What are Alkenes?
Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). This double bond makes them far more reactive than alkanes, enabling addition reactions and serving as building blocks in organic synthesis.
Alkenes have the general formula CₙH₂ₙ and contain one or more C=C double bonds. The presence of the double bond creates regions of higher electron density, making alkenes nucleophilic and prone to addition reactions.
- •Only single C−C bonds
- •Saturated
- •Unreactive (inert)
- •Examples: ethane, propane
- •One or more C=C bonds
- •Unsaturated
- •Reactive (addition reactions)
- •Examples: ethene, propene
Step-by-step worked examples
What is the molecular formula of propene?
Propene has 3 carbons (n=3) and one C=C double bond. CₙH₂ₙ = C₃H₆
An alkene contains 5 carbons and one double bond. How many hydrogens?
n = 5 H = 2(5) = 10 Formula: C₅H₁₀ (pentene)
Ethene (C₂H₄) undergoes addition with HBr. What is the product?
C₂H₄ + HBr → C₂H₅Br (bromoethane) The H adds to one carbon, Br to the other.
Flashcards
Quick quiz
Q1.General formula for alkenes?
Q2.Which statement about the C=C bond is true?
Q3.Ethene + Br₂ → ?
Q4.Alkenes are used to make…
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Common mistakes
Alkenes are the same as alkanes. — Correct: Alkenes have C=C double bonds; alkanes have only C−C single bonds.
The double bond has two σ bonds. — Correct: It has one σ bond and one π bond.
Alkenes undergo substitution reactions easily. — Correct: Alkenes undergo addition reactions; substitution is typical for alkanes.
All alkenes are liquids. — Correct: Ethene (C₂H₄) and propene (C₃H₆) are gases; larger alkenes are liquids.
FAQ
What is an alkene?
An unsaturated hydrocarbon with at least one C=C double bond, following the formula CₙH₂ₙ.
Why do alkenes undergo addition reactions?
The π bond in the C=C is relatively weak and readily breaks, allowing new atoms or groups to add to the carbons.
What is polymerization of alkenes?
Many alkene molecules link together via addition reactions to form long-chain polymers (plastics).
How can you distinguish alkene from alkane experimentally?
Alkenes decolorize bromine water (Br₂ adds to the double bond); alkanes do not.