups directly attached to a double bond. Vinyl alcohols are unstable, and they rearrange to a more stable species, either an aldehyde or ketone. This is known as tautomerization, which is the process of a vinyl alcohol converting to a carbonyl. Hydration of a terminal triple bond with hydroboration yields an aldehyde, while acid-catalyzed hydration of a terminal alkyne with HgSO4 yields a ketone. Internal alkynes convert to ketones with either hydration method. The hydration method used determines whether a ketone or an aldehyde is formed.

Two Methods for Hydration of an Alkyne

Catalytic hydrogenation of an alkyne with hydrogen (H2) in the presence of nickel (Ni), platinum (Pt), or palladium (Pd) yields an alkane. Catalytic hydrogenation (or hydrogenation) is the use of a heavy-metal catalyst to add hydrogen to an alkene or alkyne. Catalytic hydrogenation requires an excess of hydrogen (H2), and with excess hydrogen, the result is always the complete conversion of any carbon-carbon triple or double bond into a carbon-carbon single bond. Reactants with any combination of double and triple bonds are hydrogenated to single bonds.

Catalytic Hydrogenation of Alkynes

Hydrogenation of a triple bond to a double bond and not to a single bond requires a special type of hydrogenation catalyst. Poisoned catalysts, such as Lindlar's catalyst, are stoichiometrically designed to only react once. Addition of Lindlar's catalyst, and H2, yields a cis-alkene via a syn addition. Cis is the geometric arrangement in which two identical substituents are on the same side of the central part of a molecule or ion. Syn addition (or syn) is the addition of two substituents to the same side of a double bond or a triple bond.

Lindlar Catalyst

Addition of sodium in ammonia (Na, NH3) to an alkyne yields a trans-alkene via anti addition (called dissolving metal reduction). Trans is the geometric arrangement in which two identical substituents are on opposite sides of the central part of a molecule or ion. Anti addition (or anti) is the addition of two substituents to opposite sides of a double bond or a triple bond.