Alkenes are hydrocarbons, molecules composed solely of carbon and hydrogen atoms, that contain at least one carbon-carbon double bond. Like nearly all other hydrocarbons, alkenes are highly flammable. They can easily ignite in the presence of heat, sparks, or flame, leading to rapid combustion. Their chemical structure, specifically the double bond, influences how they burn compared to other hydrocarbon types.
The Chemical Basis for Hydrocarbon Combustion
The flammability of any hydrocarbon stems from combustion, a rapid reaction with atmospheric oxygen. This process is exothermic, releasing energy in the form of heat and light. Complete combustion is the ideal reaction where the hydrocarbon breaks down completely into carbon dioxide and water vapor, given a sufficient supply of oxygen.
The heat release occurs because the energy required to break the starting bonds is less than the energy released when forming the stronger bonds in the carbon dioxide and water products. Hydrocarbons contain many carbon-hydrogen (C-H) bonds, which are the main energy source. Carbon atoms are oxidized to carbon dioxide, and hydrogen atoms are oxidized to water.
Flammability Differences Between Alkenes and Saturated Hydrocarbons
The double bond in an alkene gives it a lower ratio of hydrogen atoms to carbon atoms compared to a saturated hydrocarbon (alkane) with the same number of carbons. For example, ethene has the formula C2H4, while the corresponding saturated ethane is C2H6. This higher carbon content makes alkenes more prone to incomplete combustion when burned in the open air.
Incomplete combustion occurs when there is not enough oxygen to fully convert all the carbon into carbon dioxide. Instead, some of the carbon atoms are released as fine, uncombusted solid particles, known as soot. These hot carbon particles glow in the flame, causing alkenes to burn with a smoky, luminous yellow flame, unlike the cleaner, blue flame seen with saturated alkanes. This tendency for soot formation is a visual indicator of the higher carbon percentage in unsaturated alkenes. The molecular structure of alkenes also influences the intermediate species formed during burning.
Essential Safety and Handling Procedures
Given their highly flammable nature, safety protocols for handling alkenes are essential. Smaller alkenes, such as ethene, propene, and butene, exist as gases at room temperature. Gaseous alkenes must be stored in pressurized containers and kept in a well-ventilated area to prevent the buildup of explosive mixtures with air. Liquids and solids should be stored away from any source of ignition, including heat, sparks, or open flames.
Adequate ventilation, such as the use of a chemical fume hood, is necessary to manage the vapors of liquid alkenes. This prevents the inhalation of potentially toxic or irritating fumes and minimizes the risk of fire or explosion. Personal protective equipment (PPE), including chemical splash goggles and appropriate chemical-resistant gloves, is also necessary to prevent skin and eye contact. A thorough review of the chemical’s Safety Data Sheet (SDS) is always necessary to understand the specific hazards and required precautions for any particular alkene compound.