Alkenes are hydrocarbons that have a C=C, otherwise known as a carbon-carbon double bond. They are an unsaturated hydrocarbon, because hydrogens do not fill all the available spots on the carbon chain due to the double bond. They are also called olefins, because when they react with chlorine gas (Cl₂), they form oily liquids.¹

Physical Properties

Generally speaking, the physical properties of alkenes have similar trends to that of alkanes. The main differences between the two are acidity levels and physical states. Alkenes tend to be more acidic than their alkane counterparts. The physical state of an alkene depends on the molecular mass. Ethene, Propene, and Butene are gases, while alkenes with five to sixteen carbons are liquids, and higher alkenes are waxy solids. This is different than alkanes because of the difference in molecular mass with the same number of carbons in a chain.²

Cis/Trans

Due to the double bond present in an alkene, there are two different stereoisomers: cis and trans. To distinguish which isomer an alkene is, first the molecule needs to be assigned priority. Priority can be simply expressed as the heaviest group attached to the carbon. On the carbon-carbon double bond of an alkene, if the highest priority group on each carbon is diagonal to the one on the other carbon, it is trans. If they are on the same side, it is a cis-alkene. More information (with pictures) can be found here:

http://www.chemguide.co.uk/basicorg/isomerism/geometric.html

 

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Real World Application

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One real world example of an alkene is propene. It is a by-product of oil refining and natural gas processing. Propene is produced from fossil fuels, and is a byproduct of oil refining. Propene is also very commonly used to create other products. Propene is a key ingredient (~60%) of plastic polypropylene. Propylene is a major industrial chemical intermediate that serves as one of the building blocks for an array of chemical and plastic products.⁴

Polymerization:

Alkenes are used a lot in polymerization. Polymerization is when monomers (individual molecules) react and bind together to create a larger molecule, a polymer. Thylene can be polymerized to make polyethylene. This type of reaction with alkenes is used quite often in medicine.⁵

Ethylene is widely used in chemical industry, and its worldwide production (over 109 million tonnes in 2006) exceeds that of any other organic compound.⁶ Ethylene is also an important natural plant hormone, used in agriculture to force the ripening of fruits.⁷

 

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