Alkenes 4.1.3 Flashcards
What is the general formula for alleges?
CnH2n
How does the double bond affect alleges reactivity, why?
It makes them more reactive, the carbon carbon double bond is a region of high electron density
What two distinct bonds make up the double bond in alkenes? Define each bond.
Sigma bond - overlap of orbitals between C atoms
Pie bond - sideways overlap of p-orbitals, adjacent p-orbitals above and below bonding (4 atoms)
What bond angle is present around carbon carbon double bonds?
120• , only 3 bonding regions.
Define a structural isomer.
The same molecular formula but a different structural formula.
Define a stereoisomer
Molecules with the same structural formula but a different arrangement in space.
What are the two thins that are needed for a stereoisomer to form? What are the two types of stereoisomers and how do they differ?
- Carbon carbon double bond
- Different groups attached to each carbon in the double bond
E - even, the heaviest groups are on opposite sides
Z - same, the heaviest groups are on the same side
How do we name alkenes and when must numbering be introduced?
___ene
Once you get to but. Eg but-2-ene
Why are alkenes more reactive than alkenes?
Because of the relatively low bond Enthalpy of the pie bond.
Write a word equation for the hydrogenation reaction. What are the necessary reagents and conditions for this reaction?
Alkene + H2 ➡️ Alkane
•Hydrogen
•Nickel Catalyst (finely divided)
•High Temp.
When is hydrogenation used in the industry?
Manufacturing of margarine.
Write an equation for halogenation. What are the necessary reagents and conditions for this reaction?
Alkene + X2 ➡️ Haloalkane
•Cl2, Br2, I2
•Room temp
•Non-polar solvent, eg. CCl4
If you added bromine water to a halogenation reaction what colour change would be seen?
Orange/brown to colourless. As the alkene turns to a haloalkane.
During halogenation reactions where do the halogens go in the alkene?
On either side of the double bond.
Write an equation for the addition of hydrogen halides. What are the reagents and the conditions for this reaction?
Alkene + HX ➡️ Haloalkane (mono)
- HCl or HBr or HI
- Room Temperature
- Non-polar solvent e.g CCl4
How does reactivity of the hydrogen halides change from HF to HI? Why?
It increases. Bond strength decreases as they are easier to break therefore they are more reactive.
How are minor and major products formed?
When unsymmetrical alkenes react with hydrogen halides or other non symmetrical molecules.
Write a general word equation for hydration. What are the conditions and the reagents?
Alkene + water ➡️ Alcohol
•Steam (H2O)
600K (Temp>100•)
•60 atm
Phosphoric acid catalyst (H3PO4)
(2 products if an unsymmetrical alkene.
What is the main mechanism that we study for alkenes called?
Electrophilic addition
Define an electrophile
An electron pair acceptor.
What is a carbocation?
A positively charged ion formed during electrophilic addition it is an intermediate molecule.
What is Markovinikovs rule?
When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogen a attached to it already.
How do you decide where to add the H when you react the alkene with compound HX?
Add it to the carbon which has the most H atoms directly attached to it.
Define a polymer
Polymers are long chained molecules made by the joining of many small chained molecules called monomers.
What happens to the bonds in alkenes during addition polymerisation?
A saturated chain is produced as a pie bond is broken and two new sigma bonds are made.
How many types of monomers can addition polymers be made out of?
ONE TYPE OF MONOMER ONLY!!
Draw a polymerisation equation for polyethene.
Look in pictures for the answer
What must you remember when writing out polymer equation and sub units?
Only include n if it is an equation!!
Draw a repeat unit for tetrafluoroethene (CF2CF2)
Look at pictures😊
What is the main problem with addition polymers?
They are generally non-biodegradable so they need to be recycled or reused.
What is thermal recycling?
The polymers are melted down and then used as feedstock for petroleum products.
What is mechanical recycling?
The polymers are processed and used as feedstock for other plastic products.
Give 2 advantages for recycling.
- Reduces the amount of waste sent to landfill
- Reduces energy consumption
- Reduces pollution
- Reduces global warming
- Conserves natural resources
- Promotes sustainable use of resources
Give 2 disadvantages of recycling.
- Not always cost effective
- High initial cost
- Not widespread on a large scale
- Recycling sites may not be safe and hygienic.
What are the three other main alternatives to recycling?
- Incineration - generates energy but it requires high temperatures and releases carbon dioxide
- Biodegradable and photodegradable plastics - these are being modified and produced
- Re-use!
Define an alkene.
An unsaturated hydrocarbon with a C=C double bond.
