6.3 Alkenes Flashcards
Describing, bonding, electrophillic addition + mechanism, tests, polymers
What is a 𝜋 bond?
Overlap of p orbitals sideways above and below the 𝜎 bond.
What are alkenes?
Unsaturated hydrocarbons that contain at least one C=C bond made up of a 𝜋 bond and a 𝜎 bond
What bond restricts the rotation of carbon atoms?
𝜋 bond
What is the angle and shape of a double bond?
Trigonal planar
120°
What is the qualitative test for alkanes?
This tests for C=C double bond using bromine water. If the alkane is present bromine water decolourises.
Are they more or less reactive than alkanes? Why?
More reactive due to high electron density of double bond and the fact the pi-bond is slightly easier to break
What are the types of isomers that can be formed using alkenes?
E/Z isomers - due to the restricted rotation
Cis-trans isomers - if two of the same substituents are attached to each carbon
What is an electrophile?
Species that are electron pair acceptors
What is the most stable type of carbocation intermediate? Why?
Alkyl groups have a positive inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms i.e. A tertiary carbocation
Major products will be formed from which kinds of carbocations?
Tertiary (or the most stable available)
What conditions are needed for the electrophilic addition of H 2
O 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 product(s) of the hydration reaction?
An 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 alkene?
Halogenation
How does a molecule with a non-polar bond react as if it is an electrophile?
C=C double bond with a high electron density induces a temporary dipole in the halogen molecule → δ+ atom attracted to double bond
How can an alkene be converted into alkane? What is the reaction called and what are the required conditions?
Alkene + hydrogen = Alkane
Hydrogenation
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 which combine to form a polymer
Usually have a C=C bond which breaks to leave a repeating pattern
What are the ways in which plastics can be disposed? (6)
● Landfill
● Combustion
● Electricity generation
● Reuse
● Recycle
● Organic feedstock
What are the disadvantages of recycling?
● Plastics must be sorted into different types
● Expensive
● Labour intensive
● Requires high technology
How can chemists limit the problems caused by polymer disposal?
- Developing biodegradable polymers
- Removing toxic waste gases caused by incineration of plastics
Explain what happens in organic feedstock
Plastics are separated and broken down into small organic molecules through a series of reaction. The molecules can then be used produce plastics and in other industries
