4d-Alkenes Flashcards
What are alkenes?
Alkenes are a homologous series of unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). They have the general formula CₙH₂ₙ.
What are the first four alkenes and their molecular formulas?
A:
Ethene (C₂H₄)
Propene (C₃H₆)
Butene (C₄H₈)
Pentene (C₅H₁₀)
What are the physical properties of alkenes?
They are non-polar and insoluble in water.
They have low boiling points, increasing with molecular size.
They are more reactive than alkanes due to the presence of the double bond.
How do alkenes react with oxygen Complete combustion)?
Complete Combustion (excess oxygen): Produces carbon dioxide and water.
Example: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
How do alkenes react with oxygen
Icomplete combustion)?
Incomplete Combustion (limited oxygen): Produces carbon monoxide or carbon (soot) and water.
Example: C₂H₄ + O₂ → CO + H₂O
What type of reactions do alkenes undergo?
Hydrogenation
Alkenes undergo addition reactions due to the presence of the C=C double bond.
Hydrogenation (Addition of Hydrogen)
Converts alkenes into alkanes by adding hydrogen gas.
Conditions: Nickel catalyst, 150°C
Example: C₂H₄ + H₂ → C₂H₆ (ethene to ethane)
What type of reactions do alkenes undergo?
Reaction with Halogens (Bromine Water Test)
Reaction with Halogens (Bromine Water Test)
Alkenes react with bromine water, causing an orange to colorless change.
Example: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)
This test distinguishes alkenes from alkanes (alkanes do not react, so bromine water stays orange).
What type of reactions do alkenes undergo?
Reaction with Steam (Hydration Reaction)
Reaction with Steam (Hydration Reaction)
Converts alkenes into alcohols by adding water.
Conditions: Phosphoric acid (H₃PO₄) catalyst, 300°C, 60-70 atm
Example: C₂H₄ + H₂O → C₂H₅OH (ethanol)
What type of reactions do alkenes undergo?
Reaction with Hydrogen Halides
Reaction with Hydrogen Halides
Alkenes react with hydrogen halides (HCl, HBr, HI) to form haloalkanes.
Example: C₂H₄ + HCl → C₂H₅Cl (chloroethane)
Q: What 2 of isomerism do alkenes show?
Structural isomerism
Structural Isomerism: Different connectivity of atoms.
Example: Butene (C₄H₈) has but-1-ene and but-2-ene.
What 2 of isomerism do alkenes show?
Geometric isomerism
Geometric Isomerism: Occurs due to restricted rotation around the C=C bond.
Cis-isomer: Similar groups on the same side of the double bond.
Trans-isomer: Similar groups on opposite sides of the double bond.
Practical: Investigating Alkenes
Bromine Water Test (Test for Unsaturation)
Method: Add bromine water to an alkene and shake.
Observation: Orange bromine water turns colorless.
Positive Result: Confirms the presence of a C=C bond (alkene).
Negative Result: No color change (alkane or saturated compound).
Practical: Investigating Alkenes
Hydrogenation of Alkenes
Hydrogenation of Alkenes
Method: Pass hydrogen gas over ethene with a nickel catalyst at 150°C.
Observation: Ethene is converted to ethane, confirming the reaction.
