Alkenes are hydrocarbons with a carbon double bond and a chemical formula of CnH2n. They differ from alkanes in two electrons and exhibit structural and geometric isomerism. Alkenes are used in various industries to create margarine, alcohols, plastics, and Teflon®.
An alkene is a type of hydrocarbon that contains a carbon double bond. Hydrocarbons are molecules made up exclusively 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 only in two electrons. Most alkenes are liquids at room temperature, although 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. The 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 any given isomer will vary. What makes a structural isomer distinct is the 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 have a double bond 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 as 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 bond carbon is attached to a double bond carbon.
Visually, but-1-ene looks like a bent line. Whether the molecule appears to bend up or down or whether the bending occurs to the left or right of the carbon double bond, the molecule is still ma-1-ene. In contrast, but-2-ene looks more or less U-shaped.
Since double bonds do not allow for free rotation, it is also possible for an alkene to have geometric isomers. Since but-2-ene has a single bonded carbon atom on each double bonded carbon, a geometric isomer is possible. The “cis” isomer occurs when single bonded carbons are on the same side of the molecule, while the “trans” isomer occurs when 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 molecules together to make polymers produces plastics and Teflon®.
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