Chemistry of Alkynes

Alkynes are a type of aliphatic hydrocarbon that contains one or more triple bonds in the hydrocarbon chain. The hybridization of the carbon, which has been included in double bonds, is sp. Since there are triple bonds in alkynes, they are also a type of unsaturated hydrocarbon.

Also, alkynes are more unsaturated than alkenes. Due to the triple bond, alkynes could undergo electrophilic addition reactions through the cleavage of the pi bonds.

The empirical formula of alkynes that have only one triple bond is C_nH_2n-2. Carbon atoms bond linearly to form acyclic alkynes. Sometimes there are branches in the carbon backbone.

Cyclic alkynes can also be found when carbon atoms are bonded in a cycle. However, cyclic alkynes that have carbon atoms less than 8 are unstable due to the linear nature of the triple bond. In this article, we are discussing linear alkynes that only have one triple bond.

Properties of alkynes

Alkynes are nonpolar compounds. Therefore, they have only van der Waals interactions between molecules as secondary interaction forces. When the number of carbon atoms increases, the molecule gets bigger, so it gets a higher surface area. Thus, the van der Waals forces also increase. Alkynes with 2-4 carbon atoms exist as gases at room temperature.

Alkynes release a high energy when they are combusted. Therefore, they are used as fuels. As an example, the simplest alkyne, ethyne aka acetylene (C₂H₂), is widely used in welding torches due to its high enthalpy of combustion.

When the number of carbon atoms increases in an organic compound. They could form different types of isomers. Therefore, alkynes could form position isomers and chain isomers. The isomerism also affects the boiling points and melting points of the alkynes.

Usually, when an alkyne has a linear structure, it has a higher surface area. Therefore, the van der Waals forces are high. Thus, the melting points and the boiling points of linear alkynes are higher compared to the branched alkynes with a similar number of carbon atoms.

Since alkynes are non-polar compounds, they do not dissolve in polar solvents like water. But they are soluble in organic solvents.

General nomenclature of alkynes

Linear alkynes without any pendant groups are named according to their number of carbon atoms in the carbon backbone. The stem name is added “yne” suffix to name an alkane.

IUPAC nomenclature of alkynes

There can be different structural formulas for one empirical formula. As an example, the butyne C₄H₆ empirical formula has two different structures as follows.

IUPAC nomenclature is introduced considering the structural formula as well as the empirical formula.

Steps to name an alkyne

1. Identify the longest hydrocarbon chain that includes the triple bond and has the maximum number of substituent groups.
2. Number the hydrocarbon chain so that the carbon atoms that are included in the triple bond get the lowest possible number. If it takes the same number from either side, choose the way the substitute groups get the lowest numbers.
3. Name the substituent groups according to alphabetical order. When naming a substituent group, it writes the number of the carbon atom and then puts a dash, and then writes the name of the alkyl group.
4. Then it writes the stem name of the hydrocarbon chain and then puts a dash, writes the number of carbon atom that has a double bond, then puts a dash again. Finally, add the suffix “yne” at the end.

• All the letters should be simple letters, and the name must be a single word.
• If there are similar alkyl groups in different carbon atoms, they are named as follows.

• There should be dashes between letters and numbers. And there should be commas between numbers. All the letters must be simple letters.

Hybridization of carbon in alkyne

In an alkyne, there are two types of bonds, which are pi bonds and sigma bonds. Carbon atoms that form triple bonds are sp-hybridized. Carbon atoms that only form sigma bonds are sp³ hybridized.

At ground state, carbon has four electrons in its valence shell. The electron configuration of carbon at the ground state is 1s² 2s² 2p². When it is supplied energy, an electron in the 2s orbital moves 2p orbital. Thus, four unpaired electrons are obtained. It can be represented in an energy diagram as follows.

One of the 2p atomic orbitals and the 2s orbital are hybridized and form two sp hybridized orbitals. After hybridization, there remain two unhybridized 2p orbitals that contain one lone electron each. The two sp hybridized orbitals are similar in size, shape, and energy. They have lower energy than the un-hybridized 2p orbital and higher energy than the 2s orbital.

These sp-hybridized orbitals remain linear geometry and 180° angle to each other. The un-hybridized 2p orbital remains perpendicular to the linear sp orbitals. In alkynes C-C triple bond is formed by the linear overlapping of sp-hybridized orbitals. A C-H bond is formed by the linear overlapping of the sp-hybridized orbital and the unhybridized s orbital of hydrogen.

Preparation of alkynes

The reaction of Calcium carbide (CaC₂) with water results in the simplest alkyne of ethyne, which is usually known as acetylene.

Alkyl halides, which have two halogens in the same carbon or neighboring carbon atoms, those alkyl halides are treated with alcoholic KOH to obtain alkynes. In this reaction, the halogen is eliminated with hydrogen from the neighboring carbon atom as a hydrogen halide (HF, HCl, HBr, HI). The hydrogen is eliminated from the carbon where there is less hydrogen.