Summary
Highlights
Alkenes are unsaturated hydrocarbons containing one or more double carbon-carbon bonds. Alkenes form a homologous series, with ethylene being the simplest representative. Ethylene is a colorless, nearly odorless gas, slightly lighter than air, and poorly soluble in water. Propylene and butylene are also gaseous. From pentene to octadecene, alkenes are liquids, and starting from nonadecene, they are solids. Alkenes are poorly soluble in water but better than alkanes, and well soluble in organic solvents. Ethylene and propylene burn with a sooty flame.
The double bond in alkenes consists of a strong sigma bond and a weaker, more diffuse pi bond, which is accessible for many reactions. This double bond represents an area of high electron density, making it attractive to electrophiles. This explains the high reactivity of alkenes, as they readily undergo addition reactions across the double bond via an ionic electrophilic mechanism. These reactions include hydrogenation (reaction with hydrogen in the presence of a nickel catalyst and elevated temperature), halogenation (reaction with halogens like bromine, which causes the discoloration of bromine water), and hydrohalogenation (reaction with hydrogen halides).
The addition of water (hydration) is also characteristic of alkenes. The addition of hydrogen halides and water to unsymmetrical alkenes follows Markovnikov's rule, stating that the hydrogen atom adds to the carbon atom with the most hydrogen atoms already attached to it at the double bond. Another characteristic reaction for alkenes, and all organic compounds, is oxidation. Combustion is a type of rapid oxidation. Oxidation by potassium permanganate in a neutral medium is a qualitative test for alkenes, causing the pink solution to decolorize.
Polymerization is a significant reaction for alkenes, resulting in substances like polyethylene and polypropylene, which have wide practical applications. A monomer is the substance entering the polymerization reaction, and the polymer is its product. The structural unit is the repeating formula within the polymer chain. The index 'n' represents the degree of polymerization. Ethylene hydrocarbons can interact with each other to form long chain molecules, typically under high temperature, pressure, and with a catalyst. This process of sequential joining of identical molecules into larger ones is called polymerization.
Hydrocarbons form explosive mixtures with air, which is an important safety consideration when working with them. In summary, alkenes are characterized by addition reactions, oxidation, and polymerization reactions.