Reactions of Alkenes and Alkynes
Alkenes and alkynes are generally more reactive than alkanes due to the electron density available in their pi bonds. In particular, these molecules can participate in a variety of addition reactions and can be used in polymer formation.
Unsaturated hydrocarbons can participate in a number of different addition reactions across their double or triple bonds. A generic reaction scheme is shown in Figure 1.
These addition reactions include catalytic hydrogenation, halogenation, hydrohalogenation, and others.
Cycloadditions and oxidation
Alkynes undergo diverse cycloaddition reactions. Most notable is the Diels–Alder reaction with 1,3-dienes to give 1,4-cyclohexadienes. This general reaction has been extensively developed, and electrophilic alkynes are especially effective dienophiles. The 'cycloadduct,' derived from the addition of alkynes to 2-pyrone, eliminates carbon dioxide to give the aromatic compound. Other specialized cycloadditions include multicomponent reactions such as alkyne trimerisation, which gives aromatic compounds and the [2+2+1] cycloaddition of an alkyne, alkene, and carbon monoxide in the Pauson–Khand reaction. Non-carbon reagents also undergo cyclization (Azide alkyne Huisgen cycloaddition) to give triazoles. Cycloaddition processes involving alkynes are often catalyzed by metals (enyne metathesis and alkyne metathesis), which allows the scrambling of carbyne (RC) centers.
Oxidative cleavage of alkynes proceeds via cycloaddition to metal oxides. Most famously, potassium permanganate converts alkynes to a pair of carboxylic acids. Alkenes and alkynes can be oxidized to form epoxides, vicinal diketones, and carboxylic acids.
In the presence of a catalyst—typically platinum, palladium, nickel, or rhodium—hydrogen can be added across a triple bond to take an alkyne to an alkene and an alkene to an alkane. In practice, it is difficult to isolate the alkene product of this reaction, though a poisoned catalyst—a catalyst with fewer available reactive sites—can be used. As the hydrogen is immobilized on the surface of the catalyst, the triple or double bonds are hydrogenated in a syn fashion, that is to say, the hydrogen atoms come from the same side of the molecule. For an example, see Figure 2.
Alkenes and alkynes can also be halogenated with the halogen adding across the double or triple bond. The halogenation of an alkene results in a dihalogenated alkane product, while the halogenation of an alkyne can produce a tetrahalogenated alkane.
Alkenes and alkynes can react with hydrogen halides like HCl or HBr. Hydrohalogenation gives the corresponding vinyl halides or alkyl dihalides, depending on the number of HX equivalents added. The addition of water to alkynes is a related reaction, except the initial enol intermediate converts to the ketone or aldehyde. If the alkene is asymmetric, the reaction will follow Markovnikov's rule—the halide will be added to the carbon with more alkyl substituents (Figure 3).
Water can be added across triple bonds in alkynes to yield aldehydes and ketones for terminal and internal alkynes, respectively. Hydration of alkenes via oxymercuration produces alcohols. This reaction takes place during the treatment of alkenes with a strong acid as catalyst.