What are Alkynes?
Alkynes are unsaturated hydrocarbons with one or more carbon–carbon triple bonds (C≡C). These highly reactive compounds are even more unsaturated than alkenes and play a key role in organic synthesis and industrial applications like welding.
Alkynes have the general formula CₙH₂ₙ₋₂ (for one triple bond) and contain C≡C triple bonds. Each triple bond consists of one σ bond and two π bonds, making the electron density very high and the molecule highly reactive.
- •C−C single bonds
- •σ bonds only
- •Least reactive
- •Examples: ethane
- •C≡C triple bonds
- •1 σ + 2 π bonds
- •Most reactive
- •Examples: acetylene
Step-by-step worked examples
What is the molecular formula of acetylene?
Acetylene has 2 carbons (n=2) and one C≡C triple bond. CₙH₂ₙ₋₂ = C₂H₂
A hydrocarbon has the formula C₄H₆. Is it an alkene or alkyne?
For alkene (one double): CₙH₂ₙ = C₄H₈ (no) For alkyne (one triple): CₙH₂ₙ₋₂ = C₄H₆ (yes) It is a butyne (alkyne).
Acetylene reacts with Br₂. Write the first addition product.
C₂H₂ + Br₂ → C₂H₂Br₂ (1,2-dibromoethene) The first π bond breaks; one π bond remains.
Flashcards
Quick quiz
Q1.General formula for alkynes?
Q2.What is the structure of the C≡C bond?
Q3.Acetylene (C₂H₂) is most commonly used for…
Q4.C₃H₆ could be…
The full card deck, worked steps and AI-tutor support for “What are Alkynes?” are in Notek — study by hand before your exam.
Common mistakes
Alkynes have the same formula as alkenes. — Correct: Alkynes are CₙH₂ₙ₋₂; alkenes are CₙH₂ₙ.
The triple bond is three σ bonds. — Correct: It is one σ + two π bonds.
Alkynes are less reactive than alkenes. — Correct: Alkynes are more reactive due to higher electron density.
Acetylene is used mainly in chemical industry. — Correct: Its primary use is welding and metal cutting due to its hot flame.
FAQ
What is an alkyne?
An unsaturated hydrocarbon with at least one C≡C triple bond, following the formula CₙH₂ₙ₋₂.
Why is acetylene useful for welding?
Acetylene burns with oxygen at ~3000 °C, producing a hot, intense flame that can melt and join metals.
How many times can acetylene undergo addition?
Twice — one addition per π bond. C₂H₂ + 2 Br₂ → C₂H₂Br₄.
Are alkynes aromatic?
No. Aromatic compounds (like benzene) have different bonding and special stability.