An alkene is a hydrocarbon that contains at least one carbon-carbon double bond. That double bond is what defines the entire class of molecules and drives nearly everything interesting about them, from how they react in a lab to how they ripen the fruit on your counter. The general formula for a simple, straight-chain alkene with one double bond is CnH2n, meaning each double bond reduces the hydrogen count by two compared to the corresponding single-bonded hydrocarbon (alkane).
The Double Bond Up Close
A carbon-carbon double bond is not simply two single bonds stacked together. It consists of two different types of bonds: a sigma bond and a pi bond. The sigma bond forms from a head-on overlap of orbitals between the two carbon atoms, just like a normal single bond. The pi bond, however, forms from a sideways overlap of orbitals that sit above and below the plane of the molecule. This pi bond is what makes alkenes chemically distinct from alkanes.
Each carbon in the double bond connects to three groups (the other carbon plus two additional atoms or groups), and those three connections spread out in a flat, triangular arrangement with roughly 120-degree angles between them. The geometry is called trigonal planar, and it means the atoms directly attached to the double bond all lie in the same plane. This flatness matters because it prevents the two carbons from spinning freely relative to each other. Rotating around the double bond would require breaking the pi bond, which takes substantial energy. A carbon-carbon double bond has a total bond energy of about 682 kJ/mol, compared to roughly 368 kJ/mol for a single bond. The double bond is stronger, but not twice as strong, because the pi bond component is weaker than the sigma bond component.
Naming Alkenes
Alkenes follow the same basic naming system (IUPAC nomenclature) as other organic compounds, with one key difference: the suffix changes from “-ane” to “-ene.” So a two-carbon alkene is ethene, a three-carbon alkene is propene, and so on. When naming an alkene, you find the longest continuous carbon chain that includes the double bond, number the carbons so the double bond gets the lowest possible number, and insert that number into the name. A six-carbon chain with a double bond starting at the second carbon, for example, is hex-2-ene.
Any branches or substituent groups attached to the main chain are named and numbered separately, just as they would be with alkanes. The double bond always takes priority in determining how you number the chain.
Geometric Isomers: Cis-Trans and E-Z
Because the double bond locks atoms in place and prevents rotation, alkenes can have geometric isomers. These are molecules with the same chemical formula and the same connections between atoms, but with groups arranged differently in space. For this to happen, each carbon in the double bond must carry two different groups.
When two identical groups end up on the same side of the double bond, the arrangement is called “cis” (from Latin, meaning “on this side”). When they end up on opposite sides, it’s called “trans” (meaning “across”). Cis-2-butene and trans-2-butene, for instance, are two distinct compounds with different physical properties, even though they share the same molecular formula.
The cis-trans system works well for simple cases, but it breaks down when the four groups attached to the double bond are all different. In those situations, chemists use the E-Z system, which ranks each group by a set of priority rules and then determines whether the higher-priority groups are on the same side (Z) or opposite sides (E) of the double bond.
Physical Properties
Alkenes behave a lot like alkanes in everyday physical terms. They are nonpolar, lighter than water, and insoluble in water. Their boiling points track closely with those of alkanes that have the same number of carbon atoms, typically running just a few degrees lower. Small alkenes like ethene and propene are gases at room temperature, while longer-chain alkenes are liquids.
How Alkenes React
The double bond is where the action happens. Its pi bond is electron-rich and relatively exposed, making it an attractive target for electron-poor species. The signature reaction of alkenes is addition: a molecule breaks apart, and each piece attaches to one of the double-bond carbons, converting the double bond into a single bond. After the reaction, the carbon atoms go from three connections each to four.
Strong acids like hydrochloric acid (HCl), hydrobromic acid (HBr), and sulfuric acid add across the double bond readily. In the reaction with HCl, for example, the process unfolds in two steps. First, the hydrogen attaches to one carbon, creating a positively charged intermediate on the other carbon. Then the chloride ion bonds to that positive site, completing the product. This first step is slower and determines which product forms.
Water doesn’t add to alkenes on its own because it’s too weak an acid, but in the presence of a strong acid catalyst like sulfuric acid, alkenes can be converted into alcohols through a process called hydration. This is one of the fundamental ways alcohols are manufactured industrially.
Markovnikov’s Rule
When an acid like HBr adds to an asymmetric alkene (one where the two carbons of the double bond carry different groups), there are two possible products. Markovnikov’s rule predicts which one dominates: the hydrogen atom attaches to the carbon that already has more hydrogens, and the other piece (like bromide) attaches to the more substituted carbon. This happens because the intermediate that forms at the more substituted carbon is more stable, and since the first step is both the slowest and the product-determining step, stability at that stage controls the outcome.
Ethylene: The Most Important Alkene
Ethylene (ethene, C2H4) is the simplest alkene and one of the most produced chemicals on Earth. Its double bond makes it highly reactive and extraordinarily versatile as a starting material. The largest use by far is in making polyethylene, the world’s most widely used plastic, found in everything from food packaging and water pipes to medical equipment and aerospace components.
Beyond polyethylene, ethylene is converted into key intermediates like vinyl chloride (for PVC), styrene (for polystyrene), and ethylene glycol (used in antifreeze and polyester fiber). Plastics, packaging, textiles, detergents, and medical supplies all trace back to this two-carbon molecule.
Ethylene as a Plant Hormone
Ethylene also plays a remarkable biological role. It functions as a gaseous hormone in plants, triggering fruit ripening and a range of other developmental responses. When a fruit begins to ripen, ethylene activates genes responsible for color changes, softening, sugar accumulation, and aroma production. It also triggers enzymatic activity in the abscission zone, the point where a fruit connects to the plant, leading to cell wall breakdown and eventual fruit drop.
This is why placing an unripe banana next to other fruit speeds up ripening: the banana releases ethylene gas, which signals neighboring fruit to ripen as well. Commercial fruit producers use this same principle, applying controlled ethylene exposure to ripen bananas, tomatoes, and other produce at precisely the right time for market.