Homologous series and nomenclature
Each class of organic compounds has its own homologous series. For alkynes, the general formula $C_nH_{2n-2}$ allows the composition of any representative to be derived.
The first hydrocarbon in the series is $C_2H_2$. Its systematic name is ethyne, although in medical and engineering practice, the trivial name acetylene is more commonly used. Subsequent homologues are named by replacing the suffix of the corresponding alkane with "-yne":
- $C_3H_4$ — propyne;
- $C_4H_6$ — butyne;
- $C_5H_8$ — pentyne.
Electronic structure and molecular geometry
The carbon atoms directly forming the triple bond are in an $sp$-hybridized state. The hybrid orbitals are directed precisely opposite each other in space, forming a bond angle of exactly $180^\circ$.
This rigidly dictates the geometry: the carbon atoms at the triple bond form an absolutely linear grouping. Neighboring atoms—whether hydrogen atoms or radical carbons—also align in a straight line. Typical examples of linear structures include:
- Acetylene: $H-C \equiv C-H$
- Propyne: $H-C \equiv C-CH_3$
- But-2-yne: $CH_3-C \equiv C-CH_3$
Physicochemical parameters of the triple bond
The triple bond ($-C \equiv C-$) stands out for its energetic and spatial parameters. Its bond energy ($E_{b}$) is 828 kJ/mol, which exceeds the double bond energy in alkenes by 222 kJ/mol. Meanwhile, the presence of three pairs of shared electrons pulls the atomic nuclei closer together, reducing the $C \equiv C$ bond length to 120 pm, making it noticeably shorter than a double bond.
The thermodynamic instability paradox of acetylene
Despite the colossal energy of the triple bond, the acetylene molecule is thermodynamically unstable and capable of undergoing a powerful explosive decomposition. The reaction equation is as follows:
$$2C_2H_2(g) = CH_4(g) + 3C(gr)$$
The process is accompanied by the release of a huge amount of heat ($\Delta H^\circ = -529.4$ kJ). The reason lies in the balance of chemical bonds: the decomposition products (methane and graphite) contain fewer relatively weak $\pi$-bonds, while the number of stronger $\sigma$-bonds increases. This transition is energetically favorable.
Practical applications and storage regulations
The combustion of acetylene is accompanied by the release of a colossal amount of energy, with the flame temperature reaching 3150 °C. This property makes the gas indispensable for steel cutting and welding.
However, its tendency toward explosive decomposition requires strict safety measures. Acetylene must never be stored simply in a compressed state. It is transported in special white cylinders. Inside, the gas is dissolved in acetone under a pressure of about 10 atmospheres, which prevents spontaneous decomposition.
Types of isomerism in the alkyne series
Alkynes are characterized by strictly specific types of isomerism:
- Carbon skeleton isomerism — associated with the branching of the carbon atom chain in higher homologues.
- Position isomerism of the multiple bond — depends on the positions of the carbon atoms involved in the triple bond.
- Interclass isomerism — alkynes are isomeric with dienes due to the general formula $C_nH_{2n-2}$.
A crucial feature of alkynes is that spatial (cis-trans) isomerism is entirely absent in them. This is a direct consequence of the linear structure of the $-C \equiv C-$ fragment.
Briefly: answers to topical questions
What qualitative reactions are used to detect a terminal triple bond in alkynes?
To detect a terminal triple bond in alkynes, reactions involving the formation of acetylides are used. Alkynes with a terminal triple bond exhibit weak acidic properties and react with the following reagents:
- Ammoniacal silver oxide solution (Tollen's reagent) — yields a precipitate of silver acetylide.
- Ammoniacal copper(I) chloride solution — yields a precipitate of copper propynide.
Alkadienes and alkynes with an internal triple bond do not participate in these qualitative reactions.
What is the mechanism of halogenation and hydrohalogenation reactions of alkynes?
The mechanism of halogenation and hydrohalogenation reactions of alkynes involves electrophilic addition. These addition reactions proceed with more difficulty and slower than those of alkenes.
- Halogenation — involves the successive addition of two halogen molecules: a dihaloalkene is formed first, followed by a tetrahaloalkane.
- Hydrohalogenation — proceeds in accordance with Markovnikov's rule, whereby the hydrogen atom adds to the more hydrogenated carbon atom.
What products are formed during the hydration of alkynes under Kucherov reaction conditions?
During the hydration of alkynes under Kucherov reaction conditions, carbonyl compounds are formed. The reaction proceeds in an acidic medium in the presence of a mercury(II) sulfate catalyst.
First, according to Markovnikov's rule, an unstable intermediate alcohol with a hydroxyl group at the double bond (an enol) is formed. Then, an intramolecular rearrangement takes place: the electron pair shifts from oxygen to carbon to form a double bond, and a proton transfers to the adjacent carbon atom. As a result, for propyne and other homologues, the final product is a ketone.
To permanently remember the absence of spatial isomerism in alkynes, imagine a straight pointer. This is the linear structure of $sp$-hybridization with a $180^\circ$ angle. On a perfectly straight line, it is physically impossible to attach substituents "from above" or "from below" as can be done on a plane. No plane means no cis-trans isomers.
Class and exam questions
Why is acetylene stored specifically as an acetone solution rather than simply compressed in a cylinder?
Acetylene is thermodynamically extremely unstable. If simply compressed, it can undergo explosive decomposition into methane and carbon. Dissolving the gas in acetone under a pressure of about 10 atmospheres in special white cylinders stabilizes the molecules and makes transportation safe.
Can alkynes form cis- and trans-isomers?
No, spatial isomerism is impossible for this class of hydrocarbons. The carbon atoms at the triple bond undergo $sp$-hybridization, which forms a $180^\circ$ bond angle. Due to the strictly linear structure of the molecule, substituents cannot be positioned "above" or "below" relative to the bond.
What is the reason for the release of a large amount of heat during the decomposition of acetylene?
When acetylene decomposes into methane and graphite, electronic orbitals are rearranged. The reaction products show a decrease in the number of relatively weak $\pi$-bonds and the formation of new, stronger $\sigma$-bonds. This transition is energetically favorable and is accompanied by the release of 529.4 kJ of heat.
With which class of organic compounds do alkynes exhibit interclass isomerism?
Alkynes are isomeric with diene hydrocarbons (dienes). Both of these classes share an identical general formula $C_nH_{2n-2}$, but differ in structure: alkynes contain one triple bond, whereas dienes contain two double bonds.
What else to study on this topic
- Algorithm for naming alkynes according to IUPAC systematic nomenclature.
- Comparative characteristics of bond energy and length in alkanes, alkenes, and alkynes.
- Mechanism of $\sigma$- and $\pi$-bond formation during carbon $sp$-hybridization.
- Thermodynamic calculations of the enthalpy of the acetylene decomposition reaction.
- Examples of carbon skeleton and bond position isomerism using pentyne as an example.
- Physical principles of gas solubility using the acetylene-acetone system as an example.
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Alkynes" and 3,009 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in "Chapter 15. Hydrocarbons"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.