Definitions

alkyne

alkyne

[al-kahyn]
alkyne, any of a group of aliphatic hydrocarbons whose molecules contain one or more carbon-carbon triple bonds (see chemical bond). Alkynes with one triple bond have the general formula CnH2n-2. In the International Union of Pure and Applied Chemistry (IUPAC) system of chemical nomenclature, the name of an alkyne is derived from the name of the corresponding alkane by replacing the -ane alkane suffix with -yne and, if necessary, adding a prefix to indicate the location of the triple bond in the molecule. The IUPAC name of the simplest alkyne, HC---CH, is thus ethyne, which is derived from ethane. Ethyne is more commonly known as acetylene; it is an extremely important starting material in commercial chemical synthesis. The next simplest alkyne is propyne, CH3C---CH. There are two butynes, 1-butyne and 2-butyne, which are structural isomers that differ in the location of the triple bond in their molecule. The alkynes are sometimes referred to as the acetylene series, the higher members of the series being named as derivatives of acetylene, e.g., propyne as methylacetylene, 1-butyne as ethylacetylene, and 2-butyne as dimethylacetylene. The usefulness of the alkynes in chemical synthesis is due both to the reactions of the triple bond itself and to the relative acidity of a hydrogen atom bonded to a triply bonded carbon.
Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms, with the formula CnH2n-2. The alkynes are traditionally known as acetylenes or the acetylene series, although the name acetylene is also used to refer specifically to the simplest member of the series, known as ethyne (C2H2) using formal IUPAC nomenclature.

Chemical properties

Unlike alkanes, and to a lesser extent, alkenes, alkynes are unstable and reactive. Terminal alkynes and acetylene are fairly acidic and have pKa values (25) between that of ammonia (35) and ethanol (16). This acidity is due to the ability for the negative charge in the acetylide conjugate base to be stabilized as a result of the high s character of the sp orbital, in which the electron pair resides. Electrons in an s orbital benefit from closer proximity to the positively charged atom nucleus, and are therefore lower in energy. This can also be thought of in terms of electronegativity: electrons in an hybrid orbital with high s character reside closer to the nucleus. The closer proximity of the electrons to the nucleus allows an acetylinic carbon to have a greater amount of electronegative character. As a result, a proton is more easily removed from the carbon as electrons flow more willingly to a more electronegative atom.

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 (a metal acetylide):

2 RC≡CH + 2 Na → 2 RC≡CNa + H2

More generally:

RC≡CH + B → RC≡C + HB+, where B denotes a strong base.

The acetylide anion is synthetically useful because as a strong nucleophile, it can participate in C−C bond forming reactions.

It is also possible to form copper and silver alkynes, from this group of compounds silver acetylide is an often used example.

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 839 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.

The simplest alkyne is ethyne (acetylene): H-C≡C-H

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 using 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.

The terminal Hydrogen atom is weakly acidic, and can be removed by a very strong base, to yield a salt. This property can be used as a chemical test to distinguish terminal alkynes from others, or the salt may be used to make larger alkyne molecules. A few drops of diamminesilver(I) hydroxide (Ag(NH3)2+ -OH or Ag(NH3)2OH)) solution are added to samples of a non-terminal alkyne and also a terminal alkyne. No reaction occurs for the non-terminal, but the terminal alkyne forms a characteristic white precipitate. This is the insoluble silver salt of the terminal alkyne: R-C≡CH + Ag(NH3)2+ -OH → R-C≡C- Ag+ + NH4+ + NH3 (R = general alkyl group) Warning: transition metal salts of terminal alkynes (metal; acetylides) can be explosive when dry.

Synthesis

Alkynes are generally prepared by dehydrohalogenation of vicinal alkyl dihalides or the reaction of metal acetylides with primary alkyl halides. In the Fritsch-Buttenberg-Wiechell rearrangement an alkyne is prepared starting from a vinyl bromide.

Alkynes can be prepared from aldehydes using the Corey-Fuchs reaction and from aldehydes or ketones by the Seyferth-Gilbert homologation.

Reactions

Alkynes are involved in many organic reactions.

References

See also

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