Alkene
Hydrocarbons with one or more carbon–carbon double bonds.
Alkenes, also known as olefins, are hydrocarbons that feature at least one carbon–carbon double bond. This double bond can be located internally within the carbon chain or at its terminal end; terminal alkenes are specifically referred to as α-olefins. According to IUPAC recommendations, the term "alkene" is best reserved for acyclic hydrocarbons containing a single double bond, while compounds with two or more such bonds are called alkadienes, alkatrienes, or more generally polyenes. Cyclic variants are termed cycloalkenes or cycloalkadienes, and "olefin" serves as the broad class name for both cyclic and acyclic hydrocarbons with any number of double bonds. Acyclic alkenes with one double bond and no other functional groups form a homologous series with the general formula CₙH₂ₙ, where n is a natural number greater than one. When n reaches four or more, structural isomers become possible, differing by the position and conformation of the double bond. Alkenes are generally colorless, non-polar compounds, similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature. The simplest alkene, ethylene (ethene), is the organic compound produced on the largest scale industrially. Aromatic compounds are often drawn as cyclic alkenes, but their distinct structure and properties exclude them from the alkene or olefin classification. Hydrocarbons with two overlapping double bonds are called allenes, and those with three or more overlapping bonds are known as cumulenes. The carbon–carbon double bond itself consists of a sigma bond and a weaker pi bond, and is both shorter and stronger than a single bond. Rotation around this bond is restricted, leading to stable cis and trans isomers, or more generally E–Z isomers, which have distinct physical properties. The molecular geometry around each double-bonded carbon is approximately 120°, though steric strain can alter this angle. For bridged alkenes, Bredt's rule prohibits a double bond at the bridgehead of small bridged ring systems.
- field
- Organic chemistry
- known_for
- Containing carbon–carbon double bonds; forming a homologous series with general formula CnH2n; exhibiting cis–trans and E–Z isomerism
Lore & Background
Alkenes are a class of hydrocarbons defined by the presence of at least one carbon–carbon double bond. The double bond consists of a sigma bond and a pi bond, with the pi bond being weaker and restricting rotation, leading to cis–trans isomerism. According to IUPAC, the term 'alkene' is recommended only for acyclic hydrocarbons with a single double bond; compounds with two or more double bonds are called alkadienes, alkatrienes, or polyenes, while 'olefin' is used for the general class including cyclic and acyclic forms. Aromatic compounds, though often drawn as cyclic alkenes, are not classified as alkenes due to distinct structure and properties. Allenes and cumulenes, with overlapping double bonds, are separate categories.
Reader's Guide
Alkenes are fundamental in organic chemistry due to their reactivity and structural versatility. The restricted rotation around the double bond enables stable cis and trans isomers, which have distinct properties; for more complex cases, the E–Z notation based on Cahn–Ingold–Prelog priority rules is used. Alkenes with four or more carbons exhibit structural isomerism, and many also show cis–trans isomerism. IUPAC nomenclature for straight-chain alkenes replaces the '-ane' suffix of the corresponding alkane with '-ene', and the position of the double bond is indicated by a number. The first few members of the alkene series are gases or liquids at room temperature. Their industrial importance is highlighted by ethylene being the largest-scale organic compound produced.
Did You Know?
- Rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the p orbitals.
- The simplest alkene, ethylene (C2H4), is the organic compound produced on the largest scale industrially.
- Aromatic compounds are not classified as alkenes or olefins despite often being drawn as cyclic alkenes.
Frequently Asked Questions
Who is Alkene?
Alkene, often called olefin, is a hydrocarbon defined by the presence of at least one carbon–carbon double bond. In the Chemistry Fundamentals series, they occupy entry 1-24 and belong squarely to the organic chemistry field.
What are Alkene's powers or signature traits?
Their C=C double bond is the core of their identity, making them far more chemically reactive than their single-bonded counterpart Alkane. They also come in multiple geometric forms through cis–trans and E–Z isomerism, giving them a range of structural variants.
Why is Alkene important to the series?
Alkene's reactivity makes them a central building block for countless downstream reactions in organic synthesis. Their simplest member, ethylene, is the single organic compound manufactured at the greatest industrial volume, cementing Alkene's status as a foundational entry.
How does Alkene's story arc resolve?
Alkene's defining double bond is both their signature and their vulnerability: addition reactions across that bond convert them into more saturated, less reactive products. In narrative terms, Alkene's arc is one of transformation driven by their own most distinctive feature.
What is Alkene's relationship to Alkane?
They are sibling hydrocarbons, but Alkene carries one or more double bonds while Alkane is fully saturated with only single bonds. This structural difference is what pushes Alkene into a more reactive, colorless, and non-polar profile compared to Alkane's inert stability.
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