Copper(I) acetylenide: preparation and properties

2024 Author: Angel Austin | [email protected]. Last modified: 2024-01-02 17:39

Copper acetylide is an organometallic binary compound. This formula has been known to science since at least 1856. In crystals, it forms a monohydrate with the formula Cu₂C₂×H_{2O. Thermally unstable, explodes when heated.}

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Copper acetylenide is a binary compound. It is possible to conditionally distinguish in it a negatively charged part - anion C²⁻₂, and a positively charged part - copper cations Cu^{+. In fact, such a division is conditional: in the compound there is only a fraction of the ionic bond, although it is larger compared to the H-C≡ bond. But this bond also has a very strong polarity (as for a covalent one) due to the fact that the carbon atom with a triple bond is in sp hybridization - its relative electronegativity is greater than in sp}3³ hybridizations (single bond) or sp^{2 (double bond). This is what makes it relatively easy for carbon in acetylene to split off a hydrogen atom from itself and replace it with a metal atom, that is, to exhibit the properties inherent in acids.}

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The most common way to obtain copper acetylenide in the laboratory is to pass gaseous acetylene through an ammonia solution of copper(I) chloride. As a result, an insoluble precipitate of reddish acetylenide is formed.

Instead of copper(I) chloride, you can also use its hydroxide Cu_{2O. In both cases, the important thing is that the actual reaction is with the copper ammonia complex.}

Physical properties

Copper acetylenide in its pure form - dark red-brown crystals. In fact, this is a monohydrate - in the sediment, each molecule of acetylenide corresponds to one molecule of water (written as Cu₂C₂×H_{2 O). Dry copper acetylenide is explosive: it can detonate when heated (it is less thermally stable than silver acetylenide), as well as under mechanical stress, such as upon impact.}

On this occasion, there is an assumption that copper pipes in chemical industries are of great danger, since during long-term operation acetylenide is formed inside, which can then lead to a strong explosion. This is especially true for the petrochemical industry, where copper, as well as its acetylenides, are also used as catalysts, which increases the level of risk.

Chemical properties

We have already said that carbon with a triple bond in acetylene is much more electronegative than, for example, carbon with a double bond (as in ethylene) or a single bond (in ethane). The ability of acetylene to react withsome metals, donating a hydrogen ion and replacing it with a metal ion (for example, the reaction of the formation of sodium acetylenide during the interaction of acetylene with metallic sodium) confirms this. We call this ability of acetylene one of the acidic properties in accordance with the Bronsted-Lowry theory: according to it, the acidity of a substance is determined by its ability to split off a proton from itself. The acidity of acetylene (also in copper acetylenide) can be considered relative to ammonia and water: when a metal amide reacts with acetylene, acetylenide and ammonia are formed. That is, acetylene donates a proton, which characterizes it as a stronger acid than ammonia. In the case of water, copper acetylenide decomposes to form acetylene - it accepts a proton of water, showing itself to be a less strong acid than water. So, in the relative series of acidity (according to Brönsted - Lowry), acetylene is a weak acid, being somewhere between water and ammonia.

Copper(I) acetylenide is unstable: in water (as we already know) and in acid solutions, it decomposes with the release of acetylene gas and a red-brown precipitate - copper(I) oxide or a white precipitate of copper(I) chloride in when diluted with hydrochloric acid.

To avoid an explosion, the decomposition of acetylenide is carried out by gentle heating while wet in the presence of a strong mineral acid, such as dilute nitric acid.

Use

The reaction of formation of copper(I) acetylenide can be qualitative for the detection of terminal (with a triple bond at the end) alkynes. The indicator is the precipitation of insoluble red-brown precipitate of acetylenide.

In large-capacity production - for example, in petrochemistry - copper(I) acetylenide is not used, because it is explosive and unstable in water. However, several specific reactions are associated with it in the so-called fine synthesis.

Copper(I) acetylenide can also be used as a nucleophilic reagent in organic synthesis. In particular, it plays an important role in the synthesis of polyynes - compounds with several alternating triple and single bonds. Copper(I) acetylenides in an alcoholic solution are oxidized by atmospheric oxygen, condensing to form diynes. This is the Glaser-Ellington reaction, discovered in 1870 and later improved. Copper(I) plays the role of a catalyst here, since it is not itself consumed in the process.

Later, instead of oxygen, potassium hexacyanoferrate(III) was proposed as an oxidizing agent.

Ellington improved the method for obtaining polyins. Instead of alkynes and copper (I) s alts, such as chloride, which were initially introduced into the solution, for example, he proposed taking copper (II) acetate, which would oxidize the alkyne in the medium of another organic solvent - pyridine - at a temperature of 60-70 ° С.

This modification made it possible to obtain from diynes much larger and more stable molecules - macrocycles.