A Progressive Overview of Acetylene and Its Derivatives

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welding image by matei

Acetylene gas was popular in the welding field, during the early 20th century. Image by matei.

Acetylene gas (its technical name is ethyne) is one of the simplest of hydrocarbon compounds; it has the chemical formula H-C≡C-H.

Scientists first discovered ethyne in the early 19th century, and for more than one hundred years, chemists produced it by adding water to calcium carbide.

Here’s how we write out the reactions:

CaO + 3C → CaC₂ + CO↑

CaC₂ + 2 H₂O → H-C≡C-H↑ + Ca(OH)₂

Although people used acetylene gas in very early lighting, undoubtedly the most famous use of acetylene gas is in oxy-acetylene welding; developed during the first few years of the 20th century.

In spite of the development of additional specialty welding methods, two advantages of acetylene welding derive from its use where electricity is unavailable, and the high temperatures it can reach, which surpass 6,000 degrees F.

The Carbon ‒ Carbon Triple Bond

Acetylene’s triple bond is electron-rich. It possesses so-called SP hybridization, meaning that the one atomic s-orbital of hydrogen and only one of the three atomic p-orbitals of the carbon atom make up the molecular orbital of each hydrogen atom with its respective carbon atom.

The molecule is completely linear, or straight. Since it is electron-rich, other chemical species seeking electrons favor the triple bond. However, substitution of one or both of the hydrogen atoms is also a common occurrence. When a metallic atom is responsible, the result is an acetylide.

Acetylides

Acetylides are derivative names, written as if they were salts derived from acetylene or a substituted acetylene with a hydrogen atom replaced by an organic substituent (written R-C≡C-H).

Some examples are:

HC≡C⁻ Li⁺
CH₃-C≡C⁻ Na⁺

Many acetylides are inherently unstable, even dangerously explosive. This is especially the case for copper and silver acetylides. For this reason, chemists may keep acetylides in solution for use as reaction intermediates. One example of such a reaction appears below:

CH₃-C≡C⁻ Na⁺ + CH₃Br → CH₃-C≡C-CH₃ + NaBr

Polyacetylene

Chemists can react acetylene to form polyacetylene, an electrically conductive polymer. This is not generally the way chemists produce polyacetylene now, in view of the potential danger of the synthesis. We now produce polyacetylene in safer ways, such as the opening and modification of larger conjugated double-bond ring structures like octatetraene. With manipulation, incorporating the process of doping, a highly conductive polymer may form, the discovery of which won the 2000 Nobel Prize in Chemistry for the three collaborators engaged in the study.

Trans-polyacetylene - Image: PD Wikimedia Commons by Smokefoot

Here is a Trans-polyacetylene. Image by Smokefoot.

Spin-off projects have produced interesting results in a number of cases. One is the development of a semiconductor device based on organic conduction. The so-called NOMFET device (nanoparticle organic memory field effect transistor) is an outstanding example. Chemists say that this organic transistor mimics neurological synapses. Chemists hope that its development will lead to a new generation of computers that exhibit a higher degree of artificial intelligence.

Other Special Uses

Chemists can derive a wide variety of important organic compounds from acetylene using catalytic methods. Especially is this the case in forming substituted vinyls. For example, combining acetylene and an alcohol can produce a vinyl ether.

A most unusual use of an acetylene is to combine a carbonaceous sample with lithium methyl, producing lithium carbide, which is then used to liberate acetylene that is then analyzed by mass spectrometer to carbon date the sample. Chemists have developed alternatives to this by employing carbon dioxide.

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