Alkynes

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. 1-alkynes are fairly acidic and have pKa values (25) between that of ammonia (35) or ethanol with 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.

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.

See also:

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.

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.
Enlarge picture
Alkyne hydrolysis

References

1. ^ Fukuda, Y.; Utimoto, K. "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst". J. Org. Chem. 1991, 56, 3729–3731. doi:10.1021/jo00011a058
2. ^ Mizushima, E.; Cui, D.-M.; Nath, D. C. D.; Hayashi, T.; Tanaka, M. "Au(I)-Catalyzed hydratation of alkynes: 2,8-nonanedione". Organic Syntheses, Vol. 83, p.55 (2005). Link.

Further Reading

 
hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon or hydrogen are referred to as "Pure" hydrocarbons, whereas
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Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds.
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4, 2
(mildly acidic oxide)
Electronegativity 2.55 (Pauling scale)
Ionization energies
(more) 1st: 1086.5 kJmol−1
2nd: 2352.6 kJmol−1
3rd: 4620.5 kJmol−1

Atomic radius 70 pm
Atomic radius (calc.
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Acetylene (systematic name: ethyne) is a hydrocarbon belonging to the group of alkynes. It is considered to be the simplest of all alkynes as it consists of two hydrogen atoms and two carbon atoms.
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The International Union of Pure and Applied Chemistry (IUPAC) (IPA: [aɪ ju pæk]) is an international non-governmental organization established in 1919 devoted to the advancement of chemistry.
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Alkanes, also known as Paraffins, are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) (i.e. hydrocarbons), where each of these atoms are linked together exclusively by single bonds (i.e. they are saturated compounds) without any cyclic structure (i.
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alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond. The simplest alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula
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An acid dissociation constant, denoted by Ka, is an equilibrium constant for the dissociation of a weak acid. According to the Brønsted-Lowry theory of acids and bases an acid is only recognised by its reaction with a base.
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Ammonia is a compound with the formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of the planet as a precursor to foodstuffs and fertilizers.
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Ethanol, also known as ethyl alcohol, drinking alcohol or grain alcohol, is a flammable, colorless, slightly toxic chemical compound, and is best known as the alcohol found in alcoholic beverages.
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Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
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Sodium (IPA: /ˈsəʊdiəm/) is a chemical element which has the symbol Na (Latin: natrium), atomic number 11, atomic mass 22.9898 g/mol, common oxidation number +1.
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Sodium amide, commonly called sodamide, is the chemical compound with the formula NaNH2. This solid, which is dangerously reactive toward water, is white when pure, but commercial samples are typically gray due to the presence of small quantities of metallic iron
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The chemical compound n-butyllithium (abbreviated BuLi) is the most prominent organolithium reagent. It enjoys wide use as a polymerisation initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene (SBS).
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The Grignard reaction, named for the French chemist François Auguste Victor Grignard, is an organometallic chemical reaction involving alkyl- or aryl-magnesium halides, also called Grignard reagents, with electrophiles.
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In chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a reagent that forms a chemical bond to its reaction partner (the electrophile) by donating both bonding electrons.
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Silver acetylide is an inorganic chemical compound with the formula Ag2C2, a metal acetylide. The name derived from the way it is synthesized, the alternative silver carbide is not used in literature, although the substance is similar to calcium carbide .
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hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties.
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electron configuration is the arrangement of electrons in an atom, molecule, or other physical structure (e.g., a crystal). Like other elementary particles, the electron is subject to the laws of quantum mechanics, and exhibits both particle-like and wave-like nature.
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hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties.
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sigma bonds (σ bonds) are a type of covalent chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis.
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pi bonds (π bonds) are covalent chemical bonds where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital. Only one of the orbital's nodal planes passes through both of the involved nuclei.
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Acetylene (systematic name: ethyne) is a hydrocarbon belonging to the group of alkynes. It is considered to be the simplest of all alkynes as it consists of two hydrogen atoms and two carbon atoms.
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Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule, inferred from the spectroscopic studies of the compound.
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In chemistry, bond strength is measured between two atoms joined in a chemical bond. It is the degree to which each atom linked to a central atom contributes to the valency of this central atom. Bond strength is intimately linked to bond order.
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A picometre (American spelling: picometer, symbol pm) is a unit of length in the metric system, equal to one millionth-millionth of a metre, which is the current SI base unit of length.
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alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond. The simplest alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula
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Acetylene (systematic name: ethyne) is a hydrocarbon belonging to the group of alkynes. It is considered to be the simplest of all alkynes as it consists of two hydrogen atoms and two carbon atoms.
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Acetylene (systematic name: ethyne) is a hydrocarbon belonging to the group of alkynes. It is considered to be the simplest of all alkynes as it consists of two hydrogen atoms and two carbon atoms.
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Methylacetylene (propyne) is an alkyne with the chemical formula CH3C≡CH. It is a component of MAPP gas along with its isomer 1,2-propadiene (allene), which is commonly used in gas welding.
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