The invisible gas that turns a green tomato red is also the most produced organic compound on Earth, with hundreds of millions of tonnes manufactured annually. This substance, known as ethylene or ethene, serves as the foundational building block for the modern petrochemical industry. While it exists naturally as a plant hormone that regulates growth and aging, its industrial dominance stems from its ability to polymerize into polyethylene, the plastic that wraps food, lines pipes, and forms the casing for countless consumer goods. The sheer scale of its production dwarfs that of any other organic molecule, making it the silent engine behind the global economy of plastics. Despite its ubiquity, the molecule remains colorless and odorless to the human nose, though it carries a faint sweet and musty scent that is detectable by sensitive olfactory systems. This duality of nature and industry defines the alkene family, where simple hydrocarbons with double bonds act as the precursors to complex materials that shape human civilization.
The Double Bond Architecture
The defining feature of an alkene is a carbon, carbon double bond that consists of two distinct components: a strong sigma bond and a weaker pi bond. This structural arrangement creates a rigid planar geometry where the bond angles around each carbon atom measure approximately 120 degrees, a shape predicted by the VSEPR model of electron pair repulsion. The double bond is shorter than a single bond, measuring 1.33 angstroms compared to the 1.53 angstroms of a typical single bond, yet it is not twice as strong. The sigma bond contributes 611 kilojoules per mole of strength, while the pi bond adds another 65 kilocalories per mole, making the total bond energy significant but distinct from a simple sum of two single bonds. This pi bond lies outside the main axis of the molecule, with half of the electron density on one side and half on the other, creating a region of high electron density that is susceptible to chemical attack. The rigidity of this bond prevents free rotation, locking substituents into specific spatial arrangements that give rise to isomerism. This restriction means that molecules like cis-2-butene and trans-2-butene exist as distinct entities with different physical properties, unable to interconvert at ambient conditions without breaking the pi bond.The Geometry of Isomers
When a carbon chain contains four or more atoms, the position of the double bond creates a landscape of structural isomers that can number in the dozens for a single formula. For instance, the formula C6H14 can exist as thirteen different isomers, ranging from 1-hexene to 2,3-dimethyl-2-butene. The spatial arrangement of groups attached to the double bond creates cis and trans configurations, or E and Z isomers in more complex cases. The prefixes cis and trans, derived from Latin words meaning on this side and on the other side, describe whether functional groups lie on the same side or opposite sides of the double bond plane. In more intricate molecules where all four substituents differ, chemists use the E-Z notation system, based on German words for together and opposite, to assign priority based on the Cahn, Ingold, Prelog rules. This geometric precision is not merely academic; it dictates the physical state and reactivity of the compound. For example, strained alkenes like norbornene and trans-cyclooctene possess strong, unpleasant odors due to the high energy of their pi complexes with metal ions, while their unstrained counterparts may be odorless gases. The number of potential isomers increases rapidly with additional carbon atoms, creating a vast library of molecules that must be distinguished by their specific shapes and bond angles.