Summary
Highlights
Alkenes are also called olefins because liquid alkenes are oil-like substances. Their names are derived from alkanes by changing the suffix '-ane' to '-ene'. The first member of the homologous series of alkenes is ethene (ethylene), a colorless gas with a sweet odor, slightly lighter than air, and poorly soluble in water. Propylene and butylene are also gases, while C5-C15 alkenes are colorless liquids, and C16 and above are solids.
Hydrocarbons with fewer hydrogen atoms than required for full saturation form double or triple bonds between carbon atoms, known as multiple bonds. These are called unsaturated or non-limiting hydrocarbons, distinct from saturated ones. Unsaturated hydrocarbons are classified into ethylene (alkene) and alkyne series based on the number and type of multiple bonds.
Ethylene (ethene) is the primary focus for understanding alkene structure due to its practical importance. Its symmetrical molecular structure shows each carbon atom bonded to two hydrogen atoms, with an additional double bond formed between the carbon atoms. In ethylene, one s-orbital and two p-orbitals undergo sp2 hybridization. Each carbon atom has three hybridized orbitals and one unhybridized p-orbital.
Two hybridized orbitals from carbon atoms overlap to form a sigma bond between them. The remaining four hybridized orbitals from carbon atoms overlap with four s-orbitals of hydrogen atoms, forming four sigma bonds in the same plane. The two unhybridized p-orbitals of carbon atoms overlap perpendicularly to the sigma bond plane, forming one pi-bond. The pi-bond is less stable than the sigma bond and can be easily broken by reagents.
The ethylene molecule is symmetrical, with all atomic nuclei lying in the same plane. The bond angles are close to 120 degrees, and the distance between carbon atoms is 0.134 nanometers. Significant contributions to understanding stereochemistry were made by Jacobus Henricus van 't Hoff, a Dutch chemist and the first Nobel laureate in Chemistry in 1901.
Alkenes, also known as ethylene hydrocarbons, are unsaturated hydrocarbons containing one double carbon-carbon bond in addition to single bonds. Their chemical composition is described by the general formula CnH2n. This formula is shared with cycloalkanes. To form an alkene from an alkane, two hydrogen atoms are removed from adjacent carbon atoms.