Alkenes Flashcards
What are alkenes and their general formula
Unsaturated hydrocarbons (has double bond) CnH2n
Explain the shape of an alkene with its shapes and bond angles
The carbons in the double bond have 3 bonding regions which repel equally so have a bond angle of 120 and form the shape trigonal planar
What is a pie bond
a bond formed by the Sideways overlap of adjacent p-orbital concentrated above and below the sigma bond
Explain the bonding in an alkene
- for each carbon atom of double bond, there are 3 sigma bonds Leaving one electron from the P orbital.
- which Forms a π bond by the sideways overlap of 2 P orbitals one from each carbon atom of double bond.
- The pie bond’s density is concentrated above and below the sigma bond between the carbons. This locks the carbon atoms in position preventing rotation around the double bond unlike alkanes.
what is the difference between a pie bond and a sigma bond
a sigma bond is the result of a direct overlap of orbitals whereas the pie bond is formed from a sideways overlap of p orbitals
what is stereoisomerism and Why does it happen
Stereoisomers have the same structural formula but different arrangement of atoms in space (happens as rotation around double bond is restricted)
What are the states of alkenes
The first 3 are gaseous
the next 14 are liquids
Why are alkenes more reactive than alkanes
because of the pie bonds being more exposed than a sigma bond due to it being concentrated above and below a sigma bond, so it readily breaks
What is hydrogenation
addition reaction of an alkene + hydrogen passed over a nickel catalyst at 423K/150C forming an alkane
What is halogenation
addition reaction: alkene + chlorine/bromine at RTP -> Haloalkane
How do we test for unsaturation
Add bromine water, positive test - it turns orange to colourless as any compound that has a C=C will decolorise bromine
Addition reactions of alkenes with hydrogen halides
Alkenes + gaseous hydrogen halides at RTP -> Haloalkenes.
Hydration of alkenes
Alkene + H2O(g) steam = Alcohols (with an acid catalyst)
What is an electrophilic addition
Where alkene’s take part in addition reactions with an electrophile to form saturated compounds
What is an electrophile
An electron pair acceptor
Why does electrophilic addition happen
The pie bond in the alkene contains two localised electrons above and below the sigma bond between the two carbon atoms causing an area of high electron density, attracting electrophiles
What is a carbocation
The step in the middle of the electrophilic edition where one of the carbons has a positve charge
What is a tertiary carbocation
A carbocation with three carbons attached to the carbon with a positive charge That is very stable and is likely to be the major product
How does Bromine add to an alkene by electrophilic addition
As bromine approaches the pie bond, high electron density of the bond repels electrons in bromine forming an induced dipole so the bromine atom closest to the alkene becomes partially positive and is able to accept an electron pair and react
How do you draw a repeating unit from a monomer
- You draw brackets around the monomer
- opening the double bond out so the lines go through the brackets
- Write a small n after
What is polyethene used for
Plastic bags, shampoo bottles, children’s toys
what is polychloroethene or PVC used for
pipes, flooring, bottles, insulation, Fabric treatments All due to its flexibility and rigidity
What is polyprolene used for
Children’s toys, packaging crates, guttering, windows, fibres of ropes
What is polystyrene used for
Packaging material, food trays and cups due to insulating properties
