Thermal decomposition and polymerization
Acetylene is characterized by high thermal instability. At temperatures around $500^\circ C$ or when pressure increases above $20$ atmospheres, it tends to explode, decomposing into carbon (soot) and hydrogen with the formation of methane impurities.
In addition to decomposition, alkyne molecules are capable of combining with each other (polymerization):
- Dimerization: under the action of copper(I) chloride ($CuCl$), two acetylene molecules form vinylacetylene ($CH_2=CH-C \equiv CH$).
- Trimerization: when the gas is passed over red-hot carbon, three acetylene molecules cyclize into benzene ($C_6H_6$).
Addition reactions
Despite the high degree of unsaturation, electrophilic addition reactions ($A_E$) in alkynes proceed slower and with greater difficulty than in alkenes with a double bond.
- Halogenation: the process proceeds stepwise. First, one halogen molecule adds to form a dihaloalkene, then the second adds to form a tetrahaloalkane.
- Hydrohalogenation and hydration: the addition of hydrogen halides ($HX$) and water proceeds strictly according to Markovnikov's rule (the hydrogen atom is directed to the more hydrogenated carbon atom).
A special case of hydration is the Kucherov reaction. The addition of water proceeds in the presence of mercury salts ($HgSO_4$) and sulfuric acid. The resulting intermediate enol ($CH_3-C(OH)=CH_2$) is extremely unstable due to the hydroxyl group at the $sp^2$-hybridized carbon. An intramolecular rearrangement (tautomerism) occurs: the double bond shifts to the oxygen ($C=O$), and the hydrogen migrates to the adjacent carbon. Propyne and other homologues ultimately yield ketones.
Oxidation of alkynes
The behavior of alkynes upon oxidation with potassium permanganate ($KMnO_4$) depends crucially on the $pH$ of the medium:
- In a neutral or slightly alkaline medium, mild oxidation occurs. The carbon skeleton of acetylene is not destroyed, and the reaction product is the salt of oxalic acid — potassium oxalate ($K_2C_2O_4$). Meanwhile, the $\sigma$-bond between the carbon atoms is preserved.
- In an acidic medium (with the addition of $H_2SO_4$), harsh oxidation is observed with complete cleavage of the triple bond. The terminal fragment ($\equiv CH$) is oxidized to carbon dioxide ($CO_2$), while the internal carbon ($R-C \equiv$) is oxidized to a carboxyl group, forming the corresponding carboxylic acid. For example, propyne is converted into acetic acid and $CO_2$.
Acidic properties and acetylide formation
Alkynes with a triple bond at the end of the chain (terminal alkynes) possess a unique ability to split off a proton, exhibiting properties of very weak acids.
The reason lies in the $sp$-hybridization of the carbon atoms at the triple bond. The electronegativity of carbon in this state is maximal (compared to $sp^2$ and $sp^3$), which pulls electron density toward itself and makes the $C-H$ bond highly polar. Hydrogen becomes "acidic" and is easily replaced by heavy metal ions.
Products of such substitution are called acetylides. These reactions are used for the qualitative detection of a terminal triple bond:
- Upon interaction with an ammoniacal solution of silver oxide (Tollens' reagent), a characteristic precipitate of silver acetylide ($Ag_2C_2$) drops out.
- With an ammoniacal solution of copper(I) chloride, a precipitate of copper propynide is formed (in the case of propyne).
Acetylides of alkali metals and calcium ($CaC_2$) are easily hydrolyzed by water, and their interaction with haloalkanes makes it possible to extend the carbon chain.
Briefly: answers to topical questions
What is the final product formed upon the hydration of acetylene via Kucherov's reaction?
The final product of acetylene hydration is acetaldehyde (ethanal). At the same time, for acetylene homologues such as propyne, Kucherov's reaction leads to the formation of ketones (oxo compounds). The hydration process proceeds in an acidic medium using a catalyst — mercury(II) sulfate. First, an unstable enol is formed, which then undergoes intramolecular rearrangement.
How exactly does carbon chain elongation occur using alkali metal acetylides?
Carbon chain elongation occurs via alkylation, in which acetylides react with halogen derivatives of hydrocarbons. As a result of this reaction, alkyne homologues with a longer chain are synthesized, and the corresponding metal chloride is also formed. Acetylides are formed due to the weak acidic properties of alkynes with a terminal triple bond, which are capable of replacing a hydrogen atom with a metal.
What are the products of calcium carbide hydrolysis with water?
The products of calcium carbide hydrolysis with water are two substances:
- Acetylene — released as a gas;
- Calcium hydroxide — formed as the second reaction product.
Calcium carbide belongs to alkaline earth metal acetylides. Such compounds are completely hydrolyzed by water. This reaction is one of the methods for producing alkynes.
Remember Kucherov's reaction products: "Acetylene gives an aldehyde, the rest give ketones" (although the material lists only ketones from propyne, the general rearrangement principle works precisely this way: oxygen "pulls" the double bond toward itself).
Class and exam questions
Why do electrophilic addition reactions in alkynes proceed with more difficulty than in alkenes?
Despite greater unsaturation, $sp$-hybridized carbon atoms at the triple bond possess high electronegativity. They hold $\pi$-electrons more strongly, making them less accessible to electrophilic attack.
What is the essence of tautomerism during alkyne hydration?
The intermediate product of water addition — an enol (an alcohol with an OH group at the double bond) — is thermodynamically unstable. A rearrangement occurs: the proton from oxygen transfers to carbon, and the double bond shifts, forming a stable carbonyl group (ketone).
How to distinguish a terminal alkyne from an internal alkyne?
Using qualitative reactions of acetylide formation. Terminal alkynes react with ammoniacal solutions of silver or copper(I) salts, forming insoluble precipitates, whereas alkynes with an internal bond do not give such reactions.
What else to study on this topic
- Mechanism of electrophilic addition reaction to alkynes
- Balancing redox reactions involving alkynes using the ion-electron method
- Use of acetylides in organic synthesis for chain elongation
- Specifics of $sp$-hybridization and its effect on the electronegativity of the carbon atom
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