Why Ethyne Can Lose a Hydrogen Like an Acid but Ethene Cannot
Name alkynes and prepare them from calcium carbide and vicinal dihalides, understand why terminal alkynes are acidic, and learn the addition reactions of alkynes with hydrogen, halogens, hydrogen halides and water.
What makes a triple bond different from a double bond?
Ethyne, better known as acetylene, burns in oxygen with a flame hot enough to cut steel, and it is made simply by adding water to calcium carbide. Its triple bond gives alkynes a straight shape, a surprisingly acidic hydrogen and a double helping of addition reactions.
This lesson covers naming and preparing alkynes, the acidic character of terminal alkynes, and their addition reactions.
This lesson covers naming and preparing alkynes, the acidic character of terminal alkynes, and their addition reactions.
How are alkynes named and prepared?
**Alkynes are hydrocarbons with a carbon-carbon triple bond and the general formula , named with the suffix -yne, and they are prepared from calcium carbide and water or by removing two molecules of HX from vicinal dihalides.
Structure. Each carbon of the triple bond is sp hybridised, so the H-C-C-H unit of ethyne is linear, with bond angles of 180°. The triple bond has one sigma and two pi bonds and is 120 pm long — shorter and stronger than a double bond.
Naming:**
- is ethyne and is propyne
- is but-1-yne and is but-2-yne
- is 3-methylbut-1-yne
- Alkynes with the triple bond at the end of the chain are called terminal alkynes
Methods of preparation:
- From calcium carbide — made by heating quicklime with coke, it reacts with water to give ethyne:
- From vicinal dihalides — alcoholic KOH removes one HX to give a vinylic halide, and sodamide removes the second:
Worked example. From , 64 g of calcium carbide gives 22.4 L of ethyne at STP, so 32 g gives 11.2 L.
An everyday example. Welders in roadside fabrication workshops cut and join steel with oxyacetylene torches, burning ethyne in oxygen for an extremely hot flame.
The substance. Calcium carbide must never be used to ripen fruit — commercial carbide carries toxic arsenic and phosphorus impurities, which is why the practice is banned in India.
Structure. Each carbon of the triple bond is sp hybridised, so the H-C-C-H unit of ethyne is linear, with bond angles of 180°. The triple bond has one sigma and two pi bonds and is 120 pm long — shorter and stronger than a double bond.
Naming:**
- is ethyne and is propyne
- is but-1-yne and is but-2-yne
- is 3-methylbut-1-yne
- Alkynes with the triple bond at the end of the chain are called terminal alkynes
Methods of preparation:
- From calcium carbide — made by heating quicklime with coke, it reacts with water to give ethyne:
- From vicinal dihalides — alcoholic KOH removes one HX to give a vinylic halide, and sodamide removes the second:
Worked example. From , 64 g of calcium carbide gives 22.4 L of ethyne at STP, so 32 g gives 11.2 L.
An everyday example. Welders in roadside fabrication workshops cut and join steel with oxyacetylene torches, burning ethyne in oxygen for an extremely hot flame.
The substance. Calcium carbide must never be used to ripen fruit — commercial carbide carries toxic arsenic and phosphorus impurities, which is why the practice is banned in India.
Why are terminal alkynes acidic, and what addition reactions do alkynes undergo?
The hydrogen of a terminal alkyne is attached to an sp carbon with 50 per cent s-character, which holds electrons tightly and lets that hydrogen leave as a proton with strong bases; alkynes also undergo addition reactions, often twice, because they have two pi bonds.
Acidic character of terminal alkynes:
- The s-character of carbon falls from sp (50 per cent) to (33 per cent) to (25 per cent), and so does its electronegativity
- The negative charge of the acetylide ion sits in an sp orbital close to the nucleus, so the ion is relatively stable
- Order of acidity:
- Terminal alkynes react with sodium:
- They give a white precipitate with ammoniacal silver nitrate and a red one with ammoniacal copper(I) chloride, a test that separates them from internal alkynes
- Ethyne is still a very weak acid and does not turn blue litmus red
Addition reactions:
- Hydrogen — over nickel, ethyne gives ethene and then ethane; Lindlar's catalyst stops at the alkene
- Halogens — bromine adds twice, decolourising bromine water:
- Hydrogen halides — add twice, following Markovnikov's rule:
- Water — with dilute sulphuric acid and mercury(II) sulphate at 333 K, ethyne gives ethanal and propyne gives propanone, through an unstable enol that rearranges:
- Polymerisation — ethyne passed through a red-hot iron tube at 873 K forms benzene:
Worked reasoning. Alkynes add electrophiles more slowly than alkenes, even with more pi electrons, because the sp carbons hold those electrons tightly and the vinylic carbocation formed is less stable.
An everyday example. PVC pipes carrying water in Indian homes are made from vinyl chloride, , which can be prepared by adding HCl to ethyne.
The substance. Only terminal alkynes are acidic — but-2-yne has no hydrogen on an sp carbon, so it gives no precipitate with ammoniacal silver nitrate.
Acidic character of terminal alkynes:
- The s-character of carbon falls from sp (50 per cent) to (33 per cent) to (25 per cent), and so does its electronegativity
- The negative charge of the acetylide ion sits in an sp orbital close to the nucleus, so the ion is relatively stable
- Order of acidity:
- Terminal alkynes react with sodium:
- They give a white precipitate with ammoniacal silver nitrate and a red one with ammoniacal copper(I) chloride, a test that separates them from internal alkynes
- Ethyne is still a very weak acid and does not turn blue litmus red
Addition reactions:
- Hydrogen — over nickel, ethyne gives ethene and then ethane; Lindlar's catalyst stops at the alkene
- Halogens — bromine adds twice, decolourising bromine water:
- Hydrogen halides — add twice, following Markovnikov's rule:
- Water — with dilute sulphuric acid and mercury(II) sulphate at 333 K, ethyne gives ethanal and propyne gives propanone, through an unstable enol that rearranges:
- Polymerisation — ethyne passed through a red-hot iron tube at 873 K forms benzene:
Worked reasoning. Alkynes add electrophiles more slowly than alkenes, even with more pi electrons, because the sp carbons hold those electrons tightly and the vinylic carbocation formed is less stable.
An everyday example. PVC pipes carrying water in Indian homes are made from vinyl chloride, , which can be prepared by adding HCl to ethyne.
The substance. Only terminal alkynes are acidic — but-2-yne has no hydrogen on an sp carbon, so it gives no precipitate with ammoniacal silver nitrate.
Exam tip
What earns full marks on alkynes?
Link the acidity of ethyne to s-character in one clear line — sp carbons are more electronegative — rather than simply stating that ethyne is acidic.
- Ethyne:
- Vicinal dihalide with alcoholic KOH, then sodamide, gives an alkyne
- Terminal alkynes: white precipitate with ammoniacal silver nitrate
- Hydration with and dilute : ethyne gives ethanal; other alkynes give ketones
- Red-hot iron tube: three ethyne molecules form benzene
The trap. Writing ethanol as the product of hydrating ethyne. The enol rearranges to ethanal, an aldehyde.
- Ethyne:
- Vicinal dihalide with alcoholic KOH, then sodamide, gives an alkyne
- Terminal alkynes: white precipitate with ammoniacal silver nitrate
- Hydration with and dilute : ethyne gives ethanal; other alkynes give ketones
- Red-hot iron tube: three ethyne molecules form benzene
The trap. Writing ethanol as the product of hydrating ethyne. The enol rearranges to ethanal, an aldehyde.
Did you know
Why does wet calcium carbide smell of garlic?
Drop a lump of calcium carbide into water and it fizzes, releasing ethyne with a strong, garlic-like smell.
Pure ethyne, however, has almost no smell. The odour comes from impurities: commercial carbide contains traces of calcium phosphide and sulphide, which release phosphine and hydrogen sulphide when wet.
Those same impurities make the gas poisonous — one more reason calcium carbide has no place anywhere near food.
Pure ethyne, however, has almost no smell. The odour comes from impurities: commercial carbide contains traces of calcium phosphide and sulphide, which release phosphine and hydrogen sulphide when wet.
Those same impurities make the gas poisonous — one more reason calcium carbide has no place anywhere near food.
Exam relevance
How do JEE Main and NEET test alkynes and their acidity?
Hydrocarbons is a recurring chapter in both JEE Main and NEET, and alkynes tie hybridisation, acidity and addition reactions into one topic.
What gets asked. The acidity order of ethyne, ethene and ethane, distinguishing terminal from internal alkynes with ammoniacal silver nitrate, hydration products with mercury(II) sulphate, double addition of HX and halogens, and cyclic polymerisation of ethyne to benzene.
Question types. Mostly single-correct and assertion-reason questions, often built into short reaction sequences that start from calcium carbide.
Why it matters later. Hydration products lead into Aldehydes, Ketones and Carboxylic Acids, and the formation of benzene connects to aromatic hydrocarbons in the next part of this chapter.
The trap that costs marks. Predicting an aldehyde from the hydration of propyne — Markovnikov addition of water gives propanone, a ketone; only ethyne gives an aldehyde.
What gets asked. The acidity order of ethyne, ethene and ethane, distinguishing terminal from internal alkynes with ammoniacal silver nitrate, hydration products with mercury(II) sulphate, double addition of HX and halogens, and cyclic polymerisation of ethyne to benzene.
Question types. Mostly single-correct and assertion-reason questions, often built into short reaction sequences that start from calcium carbide.
Why it matters later. Hydration products lead into Aldehydes, Ketones and Carboxylic Acids, and the formation of benzene connects to aromatic hydrocarbons in the next part of this chapter.
The trap that costs marks. Predicting an aldehyde from the hydration of propyne — Markovnikov addition of water gives propanone, a ketone; only ethyne gives an aldehyde.
Key takeaways
What must you be able to do from this lesson?
- Alkynes: with linear sp carbons, prepared from calcium carbide or vicinal dihalides
- Acidity: terminal alkynes lose a proton because sp carbons hold electrons tightly, forming acetylides
- Addition reactions: hydrogen, halogens, HX and water, often adding twice, plus the polymerisation of ethyne to benzene
Which test tells but-1-yne apart from but-2-yne, and why does only one of them respond?
- Acidity: terminal alkynes lose a proton because sp carbons hold electrons tightly, forming acetylides
- Addition reactions: hydrogen, halogens, HX and water, often adding twice, plus the polymerisation of ethyne to benzene
Which test tells but-1-yne apart from but-2-yne, and why does only one of them respond?