Alkene

Alkenes are a class of organic compounds known as hydrocarbons. A hydrocarbon is a molecule made entirely of carbon and hydrogen atoms.

What distinguishes an alkene from other hydrocarbons is the presence of at least one carbon-carbon double bond. This double bond makes alkenes more reactive than alkanes, which only have single bonds. The International Union of Pure and Applied Chemistry (IUPAC) uses specific terms to categorize these molecules. An alkene is technically an acyclic hydrocarbon with exactly one double bond. When a molecule has two or more double bonds, it is called a polyene. If the molecule forms a ring, it is known as a cycloalkene. The term "olefin" is used as a more general name for the entire class, including both cyclic and acyclic structures.

To understand the chemistry of alkenes, one must look at the specific structure of the double bond. This bond is composed of two distinct types of connections: a sigma bond and a pi bond.

The sigma bond is the primary connection between the two carbon atoms. The pi bond is formed by the overlap of unhybridized 2p atomic orbitals. These orbitals sit perpendicular to the plane of the molecule, meaning the pi bond exists on both sides of the main axis. While the double bond is stronger than a single bond, it is not twice as strong. A carbon-carbon double bond has a strength of 611 kJ/mol, whereas a single bond is 347 kJ/mol. Additionally, the double bond is shorter, measuring about 1.33 Å compared to 1.53 Å for a single bond.

The nature of the pi bond has a major effect on the shape and movement of the molecule. Because the pi bond requires the p orbitals to remain aligned, rotation around the carbon-carbon double bond is restricted. If the atoms were to rotate, it would require enough energy to break the pi bond. This restriction leads to the existence of isomers, which are molecules with the same atoms but different arrangements. In simple alkenes, this results in cis and trans configurations.

In a cis configuration, the functional groups are on the same side of the double bond. In a trans configuration, they are on opposite sides. For more complex molecules with four different groups, scientists use E-Z notation. This system uses the German words "zusammen" (together) and "entgegen" (opposite) to describe whether high-priority groups are on the same or opposite sides.

Alkenes also exhibit structural isomerism, especially as the number of carbon atoms increases. A structural isomer is a molecule that has the same chemical formula but a different connectivity of atoms. For example, ethylene has only one possible structure. Propylene also has only one structure. However, once a molecule reaches four carbon atoms, like butene, multiple isomers become possible. Butene has three different structural isomers: 1-butene, 2-butene, and isobutylene. As the chain grows to five carbons, the number of possible isomers jumps to five. By the time a molecule has six carbons, there are 13 different possible isomers. This complexity grows rapidly as more carbons are added to the chain.

Naming these molecules correctly is essential for scientific communication. The IUPAC provides a standardized system for nomenclature. To name a straight-chain alkene, the suffix of the parent alkane is changed from "-ane" to "-ene." For instance, the alkane ethane becomes the alkene ethene.

For larger or branched molecules, the process is more detailed. A scientist must first identify the longest continuous carbon chain that contains the double bond. The carbons in this chain are then numbered starting from the end closest to the double bond. This number indicates the position of the double bond. Any side groups, or substituents, are also named and assigned a number based on their position on the chain.

The physical properties of alkenes are largely determined by their molecular mass. Most alkenes are colorless, nonpolar, and combustible, meaning they can burn in the presence of oxygen. The simplest alkenes, such as ethylene, propylene, and butene, are gases at room temperature.

As the number of carbon atoms increases, the state of the matter changes. Linear alkenes with between five and nineteen carbon atoms are typically liquids. The largest alkenes are waxy solids. The melting point of these solids also increases as the molecular mass grows. Alkenes generally possess stronger odors than alkanes. While ethylene has a sweet and musty smell, some strained alkenes can produce very unpleasant odors.

Alkenes play a massive role in modern industry and chemistry. Ethylene is the organic compound produced on the largest scale industrially. Because of their reactivity, alkenes are used as building blocks for many other substances. They can undergo combustion to produce carbon dioxide and water, releasing energy in the process. They are also studied through various scientific methods, such as infrared spectroscopy, which can identify the specific stretching of the carbon-carbon double bond. Understanding the precise geometry and reactivity of these molecules allows scientists to manipulate them for everything from plastics to complex chemical syntheses.