Alkynes

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of alkynes are discussed. Alkynes are an infinite series of compounds containing only carbon and hydrogen atoms and at least one carbon-carbon triple bond. Their variety is due to the unique electronic distribution and physical size of the carbon atom. The alkynes parallel the series of structures of the alkanes.

Principal Terms

• alkylation: a combination reaction that results in the addition of an alkyl group to a molecule.
• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• hydrocarbon: an organic compound composed solely of carbon and hydrogen atoms.
• hydrogenation: a chemical reaction in which two hydrogen atoms, usually in the form of molecular hydrogen (H₂), are bonded to another molecule, almost always as a result of catalysis.
• triple bond: a type of chemical bond in which two adjacent atoms are connected by six bonding electrons rather than two.

The Nature of the Alkynes

The alkynes are a series of compounds consisting of only carbon and hydrogen atoms, or hydrocarbons. Alkynes contain at least one triple bond between two adjacent carbon atoms (C≡C) as a functional group in their molecular structure and so are highly reactive materials. The structure of alkynes derives from the geometric arrangement of atomic orbitals on the carbon atom. There are four valence electrons in each carbon atom, two in the 2s orbital and one each in two of the three 2p orbitals. By combining the 2s and one of the three 2p orbitals, the carbon atom is able to form two equivalent sp hybrid atomic orbitals, each containing a single electron. The other two electrons remain in the unhybridized 2p orbitals. The two hybrid orbitals are directed in opposite directions, with the carbon atom nucleus at the center. The sp hybrid orbitals form an angle of 180 degrees. This angle and the physical diameter of the carbon atom are well suited to the formation of innumerable structures by linkage of the carbon atoms. The sp orbitals of two adjacent carbon atoms readily form a covalent single bond, or sigma (σ) bond, and each carbon atom can form three such single bonds. The unhybridized p orbitals on the adjacent carbon atoms are able to overlap in a side-by-side orientation to form two pi (π) bonds parallel to the sigma bond. Accordingly, alkynes form a large and varied group of chemical compounds in their own right, and the reactivity of the C≡C functional group enables the formation of many other compounds. In alkynes, the carbon atoms of the triple bonds do not have all of the bonds that they can, and another way to describe the alkynes is as the series of unsaturated hydrocarbons. If a hydrocarbon molecule contains only carbon-carbon single bonds (C–C) or double bonds (C=C), it is not classed as an alkyne, but as a hydrocarbon in the alkane or alkene series, respectively.

The alkynes have two parallel families of structures, one comprising acyclic molecular structures and the other comprising cyclic molecular structures. The carbon atom skeletons of the acyclic alkyne series can be expanded infinitely, at least in principle. The simplest alkyne is ethyne (acetylene), C₂H₂, having just two carbon atoms and two equivalent bonds to as many hydrogen atoms. The next are propyne (C₃H₄), butyne (C₄H₆), and so on. All alkynes have the general chemical formula C_nH_2n−2, which is the same as the cyclic alkene series. For alkynes with four or more carbon atoms (butyne or higher), there are also numerous possible isomers, chemical species with the same molecular formula but different molecular structures. The triple bond in butyne can be between either the first and second carbon atoms or the second and third carbon atoms in the molecule, both of which have the chemical formula C₄H₆. In addition, the geometry of the C≡C bond is rigidly fixed and rotation about a C≡C bond cannot occur as it can with a C–C bond. This does not contribute to the formation of geometric isomers, however, as it does in the alkene series. Accordingly, 2-butyne has only the two isomeric structures, represented as

The two compounds have both different physical properties and chemical behaviors.

With each additional carbon atom, the number of possible isomeric forms increases. Four carbon atoms can be arranged in two isomeric alkyne forms. With five carbons atoms, there are three isomeric alkyne structures, and with six carbon atoms, there are seven isomeric alkyne structures. With seven carbon atoms, the number of isomeric alkynes increases to twelve. The presence of additional C≡C and C=C bonds further increases the number of possible skeletal arrangements. There are even more isomeric forms when optical isomers are included. Optical isomers are compounds in which a saturated carbon atom has four different substituent groups, or side chains, bonded to the same carbon atom. They are identical in every way except the order which the bonds to side chains are distributed about that central asymmetric carbon atom. While the physical properties and chemical behavior of two optical isomers are the same, they are nonetheless distinctly different structurally and therefore separate isomeric forms. Optical isomerism is not possible around a carbon atom in a single C≡C bond.

In the cyclic alkynes, the carbon atoms are bonded together in a ring structure, but the restrictions imposed by the linear geometry of the C≡C system prevents the formation of cyclic alkynes of fewer than ten carbon atoms and precludes the possibility of geometric isomers.

Nomenclature of Alkynes

Alkynes are named according to the number of carbon atoms comprising the longest unbranched chain of carbon atoms in the molecular structure, and the positions of side chains are identified accordingly. The name sequence begins with ethyne, propyne, and butyne as common names, and then progresses through names based on the number of carbon atoms: pentyne, hexyne, heptyne, octyne, nonyne, decyne, and so on. When the base chain has been determined and has three or more carbon atoms, the position of the C≡C bond is indicated by the lower of the two position numbers of the carbon atom in the bond. The position number is inserted into the name of the primary chain immediately before the portion that identifies the compound as an alkyne. (Some naming conventions place the position number before the name of the chain.) A seven-carbon chain with a C≡C bond between the second and third carbon atoms from one end would thus be named hept-2-yne (or 2-heptyne), not hept-3-yne or hept-5-yne. The order of the side chains is determined next. The numerical positions of the side chains are assigned as determined by the position assigned to the C≡C bond. For example, a six-carbon chain that has methyl groups (−CH₃) on the third and fourth carbon atoms from the end that has the C≡C bond would be named 3,4-dimethylhexyne. When different side chains are present on a hydrocarbon molecule, they are assigned by the priority of their size, again to attain the lowest numbers, and named alphabetically.

Organic structures are typically represented by structural formulas, line drawings that depict the angles between the carbon atoms in such a way that every angle vertex and line end represents a single carbon atom. Such line drawings allow chemists to communicate a large amount of chemical information in a very small space. This is a much more convenient and readily understood representation than strings of Cs connected by lines and makes the nature and identity of the side chains immediately apparent. For example, the structure

is much more easily and clearly understood than

and can be easily assigned the proper name 6-butyl-3-ethyl-3-methylundecyne, according to the rules of nomenclature prescribed by the International Union of Pure and Applied Chemistry (IUPAC).

Cyclic alkynes are named in a similar manner. The basic name reflects the number of carbon atoms in the largest ring structure in the molecule. The first position in the ring structure is assigned to the carbon atom bearing the first carbon atom of the C≡C bond, and other substituents are assigned so as to attain the lowest set of position numbers. A cyclododecyne (C-12) ring bearing a methyl substituent on the third carbon atom from the beginning of the C≡C bond and an ethyl substituent on another carbon atom two positions farther around the ring would be named 5-ethyl-3-methylcyclododecyne. Cyclic alkyne structures are slightly harder to draw due to the number of carbon atoms in the ring and the geometric constraints imposed by the C≡C system, but the structural drawings are highly informative.

Formation of Alkynes

Alkynes are so highly reactive that they do not occur under normal conditions. Interestingly, compounds containing multiple C≡C bonds have been detected in interstellar space. The C≡C bond can be produced by dehydration reactions, which eliminate the components of water molecules from adjacent carbon atoms in diols, compounds containing two hydroxyl groups. Ethylene glycol, for example, can be dehydrated to produce ethyne and water, according to the equation

HO–H₂C–CH₂–OH → HC≡CH + 2H₂O

Acetylene (ethyne) is never produced in this way, however. It is produced much more efficiently by the reaction of calcium carbide (CaC₂) with water, yielding acetylene and calcium hydroxide, Ca(OH)₂. Other alkynes are commonly produced by alkylation reactions in which the ethynyl group is added to other molecular structures as a substituent. Polyhalogenated compounds can be either dehalogenated or dehydrohalogenated to produce the C≡C bond as well. In laboratory procedures, specific methods are used to generate C≡C bonds in a particular molecular structure. Controlled dehydrohalogenation can be used to eliminate hydrogen halide molecules when two suitable –X substituents (X = chlorine, bromine, or iodine) are present. Another method is dehalogenation, which eliminates X₂ from molecules containing four halogen atoms on adjacent carbon atoms. Often such reactions produce adjacent C=C bonds instead of the C≡C bond directly, but the resulting allene system can be made to rearrange into the alkyne structure.

Reactions of Alkynes

Alkynes are highly reactive compounds. They readily undergo reduction reactions such as hydrogenation, in which two hydrogen atoms are bonded to each of the two carbon atoms of the C≡C bond. This is typically carried out using a metal catalyst. Alkynes are also readily and rapidly oxidized by good oxidizing agents, such as ozone (O₃) and permanganate ion (MnO₄⁻). Electrophilic addition reactions such as halogenation and hydrohalogenation are also facile reactions with alkynes. The C≡C bond forms an open linear region in a molecular structure that exposes the electron-rich region, facilitating bond formation with an electrophile (a species attracted to electrons). Simple alkynes are thus able to readily undergo polymerization reactions with ease. The compound dimethylacetylenedicarboxylate (DMAD) is widely used in Diels-Alder reactions to add functional groups to an existing molecular structure. DMAD typically undergoes electrocyclic self-addition reactions, in which pi bonds are changed to sigma bonds, to form a variety of polycyclic compounds.

The compounds formed by self-addition are readily reversed by heating to regenerate the material in the desired form.

As hydrocarbons, alkynes are highly combustible, and the extra energy of the C≡C bond increases the amount of energy released in combustion and other oxidation reactions. Because of the high electron density of the C≡C bond, it is sufficiently easy for the hydrogen atom of a terminal alkyne to be removed by a strong base, producing an acetylide anion. This ion can then be used in nucleophilic addition and substitution reactions to add the C≡C functional group to an existing molecular structure, a process called alkylation. Many organometallic complexes are also known in which C≡C groups coordinate to metal atoms as ligands.

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