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What are Alkenes?

By Angie Bates
Updated May 21, 2024
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An alkene is a type of hydrocarbon that contains a double carbon bond. Hydrocarbons are molecules which consist solely of hydrogen and carbon atoms. In alkenes, for every carbon atom there are two hydrogen atoms, which results in a chemical formula of CnH2n. For example, ethene and propene, the simplest alkenes, have chemical formulas of C2H4 and C3H6 respectively.

Alkenes are very similar to alkanes, differing by only two electrons. Most alkenes are liquids at room temperature, though ethene, propene and butene are gases. All alkenes are insoluble in water and soluble in organic solvents.

In an alkene, only one pair of carbon atoms will have a double bond. Atoms of any molecule are bonded together by means of a shared electron. Double bonds occur when two electrons are shared between atoms.

With the exception of ethene and propene, alkenes exhibit structural isomerism. Structural isomers have the same chemical formula, but different structures. Single-bonded atoms exhibit free rotation, so the geometric shape of a given isomer will vary. What makes a structural isomer distinct is pattern of bonds not the shape of the molecule.

For example, but-1-ene and but-2-ene both have the chemical formula C4H8. Two of these carbon molecules will be double-bonded in both but-1 and 2, but the two carbon atoms that have single bonds are arranged differently. In but-1, the carbons are arranged like a chain, the first carbon is attached to the second with a double bond, which is attached to the third with a single bond, which is attached to the fourth with a single bond. In but-2, each single-bonded carbon is attached to a double-bonded carbon.

Visually, but-1-ene looks like a bent line. Whether the molecule seems to bend upward or downward or the bend occurs to the left or the right of the double carbon bond, the molecule is still but-1-ene. Conversely, but-2-ene looks roughly U-shaped.

Since double bonds do not allow free rotation, it is also possible for an alkene to have geometric isomers. Since but-2-ene has one single bonded carbon atom on each double bonded carbon, a geometric isomer is possible. The "cis" isomer occurs when the single-bonded carbons are on the same side of the molecule, whereas the "trans" isomer occurs when the single-bonded carbons are on opposite sides of the molecule.

Alkenes react differently with many substances. For this reason, they are used in many industries. Reacting some alkenes with hydrogens can create margarine, adding water to others creates certain alcohols, such as ethanol, and linking the molecules together to make polymers produces plastics and Teflon®.

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