Alkenes Flashcards
(29 cards)
What are Alkenes?
They are unsaturated hydrocarbons containing 1 or multiple C=C bonds, which contains a pi-bond
What are pi-bonds?
They are a sideways overlap of adjacent p-orbitals above and below the bonding C atoms and a sigma bond
- restricted rotation around it
What is a sigma bond?
An overlap of orbitals directly between bonding atoms
What is the shape and bond angle around each C in the C=C of alkenes?
- Trigonal planar
- 120º
What are stereoisomers?
Compounds with the same structural formula but different arrangement in space
What is E/Z isomerism?
- Needs restricted rotation around C=C and 2 different groups attached to each C atom of the C=C group
- They have restricted rotation due to the pi-bonds electron density above and below the plane of the sigma bond
What is Cis-trans isomerism?
When 2 of the substituent groups attached to each carbon atom of the C=C group are the same
What are the CIP rules?
- Attached groups are prioritised by atomic number
- If groups of higher priority are on the same side of the double bond, the compound is a Z isomer
- If the groups of higher priority are on opposite sides of the double bond, the compound is a E isomer
What are the properties of alkenes?
-Low bond enthalpy
- Pi-bond position
How does low bond enthalpy effect an alkene?
Alkenes contain C=C double bonds which consist of a pi-bond and a sigma bond. The pi-bond has a lower bond enthalpy than the sigma bond therefore, it breaks more readily allowing alkenes to react more easily
How does the pi-bonds position effect an alkene?
There is concentrated electron density above and below the sigma bond and the pi-bonds are more exposed meaning it is more open to electrophile attacks
How can alkanes be formed from alkenes?
Hydrogenation
What is the process of hydrogenation?
- Mix the alkene with hydrogen
- Add a nickel catalyst at 423K
- Addition reaction occurs to form an alkane
- One mole of hydrogen is required per double bond
How can you test for unsaturation in a carbon chain?
Bromination
What is the process of bromination?
- Alkenes react with Cl or Br at room temperature
- Add bromine water drowse to a sample
- Bromine adds across the double bond
- Orange to colourless= C=C bond
- Unsaturated carbon chains decolourise bromine water
How haloalkanes form from alkenes?
- They react with gaseous hydrogen halides at room temperature
- If the alkene is a gas, 2 gases are mixed
- If the alkene is a liquid, the hydrogen halide is bubbled through it
- It can also react with concentrated HCl or Hbr
- Unsymmetrical alkenes react with hydrogen halides to give 2 possible products
How can alcohols be formed from alkenes?
Hydration
What is the process of hydration?
- React alkenes with steam, H20
- Occurs with phosphoric acid catalyst
- Steams adds across the double bond
- There are two possible products
What happens in electrophilic addition of a hydrogen halide?
What happens in electrophilic addition of a halogen?
What is Markownikoff’s rule?
- 2 isomeric products formed if unsymmetrical alkenes react with hydrogen halides
- Hydrogen of a hydrogen halide attaches to a carbon atom with more hydrogen atoms and less carbon atoms
- Electrophilic addition forms 2 possible carbocations either primary or secondary
- Primary carbocations have a positive charge on a carbon atom at the end of the chain
- Secondary carbocations have a positive charge on a carbon atom with the 2 carbon chains attached
What are polymers?
Large molecules formed from many monomers
What is an electrophile?
A species that are electron pair acceptors
What is an addition polymer?
Many mongers bonded together via rearrangement of bonds without the loss of any atom or molecule
