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Alkenes and Alkynes

Introduces unsaturated hydrocarbons, their nomenclature and molecular geometry.

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Alkenes and Alkynes

Acetylene torches are used to weld metal

Credit: Courtesy of Mass Communication Specialist 3rd Class Benjamin Crossley, US Navy
Source: http://commons.wikimedia.org/wiki/File:US_Navy_110803-N-BT887-204_Hull_Technician_3rd_Class_Jeffrey_Meginness,_from_Modesto,_Calif.,_cuts_sheet_metal_with_an_acetylene_torch_aboard_the_N.jpg
License: CC BY-NC 3.0

What's better than a hacksaw?

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°C can be achieved. The amount of energy released is high – the net heat of combustion is 1300 kJ/mole. Safety precautions need to be observed since the gas is very explosive. For welding and cutting, the oxy-acetylene torch is one of the best ways to go.

Alkenes and Alkynes


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.

Ethene is the simplest alkene

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

The hybridization of each carbon atom is sp2 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 (C3H6) has three carbons total, while butene (C4H8) has four. The general formula for alkenes with one double bond is CnH2n. 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:

Nomenclature of monoalkenes

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

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.

Nomenclature of dienes

Credit: User:Abpong/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:Butadi%C3%A8ne.PNG
License: CC BY-NC 3.0


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).

Ethyne is the simplest alkyne

Credit: Ben Mills (Wikimedia: Benjah-bmm27)
Source: http://commons.wikimedia.org/wiki/File:Ethyne-2D-flat.png
License: CC BY-NC 3.0

The ethyne molecule is linear, with sp hybridization for each carbon atom. The general formula of alkynes with one triple bond is CnH2n-2. Alkynes are also unsaturated hydrocarbons. Other alkynes exist, such as 2-pentyne:

Example of another alkyne

Credit: Artur Disk (De.Wikipedia: Ljfa-ag)
Source: http://commons.wikimedia.org/wiki/File:2-Pentin.svg
License: CC BY-NC 3.0


  1. What is an alkene?
  2. What is an alkyne?
  3. Can a compound have more than one carbon-carbon double bond in it?

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Image Attributions

  1. [1]^ Credit: Courtesy of Mass Communication Specialist 3rd Class Benjamin Crossley, US Navy; Source: http://commons.wikimedia.org/wiki/File:US_Navy_110803-N-BT887-204_Hull_Technician_3rd_Class_Jeffrey_Meginness,_from_Modesto,_Calif.,_cuts_sheet_metal_with_an_acetylene_torch_aboard_the_N.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  4. [4]^ Credit: User:Abpong/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Butadi%C3%A8ne.PNG; License: CC BY-NC 3.0
  5. [5]^ Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: http://commons.wikimedia.org/wiki/File:Ethyne-2D-flat.png; License: CC BY-NC 3.0
  6. [6]^ Credit: Artur Disk (De.Wikipedia: Ljfa-ag); Source: http://commons.wikimedia.org/wiki/File:2-Pentin.svg; License: CC BY-NC 3.0

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