📖 In-Depth Theory
What Are Alkenes?
ALKENES are hydrocarbons (contain only carbon and hydrogen) with a C=C DOUBLE BOND.
GENERAL FORMULA: CₙH₂ₙ
Examples: ethene C₂H₄, propene C₃H₆, butene C₄H₈, pentene C₅H₁₀.
UNSATURATED:
Alkenes are UNSATURATED — they contain fewer hydrogen atoms than the maximum possible.
The C=C double bond means two H atoms fewer than the corresponding alkane (CₙH₂ₙ₊₂).
Compare: ethane C₂H₆ (alkane, saturated) vs ethene C₂H₄ (alkene, unsaturated).
The double bond makes alkenes much MORE REACTIVE than alkanes.
Alkanes are relatively inert; alkenes undergo addition reactions readily.
FUNCTIONAL GROUP:
The C=C double bond is the functional group — it determines the characteristic reactions of alkenes.
All alkenes react in similar ways because they all share this functional group.
Representing Alkene Structures
Alkene structures can be represented in three ways:
1. MOLECULAR FORMULA: shows total atoms — e.g. C₂H₄ (ethene), C₃H₆ (propene).
2. DISPLAYED FORMULA (structural formula): shows all bonds and atoms.
Ethene: H₂C=CH₂ (each C has a double bond between them and 2 H atoms)
Propene: CH₃-CH=CH₂ (one CH₃ group and a C=C double bond)
3. SKELETAL FORMULA: shows carbon skeleton as a zigzag, H atoms implied.
Each line represents a bond; each corner/end = a carbon.
FIRST FOUR ALKENES:
Ethene: C₂H₄ — two carbons, double bond
Propene: C₃H₆ — three carbons, double bond between C1 and C2
Butene: C₄H₈ — four carbons (can have double bond in different positions)
Pentene: C₅H₁₀ — five carbons
TESTING FOR UNSATURATION (C=C double bond):
Add bromine water (orange/yellow solution).
Alkenes DECOLOURISE bromine water — goes from orange to colourless.
Alkanes do NOT decolourise bromine water — no change.
This is the standard test to distinguish alkenes from alkanes.
Homologous Series
Alkenes form a HOMOLOGOUS SERIES — a family of compounds with:
Same general formula (CₙH₂ₙ).
Same functional group (C=C).
Same general chemical properties.
Gradually changing physical properties (boiling point, density increases with chain length).
Each member differs from the next by CH₂.
COMPARING ALKENES AND ALKANES:
Alkanes (CₙH₂ₙ₊₂): saturated, single bonds only, relatively unreactive, used mainly as fuels.
Alkenes (CₙH₂ₙ): unsaturated, C=C double bond, reactive, used to make polymers and other chemicals.
CRACKING produces alkenes:
Long-chain alkanes cracked at high temperature with a catalyst → shorter alkanes + alkenes.
Ethene and propene are the most commercially important alkenes.
Ethene → polyethene (poly(ethene)) — one of the most widely produced plastics.
⚠️ Common Mistake
Alkenes are UNSATURATED — they have fewer H atoms than alkanes with the same carbon count. Don't confuse the general formula: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ. The test for C=C: bromine water decolourises (orange → colourless) with alkenes, no change with alkanes.