4.1.3 Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons that contain at least one C=C bond made up of a pi bond and a sigma bond
What is the general formula of alkenes?
Cn H2n
How is a pi bond formed?
Electrons in the adjacent p orbitals overlap above and below the carbon atoms. They can only be made after a sigma bond is formed
What bond restricts the rotation of carbon atoms?
pi 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
This is due to high electron density if double bond and the fact that the pi bond is slightly easier to break
What intermolecular forces of attraction do alkenes have?
Only london forces due to non-polar bonds
Are they soluble in water?
Why?
No
Non polar bonds
Write an equation for the complete combustion of pent-2-ene
CH3CH=CHCH2CH3 + 1.5 O2 –> 5CO2 + 5H2O
What are the types of isomers that can be formed using alkenes?
EZ isomers
due to restricted rotation
Cis-trans isomers
if two of the same substituent 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?
A tertiary carbocation
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 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 product(s) of the hydration reaction?
An alcohol
Draw a mechanism for the addition of water to ethene
mechanism
What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?
Hydrogen halide gases must be at room temperature
Draw a mechanism for the reaction of HBr and ethene
mechanism
What is the reaction called when a halogen is added to alkene?
Halogenation
How does a molecule with a non polar bond react if it is an electrophile?
C=C double bond with high electron density induces a temporary dipole in the halogen molecule
Delta positive atom is attracted to the double bond
Draw a mechanism for the reaction between bromine and ethene
mechanism
How can an alkene be converted into an alkane?
What is the reaction called?
What are the required conditions?
Alkene + hydrogen = alkane
Hydrogenation
Conditions: 150 degrees, 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
Draw how you would represent the polymerisation of ethene
drawing
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 do photodegradable polymers break down?
They are broken down chemically using energy with wavelengths similar to light
Once the break down begins, it is not possible to stop the process
Explain what happens in organic feedstock
Plastics are separated and broken down into small organic molecules through a series of reactions. The molecules can then be used to produce plastics and in other industries
Give a disadvantage of photodegradable polymers
May not be exposed to sufficient light
