Alkyne

[1] The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CnH2n−2.

The acidic hydrogen on terminal alkynes can be replaced by a variety of groups resulting in halo-, silyl-, and alkoxoalkynes.

In systematic chemical nomenclature, alkynes are named with the Greek prefix system without any additional letters.

The position of unsaturation is indicated by a numerical locant immediately preceding the "-yne" suffix, or 'locants' in the case of multiple triple bonds.

Classically, acetylene was prepared by hydrolysis (protonation) of calcium carbide (Ca2+[:C≡C:]2–): which was in turn synthesized by combining quicklime and coke in an electric arc furnace at 2200 °C: This was an industrially important process which provided access to hydrocarbons from coal resources for countries like Germany and China.

However, the energy-intensive nature of this process is a major disadvantage and its share of the world's production of acetylene has steadily decreased relative to hydrocarbon cracking.

Hundreds of millions of kilograms are produced annually by partial oxidation of natural gas:[8] Propyne, also industrially useful, is also prepared by thermal cracking of hydrocarbons.

Terminal alkynes (RC≡CH, including acetylene itself) can be deprotonated by bases like NaNH2, BuLi, or EtMgBr to give acetylide anions (RC≡C:–M+, M = Na, Li, MgBr) which can be alkylated by addition to carbonyl groups (Favorskii reaction), ring opening of epoxides, or SN2-type substitution of unhindered primary alkyl halides.

In the presence of transition metal catalysts, classically a combination of Pd(PPh3)2Cl2 and CuI, terminal acetylenes (RC≡CH) can react with aryl iodides and bromides (ArI or ArBr) in the presence of a secondary or tertiary amine like Et3N to give arylacetylenes (RC≡CAr) in the Sonogashira reaction.

For example, phenylacetylene can be generated from styrene by bromination followed by treatment of the resulting of 1,2-dibromo-1-phenylethane with sodium amide in ammonia:[9][10] Via the Fritsch–Buttenberg–Wiechell rearrangement, alkynes are prepared from vinyl bromides.

Such use was pioneered by Ralph Raphael, who in 1955 wrote the first book describing their versatility as intermediates in synthesis.

For example, acetylene has a heat of formation of +227.4 kJ/mol (+54.2 kcal/mol), indicating a much higher energy content compared to its constituent elements.

The highly exothermic combustion of acetylene is exploited industrially in oxyacetylene torches used in welding.

Partial hydrogenation, stopping after the addition of only one equivalent to give the alkene, is usually more desirable since alkanes are less useful: The largest scale application of this technology is the conversion of acetylene to ethylene in refineries (the steam cracking of alkanes yields a few percent acetylene, which is selectively hydrogenated in the presence of a palladium/silver catalyst).

For more complex alkynes, the Lindlar catalyst is widely recommended to avoid formation of the alkane, for example in the conversion of phenylacetylene to styrene.

Due to their comparable thermodynamic stabilities, the equilibrium constant of alkyne/allene isomerization is generally within several orders of magnitude of unity.

For example propyne can be isomerized to give an equilibrium mixture with propadiene: Alkynes undergo diverse cycloaddition reactions.

The "cycloadduct" derived from the addition of alkynes to 2-pyrone eliminates carbon dioxide to give the aromatic compound.

Non-carbon reagents also undergo cyclization, e.g. azide alkyne Huisgen cycloaddition to give triazoles.

Via the condensation with formaldehyde and acetylene is produced butynediol:[8][17] In the Sonogashira reaction, terminal alkynes are coupled with aryl or vinyl halides: This reactivity exploits the fact that terminal alkynes are weak acids, whose typical pKa values around 25 place them between that of ammonia (35) and ethanol (16): where MX = NaNH2, LiBu, or RMgX.

Thus, few drops of diamminesilver(I) hydroxide (Ag(NH3)2OH) reacts with terminal alkynes signaled by formation of a white precipitate of the silver acetylide.

[19] According to Ferdinand Bohlmann, the first naturally occurring acetylenic compound, dehydromatricaria ester, was isolated from an Artemisia species in 1826.

Polyynes, a subset of this class of natural products, have been isolated from a wide variety of plant species, cultures of higher fungi, bacteria, marine sponges, and corals.

Diynes and triynes, species with the linkage RC≡C–C≡CR′ and RC≡C–C≡C–C≡CR′ respectively, occur in certain plants (Ichthyothere, Chrysanthemum, Cicuta, Oenanthe and other members of the Asteraceae and Apiaceae families).

A carbon–carbon triple bond is also present in marketed drugs such as the antiretroviral efavirenz and the antifungal terbinafine.

Molecules called ene-diynes feature a ring containing an alkene ("ene") between two alkyne groups ("diyne").

These compounds, e.g. calicheamicin, are some of the most aggressive antitumor drugs known, so much so that the ene-diyne subunit is sometimes referred to as a "warhead".

Ene-diynes undergo rearrangement via the Bergman cyclization, generating highly reactive radical intermediates that attack DNA within the tumor.