4.8: Alkenes and Alkynes

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One of the most effective ways to cut metal is with an oxy-acetylene torch. Very high temperatures are obtained when acetylene burns in oxygen. Mixed 1:1 with oxygen, a temperature of over \(3000^\text{o} \text{C}\) can be achieved. The amount of energy released is high—the net heat of combustion is \(1300 \: \text{kJ/mol}\). Safety precautions need to be observed, as the gas is very explosive. The oxy-acetylene torch is one of the top tools for welding and cutting.

Alkenes and Alkynes

Alkenes

An alkene is a hydrocarbon with one or more carbon-carbon double covalent bonds. The simplest alkene is composed of two carbon atoms and is called ethene (shown below). Each carbon is bonded to two hydrogen atoms, in addition to the double bond between them.

The hybridization of each carbon atom is \(sp^2\) with trigonal planar geometry. All the atoms of the molecule lay in one plane. Like the alkane series, the names of alkenes are based on the number of atoms in the parent chain. Naming follows the same rules as for alkanes, with the addition of using a number to indicate the location of the double bond. Propene \(\left( \ce{C_3H_6} \right)\) has three carbons total, while butene \(\left( \ce{C_4H_8} \right)\) has four. The general formula for alkenes with one double bond is \(\ce{C_{n}H_{2n}}\). Alkenes are called unsaturated hydrocarbons. An unsaturated hydrocarbon is a hydrocarbon that contains less than the maximum number of hydrogen atoms that can possibly bond with the number of carbon atoms present.

The location of the carbon-carbon double bond can vary. The 4-carbon alkene generic name is butene. Since the double bond can be located in more than one place, we have 1-butene and 2-butene:

Molecules with multiple double bonds are also quite common. The formula below shows a four-carbon chain with double bonds between carbons 1 and 2, and between carbons 3 and 4. This molecule is called 1,3-butadiene.

Alkynes

An alkyne is a hydrocarbon with one or more carbon-carbon triple covalent bonds. The simplest alkyne consists of two carbon atoms and is called ethyne (common name: acetylene).

The ethyne molecule is linear, with \(sp\) hybridization for each carbon atom. The general formula of alkynes with one triple bond is \(\ce{C_{n}H_{2n-2}}\). Alkynes are also unsaturated hydrocarbons. Other alkynes exist, such as 2-pentyne:

Summary

An alkene is a hydrocarbon with one or more carbon-carbon double covalent bonds.
An alkyne is a hydrocarbon with one or more carbon-carbon triple covalent bonds.
An unsaturated hydrocarbon is a hydrocarbon that contains less than the maximum number of hydrogen atoms that can possibly bond with the number of carbon atoms present.
Structures of alkene and alkyne are given.