17:Alkenes Flashcards
What is a sigma bond (σ-bond)
2 s orbitals, each with 1 electron, overlap to form a sigma bond
or
2 p orbitals, each with 1 electron overlap end-over-end
What is a pi bond (π-bond)
2 p orbitals with 1 electron overlap sideways
Why is rotation not possible around a double bond
Rotation would destroy the sideways overlap of the p-orbitals and hence break the π-bond
What is the general formula for alkenes
Cn H₂n
Are alkenes saturated or unsaturated
unsaturated
Definition of stereoisomers
Compounds with the same structural formula but with a different arrangement in space
How do you assign priority in stereoisomers
Whichever atom bonded to the carbon has the higher atomic number.
If they are the same then go to the next atom until priority is assigned
Where are the higher priority groups in a Z-isomer
On the same side
Where are the higher priority groups in a E-isomer
On opposite sides
What does Cis mean when naming stereoisomers
two identical groups are on the same side
What does trans mean when naming stereoisomers
two identical groups are on opposite sides
What other name is dehydration called
elimination
What conditions are needed for dehydration of alcohols
Heating in the presence of an acid catalyst (usually concentrated sulfuric acid)
What happens when you add hydrogen to an alkene in the presence of a nickel catalyst
It turns into the corresponding alkane
What conditions are needed for addition of hydrogen to alkene reaction
Nickel catalyst and at (typically) 60℃
What conditions are needed for addition of halogens to alkene
Alkenes will react readily with chlorine and bromine at room temperature and in the dark
What observations will there be for addition of bromine to an alkene
The orange bromine solution will immediately decolourise
How do you test for unsaturation (alkenes)
Add bromine water.
Bromine water is orange. When it is added to an alkene, the solution will immediately become colourless
What happens when hydrogen halides are added to alkenes
Turn into a haloalkane
What happens when steam is added to an alkene
Turns into an alcohol
What conditions are needed for addition of steam to an alkene
Acid catalyst
most commonly phosphoric acid
What is the name of the mechanism for addition reactions for alkenes
Electrophilic addition
What are the 3 steps of the mechanism of adding a halogen to an alkene
1) Polarisation of the halogen
2) Formation of the intermediate
3) The carbocation intermediate rapidly reacts with the bromide ion, forming the product
What is an electrophile
An electron pair acceptor
What is heterolytic fission
Both electrons stay with one atom and the other atom retains neither
What is a carbocation
The organic intermediate formed in electrophilic addition, it is a positive ion with the charge on carbon
What is the order of stabilities of carbocations
Tertiary > secondary > primary
What is Markownikoff’s rule
The major product of the addition reaction will be the one formed via the more stable carbocation intermediate
What is a polymer
A long molecule formed by chemically linking together many smaller molecules, called monomers
What is the bracketed fragment of a polymer chain called
The repeat unit
What are the advantages of combustion of polymers
They are good fuels, like other hydrocarbons, and burning them releases energy (which can be used for heat or electricity generation) and reduces demand for landfill sites.
What are disadvantages of combustion of polymers
Toxic combustion products may be formed. This is particularly an issues for chlorinated plastics such as PVC, because toxic, acidic hydrogen chloride is formed when this burns
What are 3 sustainable uses for polymers
Combustion
Feedstock for production of new plastics and other organic chemicals
Biodegradable and photodegradable polymers
