Structure and naming of alkynes
An alkyne has a C≡C triple bond. General formula CₙH₂ₙ₋₂. First member: ethyne (acetylene) HC≡CH.
- Each triple-bond carbon is sp hybridised. Its two sp orbitals point opposite ways, so H–C≡C–H is linear (180°).
- Triple bond = one σ + two π. The two π bonds lie at right angles and together form a cylinder-shaped electron cloud around the C–C axis.
- C≡C is 120 pm long (shortest C–C bond) and has bond energy 823 kJ/mol.
- Naming: change -ane to -yne. CH₃–C≡CH is propyne. CH₃–C≡C–CH₃ is but-2-yne. Alkynes show position and chain isomerism, but no cis–trans (the line is straight).
Preparation of ethyne
- From calcium carbide: CaC₂ + 2H₂O → HC≡CH + Ca(OH)₂. Calcium carbide itself is made by heating quicklime with coke: CaO + 3C → CaC₂ + CO.
- From vicinal dihalides: remove HX twice. Alcoholic KOH removes the first HX and gives a vinylic halide; the second, harder step needs sodium amide (NaNH₂): BrCH₂–CH₂Br → CH₂=CHBr → HC≡CH.
Physical properties
Ethyne, propyne and butynes are gases; the next eight are liquids; higher ones are solids. Pure ethyne has no smell; the garlic smell of carbide ethyne comes from impurities (PH₃, H₂S). Alkynes are weakly polar, lighter than water, and do not mix with water. Boiling points rise with size.
Acidic character of alkynes
A hydrogen on a triple-bond carbon (≡C–H) is weakly acidic. It can be removed by a strong base.
- HC≡CH + Na → HC≡C⁻Na⁺ + ½H₂ (monosodium ethynide)
- HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃ (sodium acetylide)
- CH₃–C≡C–H + NaNH₂ → CH₃–C≡C⁻Na⁺ + NH₃
Why?
An s orbital stays closer to the nucleus than a p orbital. The more s-character a hybrid has, the more tightly it holds its electrons:
- sp³ (ethane): 25% s
- sp² (ethene): 33% s
- sp (ethyne): 50% s
So an sp carbon is the most electronegative. It pulls the C–H electrons towards itself, and the H can leave as H⁺. The ion left behind (acetylide, HC≡C⁻) keeps its lone pair in an sp orbital close to the nucleus, so it is fairly stable.
Order of acidity: HC≡CH > H₂C=CH₂ > CH₃–CH₃. Among alkynes: HC≡CH > CH₃–C≡CH ≫ CH₃–C≡C–CH₃. But-2-yne has no ≡C–H, so it is not acidic at all. Alkynes are still much weaker acids than water or alcohol; ethyne does not turn litmus red.
This lets us tell a terminal alkyne (≡C–H at the end) from an internal one: only the terminal one reacts with NaNH₂ (and gives a white precipitate with ammoniacal AgNO₃).
Addition reactions of alkynes
The triple bond has two π bonds, so addition can happen twice. Alkynes are less reactive than alkenes towards electrophiles because the π electrons of the tube cloud are held more tightly by the sp carbons.
1. Hydrogen
HC≡CH + H₂ → H₂C=CH₂ → (+H₂) → CH₃–CH₃ (Pt, Pd or Ni). Lindlar's catalyst stops at the (cis) alkene.
2. Halogens
HC≡CH + Br₂ → BrCH=CHBr (1,2-dibromoethene); + Br₂ → Br₂CH–CHBr₂ (1,1,2,2-tetrabromoethane). The red-brown colour of bromine fades: a test for unsaturation.
3. Hydrogen halides
Two HX add, both by the Markovnikov rule, so both X end up on the same carbon (a gem-dihalide): HC≡CH + HBr → CH₂=CHBr; + HBr → CH₃–CHBr₂ (1,1-dibromoethane). Propyne + 2HBr → CH₃–CBr₂–CH₃.
4. Water (hydration)
With 40% H₂SO₄ and 1% HgSO₄ at 333 K, one H₂O adds. First an enol forms (C=C with OH on it). An enol is unstable: its H moves from O to C and a C=O forms (tautomerism).
- HC≡CH + H₂O → [CH₂=CH–OH] → CH₃CHO (ethanal). Ethyne is the only alkyne that gives an aldehyde.
- CH₃–C≡CH + H₂O → [CH₃–C(OH)=CH₂] → CH₃COCH₃ (propanone). Other alkynes give ketones (Markovnikov: OH goes to the inner carbon).
5. Polymerisation
- Linear: many ethyne molecules join to give polyacetylene, –(CH=CH)ₙ–, a plastic that can conduct electricity.
- Cyclic: pass ethyne through a red-hot iron tube at 873 K. Three molecules join into a ring: 3HC≡CH → C₆H₆ (benzene). This links aliphatic and aromatic chemistry.
6. Combustion
2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O, with a very hot flame used for welding.
Key formulas and definitions
- Alkyne: CₙH₂ₙ₋₂
- CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂
- s-character: sp 50% > sp² 33% > sp³ 25%
- HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃
- HC≡CH + 2Br₂ → Br₂CH–CHBr₂
- HC≡CH + 2HBr → CH₃CHBr₂ (Markovnikov, twice)
- HC≡CH + H₂O → CH₃CHO (Hg²⁺, H₂SO₄, 333 K)
- 3HC≡CH → C₆H₆ (red-hot Fe tube, 873 K)
Worked examples
1. Write the formula of the alkyne with 5 carbons.
CₙH₂ₙ₋₂ with n = 5: H = 10 − 2 = 8. Pentyne is C₅H₈.
2. Which is more acidic: ethene or ethyne? Explain in one line.
Ethyne. Its carbon is sp (50% s-character) and holds the C–H electrons closer, so H leaves as H⁺ more easily than from sp² ethene (33% s).
3. Which of these react with sodium amide: ethyne, propyne, but-2-yne?
Only alkynes with an H on the triple-bond carbon react. Ethyne (HC≡CH) and propyne (CH₃C≡CH) do; but-2-yne (CH₃C≡CCH₃) has no ≡C–H, so it does not.
4. Give the final product when propyne reacts with excess HBr.
First HBr: Markovnikov → CH₃–CBr=CH₂. Second HBr: H goes to CH₂, Br to the carbon already holding Br → CH₃–CBr₂–CH₃ (2,2-dibromopropane).
5. What does propyne give with water, HgSO₄ and H₂SO₄?
OH adds to the middle carbon (Markovnikov) → enol CH₃–C(OH)=CH₂. The enol changes into the keto form → CH₃–CO–CH₃, propanone (acetone).
6. How many litres of ethyne (at STP) come from 32 g of calcium carbide?
Molar mass of CaC₂ = 40 + 24 = 64 g/mol. Moles = 32 ÷ 64 = 0.5 mol. 1 mol CaC₂ gives 1 mol C₂H₂, so 0.5 mol × 22.4 L = 11.2 L.
7. How much Br₂ (in g) can 2.6 g of ethyne decolourise?
C₂H₂ = 26 g/mol, so 2.6 g = 0.1 mol. Each triple bond takes 2 Br₂ → 0.2 mol. Br₂ = 160 g/mol → 0.2 × 160 = 32 g.
Common mistakes
- Thinking all alkynes are acidic. Only terminal alkynes (with ≡C–H) are; but-2-yne is not.
- Calling ethyne a strong acid. It is much weaker than water; it only gives up H⁺ to very strong bases like NaNH₂.
- Putting the two X atoms on different carbons when 2HX adds. By Markovnikov, both go to the same carbon (gem-dihalide).
- Saying every alkyne gives an aldehyde with water. Only ethyne gives an aldehyde (ethanal); others give ketones.