4.1.3 Alkenes Flashcards
What are alkenes?
- Unsaturated hydrocarbons that contain at least one C=C bond
- C=C bond is made up of a sigma (σ) bond and a pi (π) bond
What is the general formula of alkenes?
- CnH₂n
How is a pi (π) bond formed?
- Electrons in the adjacent p orbitals overlap above and below the carbon atoms
- Pi bonds are only formed after a sigma bond is formed
What bond restricts the rotation of carbon atoms?
- Pi (π) bond
What is the angle and shape of a double bond?
- Trigonal planar
- 120°
Are alkenes more or less reactive than alkanes? Why?
- More reactive
- Due to the high electron density of the double bond
- Pi bonds are slightly easier to break
What intermolecular forces of attraction do alkenes have?
- Only London forces due to non-polar bonds
Are alkenes soluble in water? Why?
- No, because they have non-polar bonds
Write an equation for the complete combustion of pent-2-ene.
- C₅H₁₀ + 7½O₂ -> 5CO₂ + 5H₂O
What are the types of isomers that can be formed using alkenes?
- E/Z isomers (due to restricted rotation)
- Cis-trans isomers (if two of the same substituents are attached to each carbon)
What is an electrophile?
- A species that accepts an electron pair
What is the most stable type of carbocation intermediate? Why?
- Tertiary carbocation
- Alkyl groups have a positive inductive effect, stabilizing the carbocation
Major products will be formed from which kinds of carbocations?
- Tertiary (or the most stable available carbocation)
What conditions are needed for the electrophilic addition of H₂O to an alkene? What is this type of reaction called?
- Steam in the presence of an acid catalyst (usually phosphoric acid)
- Reaction is called hydration
What are the products of the hydration reaction?
- Alcohol
What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?
- Hydrogen halide gases must be at room temperature
What is the reaction called when a halogen is added to an alkene?
- Halogenation
How can an alkene be converted into an alkane? What is the reaction called and what are the required conditions?
- Reaction: Hydrogenation
- Alkene + hydrogen -> alkane
- Conditions: 150°C, nickel catalyst
What is an addition polymer?
- Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
What are monomers? What form do they usually take?
- Molecules that combine to form a polymer
- Usually have a C=C bond that breaks to leave a repeating pattern
What are the ways in which plastics can be disposed?
- Landfill
- Combustion
- Electricity generation
- Reuse
- Recycling
- Organic feedstock
What are the disadvantages of recycling plastics?
- Plastics must be sorted into different types
- Expensive
- Labour intensive
- Requires high technology
How do photodegradable polymers break down?
- Broken down chemically using energy with wavelengths similar to light
- Once the breakdown begins, it cannot be stopped
Explain what happens in organic feedstock.
- Plastics are separated and broken down into small organic molecules through reactions
- These molecules are used to produce plastics or in other industries
Give a disadvantage of photodegradable polymers.
- May not be exposed to sufficient light
