Alkenes
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Alkenes Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. They are much more reactive than alkanes and are essential for making plastics and other chemicals. Alkene An unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C).
Called "unsaturated" because they contain fewer hydrogen atoms than alkanes and can add atoms across the double bond. General Formula The general formula for alkenes is: C n H 2n where n = number of carbon atoms (n ≥ 2) Important Note that methene (CH₂) does not exist - you need at least 2 carbon atoms to form a C=C double bond.
The first alkene is ethene (C₂H₄). The Carbon-Carbon Double Bond The C=C double bond is the functional group that determines the properties of alkenes. Structure of the Double Bond One sigma (σ) bond - strong bond formed by direct overlap of orbitals One pi (π) bond - weaker bond formed by sideways overlap of p orbitals The double bond is the reactive part of alkene molecules Prevents rotation around the bond (unlike single bonds) Key Concept The pi bond is weaker than the sigma bond and can be broken more easily, which is why alkenes are more reactive than alkanes.
Homologous Series Alkenes form a homologous series with predictable patterns in their properties. The same general formula (C n H 2n ) Similar chemical properties Gradual change in physical properties Each member differs from the next by CH₂ The First Six Alkenes The alkene homologous series begins with ethene and follows a regular pattern.
Name Molecular Formula Structural Formula State at Room Temp Ethene C₂H₄ CH₂=CH₂ Gas Propene C₃H₆ CH₃-CH=CH₂ Gas Butene C₄H₈ CH₃-CH₂-CH=CH₂ Gas Pentene C₅H₁₀ CH₃-CH₂-CH₂-CH=CH₂ Liquid Hexene C₆H₁₂ CH₃-CH₂-CH₂-CH₂-CH=CH₂ Liquid Heptene C₇H₁₄ CH₃-(CH₂)₃-CH=CH₂ Liquid Naming Alkenes Alkenes are named similarly to alkanes, but with the suffix -ene instead of -ane to indicate the presence of a double bond.
Naming Pattern Eth-ene = 2 carbons with a double bond Prop-ene = 3 carbons with a double bond But-ene = 4 carbons with a double bond Pent-ene = 5 carbons with a double bond And so on... Key Point The suffix -ene indicates the presence of a C=C double bond and identifies the compound as an alkene.
Physical Properties of Alkenes Alkenes have similar physical properties to alkanes with the same number of carbon atoms. Boiling points increase with chain length (similar to alkanes) First three alkenes (ethene, propene, butene) are gases at room temperature Insoluble in water but soluble in organic solvents Less dense than water Comparison Alkenes have slightly lower boiling points than alkanes with the same number of carbon atoms due to the slightly different molecular shapes.
Chemical Properties of Alkenes Alkenes are much more reactive than alkanes because of the C=C double bond. Addition Reactions The most important reactions of alkenes are addition reactions . The π bond breaks and two new atoms/groups add across the double bond, converting it to a single bond.
General pattern: Alkene + Reagent → Product (saturated compound) Key Point Addition reactions are covered in detail in the Addition Reactions topic. Combustion Like alkanes, alkenes burn in oxygen: Complete combustion: produces CO₂ and H₂O Burns with a more smoky flame than alkanes (higher C:H ratio) Not commonly used as fuels - too valuable for making other chemicals Example for ethene: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O Test to Distinguish Alkenes from Alkanes The bromine water test is the standard test to detect the presence of a C=C double bond.
Alkene + Bromine Water Orange/brown bromine water is decolorized (turns colorless) Rapid reaction at room temperature Addition reaction occurs: Br₂ adds across the C=C bond Example: C₂H₄ + Br₂ → C₂H₄Br₂ Alkane + Bromine Water Orange/brown color remains unchanged No reaction at room temperature Alkanes don't have a double bond to react with bromine Important Test This test is the key way to distinguish between alkenes and alkanes.
The decolorization of bromine water is immediate evidence of a C=C double bond. Production of Alkenes Alkenes are produced industrially by two main methods. 1. Cracking of Alkanes Alkenes are produced by cracking - breaking down larger alkane molecules from crude oil into smaller, more useful molecules.
Thermal Cracking High temperature (700-900°C) High pressure (70 atm) Produces mainly alkenes Catalytic Cracking Lower temperature (450-500°C) Zeolite catalyst More efficient and economical Produces branched alkanes and alkenes Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene) Why is Cracking Important?
Produces shorter chain hydrocarbons which are more useful as fuels Produces alkenes which are valuable raw materials for making plastics and other chemicals Matches supply with demand - crude oil contains more long-chain hydrocarbons than needed 2.
Dehydration of Alcohols Alkenes can also be produced by removing water from alcohols. Heat alcohol with concentrated sulfuric acid or p…
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