Alkyne

Acetylene-3D-vdW

Alkyne General Formulae V.2

Phenylacetylene prepn

PhC2HH2

Sonogashira reaction scheme ACS

Alkyne is a type of hydrocarbon in which one or more pairs of carbon atoms are connected by a triple bond. Alkynes are part of the larger class of unsaturated hydrocarbons, which also includes alkenes, where the carbon atoms are connected by double bonds. Alkynes are characterized by the general molecular formula C_nH_2n-2, where n is the number of carbon atoms in the molecule.

Properties

Alkynes are known for their distinctive chemical properties. The triple bond makes alkynes more reactive than alkenes or alkanes. This high reactivity is due to the presence of π (pi) bonds, which are weaker than the σ (sigma) bond and thus more susceptible to chemical reactions. Alkynes can undergo a variety of chemical reactions, including hydrogenation, halogenation, and hydration, often resulting in the formation of new compounds.

Structure

The triple bond in alkynes consists of one σ bond and two π bonds. The linear geometry of the triple bond forces the bonded carbon atoms and their attached groups to align in a straight line, leading to a linear molecular shape. This structural feature distinguishes alkynes from alkenes, which have a planar geometry due to their double bonds.

Types of Alkynes

Alkynes can be categorized into two main types: terminal alkynes and internal alkynes. Terminal alkynes have a triple bond at the end of the carbon chain, with the formula RC≡CH, where R is any alkyl group. Internal alkynes have their triple bond located between two carbon atoms within the chain, and their formula is R₁C≡CR₂, where R₁ and R₂ are alkyl or aryl groups.

Synthesis

Alkynes can be synthesized through various methods, including the dehydrohalogenation of vicinal or geminal dihalides, and the alkyne metathesis. Another common method is the partial reduction of alkynes from acetylenic compounds, which can be achieved using specific catalysts.

Applications

Alkynes find applications in a wide range of chemical syntheses. They are precursors to many types of compounds, including alcohols, ketones, and aldehydes, through various addition reactions. Alkynes are also used in the synthesis of complex natural products and in the pharmaceutical industry for the production of drugs. Additionally, the simplest alkyne, acetylene, is widely used as a fuel and a building block in organic synthesis.

Safety

Handling alkynes requires caution due to their reactivity. Many alkynes are flammable and can form explosive mixtures with air. Proper storage and handling procedures are essential to prevent accidents.