3) Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons containing a C=C bond comprising a π-bond and a σ-bond.
What is a π-bond?
Sideways overlap of adjacent p-orbitals above and below the bonding C atoms.
What is the shape and angle around a double bond?
- There are three regions of electron density around each carbon atom.
- The three repel as far as possible, so the bond angle created is 120º.
- All of the atoms are in the same plane.
Explain what is meant by the term stereoisomers.
Compounds with the same structural formula but with a different arrangement of atoms in space.
What 2 conditions must a molecule satisfy in order to have E/Z isomerism?
- A C=C double bond.
- Different groups attached to each carbon atom of the double bond.
What are the Cahn-Ingold-Prelog priority rules to identify the E and Z stereoisomers?
- If the group of higher priority are on the same side of the double bond, the compound is the Z isomer.
- If the groups of higher priority are diagonally placed across the double bond, the compound is the E isomer.
Why are alkenes more reactive?
- Because of the presence of the π-bond.
- π-bond is weaker than a σ-bond, therefore, is broken more readily.
What 4 things do alkenes have addition reactions with?
- Hydrogen.
- Halogens.
- Hydrogen Halides.
- Steam, in the presence of an acid catalyst.
What is made when alkenes react with Hydrogen? Give a suitable catalyst.
- Alkanes are made.
- Nickel catalyst.
What is made when alkenes react with halogens?
Dihaloalkanes.
What is the test for a C=C double bond?
Any compound containing a C=C double bond will turn bromine water orange to colourless.
What do alkenes react with hydrogen halides to form?
Haloalkanes.
What is made when alkenes react with steam? Give a suitable catalyst.
- Alcohols are formed.
- H3PO4 catalyst, phosphoric acid.
Use the ‘Alkenes’ card to test the knowledge of the hydrogenation of propene.
Rate knowledge 1-5.
Define what is meant by an electrophile.
An electron pair acceptor.
What are curly arrows used to show in reaction mechanisms?
The movement of a pair of electrons.
Use the ‘Alkenes’ card to test the knowledge of electrophilic addition.
Rate knowledge 1-5.
State the type of bond fission involved in the electrophilic addition.
Heterolytic fission.
Describe three ways in which chemists can reduce this environmental damage caused by the disposal of polymers.
- Develop biodegradable polymers.
- Develop ways of making polymers from plant-based substances.
- Designing processes with high atom economy.
Once polymers have been used, they become waste. Outline two ways that waste polymers are processed usefully, rather than just dumped in landfill sites.
- Separation into types and recycling.
- Combustion for energy generation.
How does the structure of trans-hex-3-ene differ from that of cis-hex-3-ene?
H’s are diagonal to each other in the trans.
Explain why both cis and trans hex-3-ene react with Br2 to produce the same structural isomer.
The product is saturated hence there is no restricted rotation.
State two difficulties in the disposal of polymers like poly(propene).
- Non-biodegradable.
- Produces toxic fumes when burnt.
State the problem associated with the combustion of polymers such as PVC.
Toxic fumes are produced.
