Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond (C=C), giving them distinct properties and reactivity patterns compared to their saturated counterparts, the alkanes.
Their general formula is CnH2n, indicating they have fewer hydrogen atoms relative to alkanes.
This section delves into the nomenclature, physical and chemical properties, and the various applications of alkenes.
Nomenclature of Alkenes
The naming of alkenes follows the International Union of Pure and Applied Chemistry (IUPAC) guidelines, with specific steps to ensure accurate identification and nomenclature:
Identify the Longest Chain: Find the longest carbon chain that includes the double bond.
Prefix Assignment: Use the same numerical prefixes as alkanes to indicate the number of carbons in the main chain.
Suffix Change: Replace "-ane" with "-ene" to denote the presence of a double bond.
Numbering: Number the chain from the end nearest to the double bond, assigning the lowest possible number to the double bond.
Substituents: Name and number any branches or substituents as in alkanes, placing them before the main chain name.
Full Name Construction: Assemble the name by combining the substituent names, numbers, and the alkene base name.
Physical Properties of Alkenes
Molecular Size and Mass: Increase with more carbon atoms.
Boiling and Melting Points: Generally, increase with molecular size, though slightly lower than alkanes of similar size due to the electron distribution around the double bond.
Density: Increases with molecular mass but remains under 1 g/cm³, indicating alkenes are lighter than water.
Solubility: Alkenes do not dissolve in polar solvents like water but are soluble in nonpolar solvents (e.g., hexane, benzene).
Chemical Properties of Alkenes
Reactivity: The presence of the C=C double bond makes alkenes more reactive than alkanes.
Addition Reactions: The double bond can open up to add atoms or groups across it, including hydrogenation, halogenation, hydration, and hydrohalogenation processes.
Polymerization: Alkenes can link into long chains or polymers, a reaction exploited in the production of many commercial polymers.
Applications of Alkenes
Plastics Production: Serving as precursors for polyethylene, polypropylene, and PVC.
Organic Synthesis: Intermediate in creating alcohols, aldehydes, and acids.
Detergents and Surfactants: Conversion into fatty acids for detergent manufacturing.
Fuel Additives: Enhancing gasoline octane ratings and engine performance.
Synthetic Rubber Production: Butadiene and isoprene are used in making various types of synthetic rubber, crucial for the automotive and manufacturing industries.