Alkynes

Introduction

Alkynes are a class of hydrocarbons characterized by the presence of a carbon-carbon triple bond. They follow the general formula and are also known as acetylenes. Alkynes exhibit two elements of unsaturation for each triple bond, which influences their chemical reactivity.

• Triple bond: The defining feature of alkynes, consisting of one sigma and two pi bonds.

• General formula:

• Alternative name: Acetylenes

• Reactivity: Alkynes undergo addition and oxidation reactions similar to alkenes, but also have unique reactions due to the triple bond.

Nomenclature: IUPAC

The systematic naming of alkynes follows specific rules to ensure clarity and consistency.

• Longest chain: Identify the longest carbon chain containing the triple bond.

• Suffix change: Replace the -ane ending of the parent alkane with -yne to indicate the presence of a triple bond.

• Numbering: Number the chain from the end nearest the triple bond to assign the lowest possible locant to the triple bond.

• Substituents: Assign numbers to branches or other substituents to indicate their positions on the main chain.

Examples

• Propyne:

• 5-Bromo-2-pentyne:

• 2,6-Dimethyl-3-heptyne:

Additional Functional Groups and Naming Priority

When multiple functional groups are present, IUPAC nomenclature assigns priority based on reactivity and importance.

• Priority order: acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > (alkane) > ether > halide

• Functional groups with higher priority than alkynes will determine the suffix and numbering of the parent chain.

Examples

• 4-methyl-2-hexyne: A methyl group at position 4, triple bond at position 2.

• 4-Hexyn-2-ol: Alcohol at position 2, triple bond at position 4.

• 4-Ethoxy-2-pentyne: Ethoxy group at position 4, triple bond at position 2.

Common Names

Some alkynes are named as substituted acetylenes, especially in older literature or for simple molecules.

• Methylacetylene:

• Isobutylisopropylacetylene: Complex alkynes may use common names based on their substituents attached to the acetylene core.

Physical Properties

Alkynes exhibit characteristic physical properties due to their nonpolar nature and molecular structure.

• Nonpolar: Alkynes are insoluble in water but soluble in most organic solvents.

• Boiling points: Similar to alkanes of comparable molecular size.

• Density: Less dense than water.

• State at room temperature: Alkynes with up to four carbons are gases at room temperature.

Electronic Structure

The bonding in alkynes is distinct due to the triple bond, which consists of one sigma and two pi bonds.

• Sigma bond: Formed by sp-sp orbital overlap between the two carbon atoms.

• Pi bonds: Two unhybridized p orbitals overlap at 90°, creating a cylindrical region of electron density around the sigma bond.

Example: In ethyne (), the carbon atoms are sp hybridized, resulting in a linear geometry with a bond angle of 180°. Additional info: The high s-character of the sp hybrid orbitals leads to shorter bond lengths compared to alkanes and alkenes.

Bond Lengths Table

Additional info: The shorter bond length in alkynes is due to increased s-character (50%) in sp hybridization.