Alkyne

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Image:Alkyne.png Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms. The alkynes are traditionally known as acetylenes, although the name acetylene is also used to refer specifically to the simplest member of the series, known officially as ethyne.

1 Structure
2 Chemical properties
3 Examples
4 Terminal and internal alkynes
5 Metal acetylides
6 Synthesis
7 Reactions

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Structure

The carbon atoms in an alkyne bond are sp hybridized: they each have 2 p orbitals and 2 sp hybrid orbitals. Overlap of an sp orbital from each atom forms one sp-sp sigma bond. Each p orbital on one atom overlaps one on the other atom, forming two pi bonds, giving a total of three bonds. The remaining sp orbital on each atom can form a sigma bond to another atom, for example to hydrogen atoms in the parent compound acetylene. The two sp orbitals on an atom are on opposite sides of the atom: in acetylene, the H-C-C bond angles are 180°. Because a total of 6 electrons take part in bonding this triple bond is very strong with a bond strength of 837 kJ/mol. The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol and the second pi bond is weak with 202 kJ/mol bond strength. The CC bond distance with 121 picometers is also much less than that of the alkene bond which is 134 pm or the alkane bond with 153 pm.

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Chemical properties

Unlike alkanes, alkynes are unstable and very reactive. This gives rise to the intense heat of the acetylene flame used in welding.

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Examples

The simplest alkyne is ethyne (acetylene):

Image:Ethyne.png

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Terminal and internal alkynes

Terminal alkynes have a hydrogen atom bonded to at least one of the sp hybridized carbons (those involved in the triple bond. An example would be methylacetylene (1-propyne under IUPAC nomenclature).

Internal alkynes have something other than hydrogen attached to the sp hybridized carbons, usually another carbon atom, but could be a heteroatom. A good example is 2-pentyne, in which there is a methyl group on one side of the triple bond and an ethyl group on the other side.

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Metal acetylides

A terminal alkyne with a strong base such as sodium, sodium amide, n-butyllithium or a grignard reagent gives the anion of the terminal alkyne and a metal acetylide. Acetylenes are fairly acidic and have pK_a values (25) between that of ammonia (35) or ethanol with 16. The explanation for this acidity is that the negative charge in acetylide is stabilized as a result of the high s character of the sp orbital in which the electron pair resides. Electrons in a s orbital benefit from closer proximity to the positively charged atom nucleus and therefore lower in energy.