Structure of Alkynes

An alkyne is a hydrocarbon that contains at least one carbon-carbon triple bond (${\rm {C{\equiv}C}}$) with C_nH_2n-2 stoichiometry. The carbon-carbon triple bond forms from two sets of overlapping p orbitals, which results in sp hybridization on the alkyne carbons. Alkynes have linear geometry, and the bond angle is 180 degrees. Triple bonds are short, strong, and reactive. Alkyne nomenclature follows the basic IUPAC (International Union of Pure and Applied Chemistry) nomenclature rules. Suffixes in the parent name differentiate the degree of saturation or unsaturation in the carbon chains. A saturated hydrocarbon is an organic compound that contains only carbon-carbon single bonds and ${\rm {C{-}H}}$ bonds. An unsaturated hydrocarbon is an organic compound that contains ${\rm {C{-}H}}$ bonds in addition to one or more carbon-carbon ${\rm {C{=}C}}$ or ${\rm {C{\equiv}C}}$ bonds.

The -ane suffix indicates a saturated hydrocarbon (or alkane) with only single bonds. The -ene suffix indicates the presence of a carbon-carbon double bond. The -yne suffix indicates the presence of a carbon-carbon triple bond. For example, a six-carbon alkane is hexane, a six-carbon alkene is hexene, and a six-carbon alkyne is hexyne. Alkenes and alkynes have numbering priority over other groups except for alcohols, amines, and carbonyls. If a molecule contains an alkene and an alkyne, the group closest to the terminal carbon takes priority. If the alkene and alkyne are the same distance from the terminal carbon, the alkene has priority. If a compound contains an alkene and an alkyne, the ending is -enyne.

The position of the alkyne in the carbon chain is indicated by a number that identifies its location. For example, a seven-carbon chain with a triple bond on the first carbon would be called hept-1-yne. Alkynes do not have stereochemistry and do not use the E/Z nomenclature; however, if a compound contains and alkene and an alkyne, the alkene will still use E/Z nomenclature. New IUPAC nomenclature incorporates the location immediately before the suffix. For example, hept-1-yne is currently preferred to 1-heptyne, although both terms are still common. Alkynes have similar physical properties to alkanes and alkenes. They are insoluble in water and soluble in low-polar solvents, but they are less dense than water. The boiling points of alkynes tend to increase with the number of carbons. Alkynes are not as prevalent in nature, medicine, and industry as alkenes, with the exception of acetylene, which is used in industries such as welding. Acetylene is also useful in the production of polymers. Alkynes are also used in the pharmaceutical industry as contraceptives, antivirals, and antifungals, among other uses. Terminal alkynes are fairly acidic because of orbital effect. Orbital effect is the O of ARIO (atom, resonance, inductive, and orbital) effects. A terminal alkyne is an alkyne with the carbon-carbon triple bond at the end of the chain. Alkynes have sp hybridization, which is 50% s and 50% p. The $\sigma$ bond shifts closer to the carbon atom, causing the hydrogen to become slightly positive. The slightly positive hydrogen becomes an easy target for abstraction by a base. Abstraction is the removal of a hydrogen atom or a group by a radical.

Terminal alkynes have a pK_a of 25; therefore, a base with a pK_a greater than 25, such as NaNH₂ with a pK_a of 36, is needed to deprotonate an alkyne and convert it into an alkynide. The reaction, deprotonation, results in a loss of a hydrogen (proton) from an organic molecule. An alkynide is a deprotonated alkyne with the formula ${\rm{MC{\equiv}CR}}$. Alkynides are great nucleophiles for S_N2 reactions.

Propyne, a three-carbon alkyne, has a pK_a of 25. Propyne can be deprotonated with a base that has a pK_a greater than 25, such as sodium amide (NaNH₂, ${\rm {p}}K_{\rm {a}}=36$), to produce sodium propynide, which can be used as a nucleophile to attack an alkyl halide and make a longer chain of carbons.