Alkenes are hydrocarbons that contain at least one carbon–carbon double bond. This structural feature makes them more chemically reactive than many related compounds, particularly alkanes, which contain only single bonds. Alkenes are important building blocks in the chemical industry, where they are used to manufacture plastics, synthetic fibers, detergents, solvents, and numerous other products.
Understanding alkenes begins with their molecular structure. The double bond affects how their atoms are arranged, how they react with other substances, and what kinds of products they can form. These properties make alkenes a fundamental topic in organic chemistry, the study of carbon-containing compounds.
What are alkenes?
Alkenes are a class of organic compounds made primarily of carbon and hydrogen. They belong to a larger group called hydrocarbons, which also includes alkanes and alkynes.
The defining feature of an alkene is a carbon–carbon double bond, written as C=C. In this bond, two carbon atoms share two pairs of electrons. By comparison, an alkane contains only single carbon–carbon bonds, while an alkyne contains at least one carbon–carbon triple bond.
The simplest alkene is ethene, also known as ethylene, with the molecular formula C₂H₄. Its two carbon atoms are connected by a double bond, and each carbon atom is bonded to two hydrogen atoms.
Other familiar alkenes include propene, with three carbon atoms, and butene, with four. As the number of carbon atoms increases, alkenes can develop more varied structures, including different double-bond positions and branching patterns.
For an acyclic alkene containing one carbon–carbon double bond and no other rings or multiple bonds, the general molecular formula is CₙH₂ₙ, where n is the number of carbon atoms. Ethene follows this pattern with C₂H₄, and propene follows it with C₃H₆.
This formula has an important limitation: it does not uniquely identify an alkene. Cycloalkanes, which contain carbon rings but no double bonds, can also have the formula CₙH₂ₙ. Molecular structure must therefore be considered alongside molecular formula when identifying a compound.
How the carbon–carbon double bond works
A carbon–carbon double bond consists of two different types of covalent bonds: one sigma bond and one pi bond. A covalent bond forms when atoms share electrons.
The sigma bond results from the direct overlap of atomic orbitals along the line connecting the two carbon nuclei. It provides the primary connection between the carbon atoms. The pi bond forms through the sideways overlap of parallel p orbitals, creating electron density above and below the plane of the atoms.
The pi bond is particularly important to the chemical behavior of alkenes. Its electron density is more exposed than that of the sigma bond, making it accessible to certain reacting substances. In many common alkene reactions, the pi bond is broken while the sigma bond between the carbon atoms remains intact.
This does not mean that the carbon–carbon double bond simply falls apart. Instead, the electrons involved in the pi bond participate in forming new bonds with other atoms or groups. The original double bond becomes a single bond as the reaction proceeds.
A double bond is stronger overall than a single bond, but it is not twice as strong. Its two components contribute differently to the bond’s strength and reactivity. The pi component is generally easier to disrupt than the sigma component, helping explain why alkenes undergo reactions that are less characteristic of alkanes.
The double bond also restricts rotation. A single carbon–carbon bond can usually rotate without breaking the bond, allowing many alkanes to adopt different three-dimensional arrangements. Rotating around a carbon–carbon double bond, however, would disrupt the pi bond. As a result, alkenes maintain a more fixed arrangement of atoms around the double bond.
Molecular shape and geometric isomerism
The carbon atoms in a typical carbon–carbon double bond are approximately trigonal planar, meaning that each is surrounded by three regions of electron density arranged at angles close to 120 degrees. This geometry results from the carbon atoms’ bonding arrangement and orbital structure.
The restricted rotation around the double bond can produce geometric isomers. These are compounds with the same molecular formula and the same connections between atoms but different spatial arrangements.
Consider 2-butene, which has the structure CH₃–CH=CH–CH₃. Its two methyl groups can lie on the same side or opposite sides of the double bond.
In cis-2-butene, the methyl groups are on the same side. In trans-2-butene, they are on opposite sides. These are distinct compounds with different physical properties, even though their molecular formulas and basic connectivity are identical.
The cis/trans naming system works for relatively simple cases. More complex alkenes are described using the E/Z system, which assigns priorities to the groups attached to each carbon of the double bond according to established rules. The letter Z indicates that the higher-priority groups are on the same side, while E indicates that they are on opposite sides.
Not every alkene exhibits geometric isomerism. Each carbon of the double bond must have two different substituents, meaning two different atoms or groups attached to it. If either carbon has two identical substituents, the double bond cannot produce the usual E/Z pair.
These structural differences can influence melting points, boiling points, molecular interactions, and chemical behavior. In biological and industrial systems, the spatial arrangement of a molecule can be especially important because other molecules often recognize specific shapes.
Naming alkenes
The systematic naming of alkenes follows rules designed to communicate their carbon skeleton and the position of the double bond.
The longest continuous carbon chain containing the double bond forms the basis of the name. The chain is numbered from the end that gives the double bond the lowest possible number. The suffix -ene indicates the presence of a carbon–carbon double bond.
For example, ethene contains two carbon atoms, propene contains three, and butene contains four. In a four-carbon chain, the double bond can occur between carbon atoms 1 and 2 or between carbon atoms 2 and 3. These arrangements produce 1-butene and 2-butene, respectively.
When the molecule contains more than one double bond, the name identifies their positions and uses endings such as -diene for two double bonds and -triene for three. A compound with five carbon atoms and double bonds at positions 1 and 3, for example, is named 1,3-pentadiene.
Branches and other functional groups must also be included in a complete systematic name. The goal is to identify the structure unambiguously rather than merely describe the number of carbon atoms.
Common names remain widely used in industry and everyday chemistry. Ethene is commonly called ethylene, and propene is commonly called propylene. Both names refer to the same molecules as their systematic counterparts.
Why alkenes are chemically reactive
Alkenes are particularly known for addition reactions, in which atoms or groups add across a carbon–carbon double bond. During these reactions, the pi bond is disrupted and new sigma bonds form.
The electron-rich pi bond can interact with substances that accept electron density. Many important alkene reactions begin when an electrophile, a species attracted to electron-rich regions, interacts with the double bond. This interaction can initiate a sequence of bond-forming steps that converts the alkene into a new compound.
Alkanes generally lack this accessible pi bond and are therefore less susceptible to many of the same reactions under ordinary conditions. They can still react, including through combustion and certain substitution reactions, but their characteristic chemistry differs from that of alkenes.
The reactivity of an alkene depends on more than the presence of a double bond. The groups attached to the double-bonded carbons influence electron distribution, steric crowding, and the stability of reaction intermediates. Temperature, solvents, catalysts, and the identities of the reacting substances can also affect which products form and how quickly the reaction proceeds.
These factors explain why two alkenes may undergo the same general type of reaction but differ in reaction rate, product distribution, or preferred pathway.
Addition reactions of alkenes
Addition reactions are among the most important reactions of alkenes. They transform a double bond into a single bond while attaching new atoms or groups to the two carbon atoms that previously formed the double bond.
The exact outcome depends on the reagent, reaction conditions, and structure of the alkene. Several types of addition reactions are especially important in organic chemistry.
Hydrogenation
Hydrogenation adds hydrogen across the carbon–carbon double bond. In a typical catalytic hydrogenation, hydrogen gas reacts with an alkene in the presence of a metal catalyst such as nickel, palladium, or platinum.
For example:
CH₂=CH₂ + H₂ → CH₃–CH₃
Ethene becomes ethane. The double bond is converted into a single bond, and each carbon atom gains a hydrogen atom.
Hydrogenation is used in chemical manufacturing and in some food-processing applications. In food chemistry, partial hydrogenation of unsaturated oils can produce trans fats, depending on the process. Modern food production practices and regulations have reduced the use of partially hydrogenated oils in many applications because industrial trans fats are associated with adverse cardiovascular effects.
Complete hydrogenation of an alkene, by contrast, adds hydrogen across its double bond without leaving that particular unsaturation behind.
Halogenation
Halogenation adds a halogen, such as bromine or chlorine, across an alkene double bond. In a common reaction with bromine, one bromine atom bonds to each of the two carbon atoms that originally formed the double bond.
For ethene:
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
The product is 1,2-dibromoethane.
This reaction is useful for demonstrating the difference between alkenes and alkanes. Bromine can react readily with many alkenes under suitable conditions, and its reddish-brown color may disappear as the bromine is consumed. The observation is a traditional qualitative test for unsaturation, although other compounds and reaction conditions can affect the result, so it is not universally definitive.
Halogenation commonly proceeds through an intermediate involving the halogen and the alkene rather than by a simple, one-step addition of two independent atoms. The details depend on the halogen and the reaction environment.
Hydrohalogenation
Hydrohalogenation adds a hydrogen halide, such as hydrogen chloride (HCl) or hydrogen bromide (HBr), across the double bond. One carbon gains hydrogen, while the other gains the halogen.
For propene reacting with HBr under ordinary conditions:
CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃
The major product is typically 2-bromopropane.
This outcome illustrates Markovnikov’s rule, a useful guideline for many additions of hydrogen halides to unsymmetrical alkenes. In a typical case, hydrogen attaches to the double-bonded carbon that already has more hydrogen atoms, while the halide attaches to the other carbon.
The rule reflects the relative stability of the intermediates formed during the reaction. In many such reactions, the pathway that produces the more stable carbocation intermediate is favored. A carbocation is a positively charged carbon-containing species.
Markovnikov’s rule is not universal for every alkene addition. For example, hydrogen bromide can add to certain alkenes by a radical-chain mechanism under peroxide conditions, leading to anti-Markovnikov orientation. The mechanism and conditions determine the outcome.
Hydration
Hydration adds water across an alkene double bond, introducing hydrogen and a hydroxyl group (–OH). Under acid-catalyzed conditions, the reaction can convert an alkene into an alcohol.
For ethene:
CH₂=CH₂ + H₂O → CH₃–CH₂OH
The product is ethanol.
In practice, hydration often requires appropriate catalysts, temperature, pressure, or other controlled conditions. Ethene hydration is an important industrial route to ethanol.
For unsymmetrical alkenes, the position of the hydroxyl group depends on the reaction pathway and conditions. Acid-catalyzed hydration often follows Markovnikov orientation because of the stability of the carbocation intermediate, while other hydration methods can provide different selectivity.
Oxidation reactions
Alkenes can undergo oxidation, a broad class of reactions in which their bonding to oxygen or other elements changes, or their electron distribution shifts toward a more oxidized state.
One familiar example is the formation of diols, compounds with two hydroxyl groups, from an alkene. Under suitable conditions, an oxidizing reagent can add hydroxyl groups to the two carbons that previously formed the double bond.
Ethene can be converted into ethane-1,2-diol, also called ethylene glycol, through appropriate oxidation chemistry. Ethylene glycol is used in antifreeze formulations and in the manufacture of polyester materials.
More vigorous oxidation can break the carbon–carbon double bond entirely. Depending on the alkene and the reaction conditions, the resulting products may include aldehydes, ketones, or carboxylic acids. In some cases, further oxidation produces carbon dioxide and water.
The products are determined by the structure of the starting alkene and the strength and conditions of the oxidation process. Consequently, the presence of a double bond alone is not enough to predict the final products without knowing how the reaction is carried out.
Polymerization: How alkenes become plastics
One of the most economically important reactions of alkenes is addition polymerization. In this process, many small molecules called monomers join to form a much larger molecule called a polymer.
Ethene is a simple example. Under suitable conditions, its double bond opens so that each ethene molecule can form new single bonds with neighboring molecules. Repeating this process produces polyethylene, a polymer widely used in plastic bags, packaging films, containers, pipes, and other products.
The simplified reaction is:
n CH₂=CH₂ → (–CH₂–CH₂–)ₙ
Here, n represents the number of repeating units in the polymer chain. The equation shows the overall structural transformation rather than every step of the reaction mechanism.
Propene undergoes a similar process to form polypropylene, which is used in packaging, fibers, household goods, and many manufactured components.
Polymerization requires suitable reaction conditions and, in many industrial processes, a catalyst or an initiator. Different catalysts can influence the arrangement of groups along the polymer chain, which in turn affects properties such as stiffness, flexibility, density, and melting behavior.
The usefulness of alkene polymers comes from the ability to build long chains with controllable structures. However, many conventional plastics persist in the environment, and their disposal and degradation can create environmental challenges. The chemical versatility that makes alkenes valuable industrial feedstocks also contributes to the wide range of materials that must be managed after use.
Physical properties of alkenes
The physical properties of alkenes depend on molecular size, shape, branching, and the arrangement of atoms around double bonds.
Small alkenes, such as ethene and propene, are gases at room temperature. As the number of carbon atoms generally increases, molecular size and intermolecular attractions increase, so boiling points tend to rise. Larger alkenes may be liquids or solids under ordinary conditions.
Alkenes are generally nonpolar or only weakly polar hydrocarbons. They do not mix readily with water, which is strongly polar, but many dissolve in nonpolar or relatively weakly polar organic solvents.
Their densities are generally lower than that of water, although exact values depend on the compound and temperature. Their boiling points and melting points also depend on molecular shape. Geometric isomers, for example, can have different physical properties because their structures influence how molecules interact and pack together.
Alkenes are often colorless, and many have characteristic odors. However, odor is not a reliable method for identifying them, and direct exposure to unknown chemicals should be avoided.
Flammability is another important property of many alkenes. Like other hydrocarbons, they can burn in oxygen, releasing energy. Complete combustion generally produces carbon dioxide and water, while incomplete combustion can generate carbon monoxide, soot, and other products.
How alkenes are produced
Alkenes can be obtained from petroleum and natural gas through industrial processes that convert larger hydrocarbons into smaller molecules.
One major method is cracking, which breaks larger hydrocarbon molecules into smaller ones through thermal or catalytic processes. Depending on the feedstock and operating conditions, cracking can produce ethene, propene, and other valuable chemical building blocks.
Another method is dehydrogenation, in which hydrogen is removed from an alkane to form an alkene. For example, propane can be converted into propene by removing hydrogen:
C₃H₈ → C₃H₆ + H₂
This transformation generally requires suitable catalysts and elevated temperatures. Industrial processes are designed to favor the desired product while limiting unwanted reactions.
Alkenes can also be prepared in laboratories through reactions such as elimination, in which atoms or groups are removed from neighboring carbon atoms to create a double bond. Dehydration of an alcohol, for example, removes the elements of water and can form an alkene under appropriate conditions.
These production methods illustrate a central principle of organic chemistry: molecular structures can be transformed in predictable ways by breaking selected bonds and forming new ones. The practical challenge is to obtain the desired product efficiently and selectively.
The role of alkenes in everyday life and industry
Alkenes are important not only because of their reactions but also because they serve as starting materials for many substances used in daily life.
Ethene is used to make polyethylene and is also a natural plant hormone involved in fruit ripening and other aspects of plant development. Controlled exposure to ethene can help coordinate the ripening of certain fruits during commercial handling.
Propene is a major feedstock for polypropylene and can be converted into chemicals used to manufacture products such as propylene oxide and acrylic-related materials. Other alkenes serve as starting materials for detergents, solvents, synthetic rubber, and specialized polymers.
Some unsaturated compounds also occur naturally in biological systems. Certain plant pigments, fragrances, and natural products contain carbon–carbon double bonds. Their structures influence how they absorb light, interact with other molecules, or participate in biological processes.
Not every compound containing a double bond behaves identically. The surrounding molecular structure matters, and compounds with additional functional groups can have properties that differ substantially from those of simple hydrocarbons. Nevertheless, the chemistry of alkenes provides a foundation for understanding a wide range of natural and manufactured substances.
How alkenes differ from alkanes and alkynes
Alkenes, alkanes, and alkynes are all hydrocarbon families, but their bonding patterns distinguish them.
Alkanes contain only carbon–carbon single bonds. For acyclic alkanes, the general molecular formula is CₙH₂ₙ₊₂. They are relatively unreactive toward many reagents under ordinary conditions, although they can undergo combustion, substitution, and other reactions under suitable conditions.
Alkenes contain at least one carbon–carbon double bond. Acyclic alkenes with one double bond have the general formula CₙH₂ₙ. Their pi bonds make them especially suitable for addition reactions and polymerization.
Alkynes contain at least one carbon–carbon triple bond. Acyclic alkynes with one triple bond have the general formula CₙH₂ₙ₋₂. Their triple bonds consist of one sigma bond and two pi bonds, and they exhibit their own characteristic reactions.
These formulas apply to the specified acyclic compounds without additional rings or multiple bonds. Rings and additional unsaturation change the relationship between the number of carbon and hydrogen atoms.
The distinction among the three families is more than a naming convention. Bond type influences molecular geometry, electron distribution, reactivity, and the range of possible products. Recognizing these differences makes it easier to predict how an organic molecule might behave in a chemical reaction.
Why understanding alkenes matters
Alkenes provide a clear example of how molecular structure determines chemical behavior. Their carbon–carbon double bonds restrict rotation, influence molecular shape, and create a reactive region that can participate in addition, oxidation, and polymerization reactions.
These transformations explain how relatively small hydrocarbon molecules can become alcohols, halogen-containing compounds, and long-chain plastics. They also help chemists design industrial processes, develop useful materials, and understand naturally occurring molecules.
The central idea is straightforward: an alkene’s double bond is both a structural feature and a source of chemical possibilities. By understanding how its electrons are arranged and how the bond changes during reactions, readers can connect the structure of a molecule to its properties, transformations, and practical uses.
