Hydrocarbons are the most fundamental class of compounds, composed solely of carbon and hydrogen atoms. Molecules are categorized by the types of bonds linking their carbon atoms, which dictates their structure and properties. Alkenes are a foundational family of hydrocarbons, distinguished by a specific structural feature that makes them highly reactive and industrially significant. Understanding alkenes requires examining their unique structure, systematic naming, and chemical behavior.
Defining the Alkene Structure
Alkenes are unsaturated hydrocarbons, meaning they contain fewer hydrogen atoms than the maximum possible. The defining feature is the presence of at least one carbon-carbon double bond (C=C), which acts as the functional group dictating the molecule’s chemical identity. The general formula for non-cyclic alkenes with a single double bond is \(C_nH_{2n}\), where \(n\) is the number of carbon atoms. For example, the simplest alkene, ethene, has two carbons (\(n=2\)), resulting in the formula \(C_2H_4\).
The double bond consists of two shared electron pairs: one strong sigma (\(\sigma\)) bond and one weaker pi (\(\pi\)) bond. This forces a specific, flat geometry around the two carbon atoms involved. Each carbon lies at the center of a trigonal planar arrangement, with bond angles of approximately 120 degrees.
This planar geometry is rigid because the \(\pi\) bond prevents free rotation around the carbon-carbon axis. This lack of rotation leads to a special type of isomerism, where molecules with the same formula have different arrangements of atoms in space. The exposed electrons in the \(\pi\) bond also contribute directly to the molecule’s high chemical reactivity.
How Alkene Names Are Determined
The naming of alkenes follows systematic rules established by the International Union of Pure and Applied Chemistry (IUPAC). The primary difference from naming saturated hydrocarbons is the change in the suffix. Alkanes end in “-ane,” but alkenes replace this suffix with “-ene” to denote the double bond.
The root name is determined by the longest continuous carbon chain that must include both carbon atoms of the double bond. For alkenes with four or more carbon atoms, the position of the double bond must be indicated by a number. Numbering begins at the end that gives the double bond the lowest possible number, which is placed before the “-ene” suffix.
The simplest examples are ethene (two carbons) and propene (three carbons). When a longer chain is present, such as a four-carbon chain with the double bond starting at the first carbon, the compound is named 1-butene. This systematic approach allows chemists worldwide to precisely communicate the structure of any alkene.
Why the Double Bond Matters: Chemical Reactivity and Uses
The carbon-carbon double bond is the center of an alkene’s chemical reactivity. The weak \(\pi\) bond is easily broken, making alkenes significantly more reactive than alkanes. Alkenes characteristically undergo addition reactions, where the double bond is converted into two new single bonds.
In these reactions, the \(\pi\) bond breaks, and new atoms or groups add across the double bond, turning the unsaturated alkene into a saturated molecule. Examples include the addition of hydrogen (hydrogenation) to form an alkane, or water (hydration) to form an alcohol. This high reactivity allows alkenes to serve as versatile starting materials for synthesizing organic compounds.
The most important industrial application is polymerization, a process where thousands of small alkene molecules (monomers) link together to form long polymer chains. Ethylene, the simplest alkene, is the primary monomer used in the production of polyethylene. During polymerization, the \(\pi\) bond breaks, allowing the carbon atoms to join together and form the long, repeating chain.
Polyethylene is the most widely produced plastic globally. The ability of simple alkenes to be transformed into durable, complex materials demonstrates their fundamental role in modern manufacturing and technology.