Acidity & Reactions
Alkynes are hydrocarbons containing at least one carbon–carbon triple bond (C≡C). Unlike alkanes and alkenes, terminal alkynes — those with the triple bond at the end of the carbon chain, bearing a hydrogen directly on the sp-hybridised carbon — exhibit a surprising chemical property: measurable acidity. The C–H bond in a terminal alkyne has a pKa of approximately 25, making it far more acidic than the C–H bonds of alkenes (pKa ≈ 44) or alkanes (pKa ≈ 50). This enhanced acidity arises because the bonding electrons in sp-hybridised orbitals are held closer to the carbon nucleus than in sp² or sp³ orbitals, stabilising the resulting carbanion (alkynide ion) after proton loss. This acidity has profound practical consequences. Terminal alkynes can react with strong bases such as sodium amide (NaNH₂) or butyllithium to form metal acetylides — powerful nucleophiles and bases used widely in organic synthesis. Beyond acid–base chemistry, alkynes undergo polymerisation reactions that produce either linear conjugated polymers or cyclic aromatic products such as benzene, as well as hydration reactions that yield carbonyl compounds (ketones or aldehydes) through Markovnikov addition of water. Understanding the reactivity of alkynes — their acidity, their capacity to polymerise, and their behaviour towards water — provides a foundation for appreciating how simple hydrocarbons can be transformed into complex, functional organic molecules. These reactions are not merely academic: acetylene (ethyne) is one of the most commercially important organic chemicals, used in welding, lighting, and as a feedstock for producing vinyl chloride, acetic acid, and synthetic rubber.
- Why Terminal Alkynes Are Acidic
- Formation of Metal Acetylides
- Linear Polymerisation of Acetylene
- Cyclic Trimerisation — Formation of Benzene
- Hydration of Alkynes — Markovnikov Addition
Why Terminal Alkynes Are Acidic
In a terminal alkyne such as ethyne (HC≡CH), each carbon is sp-hybridised. The sp orbital used to form the C–H bond has 50% s-character, compared to 33% in sp² (alkenes) and 25% in sp³ (alkanes). Greater s-character means the electron pair is held in a smaller, more electronegative orbital closer to the nucleus. When the proton is removed by a base, the resulting alkynide carbanion (RC≡C⁻) is more stable because its lone pair occupies this compact sp orbital. The pKa values follow the order: alkyne (≈25) < alkene (≈44) < alkane (≈50), confirming increasing acidity with increasing s-character.
Formation of Metal Acetylides
A terminal alkyne reacts with a strong base — typically sodium amide (NaNH₂, pKa of NH₃ ≈ 38) or sodium metal — to produce a sodium acetylide and release ammonia or hydrogen gas. The reaction HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃ is thermodynamically favourable because the product acetylide is a weaker base than the amide ion. Sodium acetylide is a strong nucleophile that can attack electrophiles such as alkyl halides or carbonyl compounds, making it invaluable for building longer carbon chains — a process called alkylation.
Linear Polymerisation of Acetylene
Under the influence of certain catalysts (e.g. Ziegler–Natta-type systems or Reppe catalysts), acetylene molecules can link together to form polyacetylene, a conjugated polymer with alternating single and double bonds: —(CH=CH)ₙ—. When doped with electron acceptors (like iodine) or donors, polyacetylene becomes electrically conducting — a discovery that earned Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa the 2000 Nobel Prize in Chemistry. This linear polymerisation proceeds through successive insertion of acetylene units into a metal–carbon bond on the catalyst surface.
Cyclic Trimerisation — Formation of Benzene
When three molecules of acetylene are joined in a cyclic fashion using a metal catalyst such as nickel (Ni) or cobalt (Co) complexes, the product is benzene (C₆H₆). The overall equation is: 3 HC≡CH → C₆H₆. This cyclic trimerisation was first demonstrated by Marcellin Berthelot in 1866 by passing acetylene through a red-hot tube. Mechanistically, modern metal-catalysed routes involve coordination of two acetylene units to the metal centre, oxidative coupling to form a metallacyclopentadiene intermediate, followed by insertion of the third acetylene and reductive elimination to give benzene. This reaction beautifully illustrates how a simple molecule (acetylene) can spontaneously organise into the aromatic ring system.
Hydration of Alkynes — Markovnikov Addition
Alkynes react with water in the presence of an acid catalyst (H₂SO₄) and a mercury(II) catalyst (HgSO₄) to add H and OH across the triple bond. For a terminal alkyne RC≡CH, Markovnikov's rule dictates that the OH group adds to the more substituted carbon (the internal carbon), and H adds to the terminal carbon. The initial product is an enol (a vinyl alcohol: RC(OH)=CH₂), which is unstable and immediately tautomerises to a methyl ketone (RCOCH₃) via keto–enol tautomerism. This overall transformation is called the Markovnikov hydration. Exception: ethyne (HC≡CH) gives acetaldehyde (CH₃CHO) rather than a ketone, because both carbons are equivalent. Anti-Markovnikov hydration can be achieved using hydroboration–oxidation (with 9-BBN or diborane), yielding an aldehyde from a terminal alkyne.
The Key Equation
- \ce{HC≡CH} — Ethyne (Acetylene) (—): The simplest alkyne; two sp-hybridised carbons joined by a triple bond.
- \ce{H2O} — Water (—): The nucleophile that adds across the triple bond.
- \ce{HgSO4} — Mercury(II) sulfate (—): Catalyst that activates the triple bond towards nucleophilic attack by water by coordinating to the π system.
- \ce{CH2=CHOH} — Vinyl alcohol (Enol) (—): The unstable enol intermediate formed immediately after water addition.
- \ce{CH3CHO} — Acetaldehyde (Ethanal) (—): The stable carbonyl tautomer that results from keto–enol tautomerism of vinyl alcohol.
Worked Example
Problem. Propyne (CH₃C≡CH) is treated with dilute H₂SO₄ and HgSO₄ catalyst in water. (a) Identify the initial enol intermediate. (b) Identify the final organic product. (c) Would anti-Markovnikov hydration of propyne with 9-BBN followed by H₂O₂/OH⁻ give the same product? Explain.
Answer. Markovnikov hydration of propyne gives acetone (propanone, CH₃COCH₃). Anti-Markovnikov hydroboration–oxidation gives propanal (CH₃CH₂CHO). The two products are constitutional isomers.
Origins · 1 minMarcellin Berthelot and how this was discovered
Berthelot was a pioneering French chemist who first synthesised numerous organic compounds from inorganic starting materials, establishing that organic chemistry need not rely on 'vital force.' In 1866 he demonstrated the cyclic trimerisation of acetylene to benzene by passing the gas through a heated tube — one of the first examples of what we now recognise as a metal-free thermal cycloaddition/polymerisation. He also contributed foundational work on thermochemistry and introduced the concept of bomb calorimetry. Berthelot insisted that his ashes and those of his wife be buried together in a single ceremony — an unusual request for the time that was honoured in a joint state funeral at the Panthéon in Paris.
Related Concepts
- Alkyne Structure and sp Hybridisation — The acidity of terminal alkynes and their reactivity directly depend on the sp hybridisation of the alkyne carbons and the resulting orbital geometry.
- Addition Reactions of Alkenes — Hydration of alkynes follows the same electrophilic addition mechanism and Markovnikov's rule seen in alkene hydration, making alkene addition an essential precursor concept.
- Keto–Enol Tautomerism — The hydration of alkynes produces enol intermediates that immediately tautomerise to carbonyl compounds; understanding this equilibrium is essential to predicting alkyne hydration products.
- Benzene and Aromaticity — Cyclic trimerisation of acetylene produces benzene, making aromaticity a directly connected concept that explains why benzene is the thermodynamically favoured cyclic product.
- Acid–Base Equilibria and pKa — Comparing pKa values of different C–H bonds is the quantitative basis for understanding why terminal alkynes are stronger acids than alkenes or alkanes.