Alkynes

Alkynes are hydrocarbons with carbon-carbon triple bonds. They are unsaturated compounds and are written with the general formula CnH2n-2. This group includes simple molecules like ethyne, also called acetylene, which is a colorless gas used as a fuel for welding because it burns at high temperatures.

Alkynes also appear in natural and synthetic compounds. Some natural alkynes are found in the poison from South American tree frogs. Synthetic alkyne-containing compounds are found in medicines...

Alkynes are unsaturated hydrocarbons with carbon-carbon triple bonds. Their general formula is CnH2n-2, and their names use the suffix "-yne" instead of "-ane" or "-ene." Alkynes can be named with IUPAC names or common names.

The IUPAC system uses a step-by-step method based on the molecule’s structure. First, find the longest carbon chain that contains the triple bond. Add the suffix "-yne" to the parent chain name. In the example shown, the longest chain has seven carbon atoms, so the parent...

Terminal alkynes are more acidic than alkenes and alkanes. Acid strength is usually measured with pKa, and a lower pKa means a stronger acid. Ethane has a pKa of 51, ethene has a pKa of 44, and acetylene has a pKa of 25. That means acetylene is far more acidic than the other two hydrocarbons.

This difference comes from the stability of the carbanion formed after deprotonation. A carbanion is a negatively charged carbon species. In ethane, ethene, and acetylene, the lone pair sits in an sp3,...

Terminal alkynes can be made into longer carbon-chain alkynes by alkylation with primary alkyl halides. This reaction uses a strong base such as sodium amide. One example is 1-propyne, which reacts with sodium amide and then ethyl bromide to give 2-pentyne.

The reaction happens in two steps. First, the strong base removes the acidic hydrogen from the terminal alkyne and forms an acetylide ion. An acetylide ion is the carbon-based species that then takes part in the next step.

Next, the...

Alkynes can be made from vicinal or geminal dihalides by dehydrohalogenation. This reaction uses a strong base, such as sodium amide, in liquid ammonia. Two equivalents of hydrogen halide are lost through two successive E2 elimination reactions.

In a vicinal dihalide, the two halogen atoms are on neighboring carbon atoms. The first E2 step removes a proton that is anti to the leaving group. Because E2 reactions are concerted, the proton is removed and the halide leaves at the same time. This...

Alkyne halogenation is an electrophilic addition reaction where a halogen molecule adds across a triple bond. Because alkynes have two π bonds, they can add two equivalents of bromine or chlorine. The first addition usually gives a trans-dihaloalkene as the major product and the cis isomer as the minor product. A second equivalent of halogen then forms the tetrahalide.

The reaction begins when a π bond in the alkyne acts as a nucleophile and attacks the electrophilic end of the polarized...

Hydrohalogenation of alkynes adds hydrogen halides, HX, across a carbon-carbon triple bond. In this reaction, X can be chlorine, bromine, or iodine. The first addition gives a vinyl halide, and the second addition gives a geminal dihalide.

When HCl reacts with an alkyne, one pathway begins with proton transfer to the triple bond. The pi electrons attack the hydrogen in hydrogen chloride and push out chloride ion. This forms a secondary vinylic carbocation, and chloride then attacks to make a...

Alkyne hydration uses acid, water, and a mercuric salt to convert alkynes into aldehydes and ketones. It is similar to alkene hydration, but the product type is different. A mercuric salt such as mercuric sulfate, HgSO4, is usually added because the reaction is much slower than alkene hydration.

Terminal alkynes follow Markovnikov’s rule during hydration. In contrast, internal alkynes do not add water in a regioselective way. That means the product outcome depends on the alkynes structure.

Alkyne hydration can make aldehydes and ketones through hydroboration-oxidation. In this indirect hydration reaction, an alkyne first reacts with borane, then with alkaline peroxide. The alkyne forms an enol, and that enol quickly changes into a carbonyl product. Terminal alkynes give aldehydes, while internal alkynes give ketones.

The reaction happens in two steps. The hydroboration step is a concerted syn addition, which means the boron and hydrogen add to the same side of the carbon-carbon...

Alkynes can be broken apart by oxidative cleavage. This reaction uses strong oxidizing reagents such as potassium permanganate and ozone. It cuts the triple bond, which has one sigma bond and two pi bonds, and turns the alkyne into carboxylic acids.

With warm, basic aqueous potassium permanganate, the alkyne first forms an unstable alpha-diketone intermediate. The reaction then gives carboxylate salts. A mild acid treatment protonates these carboxylate anions and produces the free carboxylic...

Alkyne hydrogenation can stop at a cis-alkene instead of going all the way to an alkane. Transition metal catalysts such as Pt, Pd, or Ni add hydrogen in two syn steps, which means both hydrogens add to the same side. The cis-alkene is the key intermediate in this process.

Hydrogenation also helps compare the thermodynamic stability of hydrocarbons. Acetylene has a heat of hydrogenation of −176 kJ/mol, while ethylene has a value of −137 kJ/mol. The larger heat released for acetylene shows that...

Dissolving metal reduction turns alkynes into trans-alkenes with sodium or lithium in liquid ammonia. This reaction gives the alkene with anti addition across the carbon-carbon triple bond, which leads to the trans product.

The reaction must be kept very cold because ammonia is a gas at room temperature. It is run with a dry ice and acetone bath, which holds the mixture at low temperature. When sodium dissolves in liquid ammonia, it forms sodium ions and free electrons. Ammonia molecules...