Alkene

Ethylene-3D-vdW

Ethylene 3D

Ear

Biadamantylidene-bromonium-ion-from-xtal-1994-2D-skeletal

HBr-addition

Alkenes are a class of hydrocarbons that contain at least one carbon-to-carbon double bond. They are a major subset of the unsaturated hydrocarbons, which means they have fewer hydrogen atoms attached to the carbon chain than alkanes, the saturated hydrocarbons. Alkenes are characterized by the general formula C_nH_2n, indicating that they have two fewer hydrogen atoms than an alkane with the same number of carbon atoms.

Properties

Alkenes are known for their reactivity, particularly in chemical reactions such as hydrogenation, halogenation, and polymerization. The double bond is a region of high electron density, making it susceptible to attack by electrophiles. This reactivity is exploited in various industrial and synthetic processes. Alkenes are generally more reactive than alkanes due to the presence of this double bond.

Physical Properties

The physical properties of alkenes vary with the size of the molecule. Smaller alkenes are gases at room temperature, while medium and larger alkenes are liquids or solids. Alkenes are less dense than water and are insoluble in water due to their nonpolar nature. However, they are soluble in organic solvents.

Chemical Properties

Alkenes undergo addition reactions at the double bond. This includes:

• Hydrogenation – the addition of hydrogen, which converts alkenes into alkanes.
• Halogenation – the addition of halogens, forming dihalogenated compounds.
• Hydrohalogenation – the addition of hydrogen halides, producing alkyl halides.
• Hydration – the addition of water, in the presence of an acid catalyst, to form alcohols.

Nomenclature

The IUPAC system for naming alkenes uses the suffix "-ene" to indicate the presence of a double bond. The position of the double bond is specified by a number placed before the name. For example, ethene (C₂H₄) is the simplest alkene, and propene (C₃H₆) has a double bond between the first and second carbon atoms.

Isomerism

Alkenes exhibit both structural isomerism and geometric isomerism. Structural isomers have the same molecular formula but different structural formulas. Geometric isomerism, also known as cis-trans isomerism, occurs due to the restricted rotation around the double bond. Cis isomers have substituents on the same side of the double bond, while trans isomers have them on opposite sides.

Synthesis

Alkenes can be synthesized through various methods, including:

• Dehydration of alcohols – where water is removed from an alcohol, forming an alkene.
• Dehydrohalogenation of alkyl halides – where a hydrogen halide is removed from an alkyl halide.
• Cracking of alkanes – a process that breaks down larger alkane molecules into smaller alkenes and alkanes.

Applications

Alkenes are used in the manufacture of a wide range of chemicals and materials. They serve as starting materials for the synthesis of alcohols, plastics, detergents, and rubbers. Ethene, for example, is a key raw material in the production of polyethylene, a common plastic.

Environmental and Health Aspects

While alkenes are crucial in various industrial applications, they can pose environmental and health risks. They are volatile organic compounds (VOCs) and can contribute to air pollution and the formation of ground-level ozone. Exposure to high levels of certain alkenes can be harmful to human health, causing respiratory issues and other adverse effects.