3.3.4 Alkenes Flashcards
What is an alkene?
An unsaturated hydrocarbon
It is a homologous series of hydrocarbon.
What are the characteristics of alkenes?
They are hydrocarbons, so consist of hydrogen and carbon atoms only
They are unsaturated so all have a C=C double bond which is the functional group
The double bonds have high electron density which makes them reactive
They have the general formula CnH2n (if there is only one double bond).
What is the different feature of cycloalkenes?
Different general formula as they have 2 fewer hydrogens.
What is the test and result for alkenes?
Bromine water test
Add bromine water and shake
If an alkene is present, the solution will turn from brown to colourless.
This is an example of electrophilic addition.
By what reaction mechanism can alkenes be formed?
Nucleophilic elimination (of halogenoalkanes).
What is a sigma bond?
When electron orbitals directly overlap.
What is the shape and bond angles of alkene molecules?
Planar
120*
What type of bonding is in the double C=C bond of an alkene and why?
One sigma bond and one pi bond.
One sigma bond forms but carbons still have one electron each in a p-orbital.
P-orbitals lie above and below the plane of the molecule, and overlap sideways to form a pi bond.
Why does stereoisomerism occur in alkenes?
Pi bond (in the double C=C bond) cannot rotate so E-Z isomerism occurs.
Why are alkanes fairly reactive?
The pi bond has a low bond enthalpy so requires little energy to overcome.
2 electrons in sigma bond and 2 electrons in pi bond means the C=C bound has a high electron density, making it reactive.
What is electrophilic addition of alkenes?
When alkenes are attacked by electrophiles due to their double bond. The C=C bond becomes saturated to form a C-C bond instead.
What is an electrophile and hwy is it attracted to the double carbon bond?
Electron pair acceptor, so is deficient in electrons (therefore attracted to the C=C double bond as it is high density).
They can be positive ions or polar molecules.
What is the test for alkenes?
Bromine water
Turns from orange/brown to colourless.
How does the bromine test for alkenes work?
Bromine is an electrophile and adds to the alkene (electrophilic addition) to form a dibromoalkane, which are colourless.
What is the reagent and conditions for electrophilic addition of alkenes using bromine?
Aqueous bromine
What is the reagent and conditions for electrophilic addition of alkenes using hydrogen halides?
Gaseous hydrogen halide
What is the reagent and conditions for electrophilic addition of alkenes using sulfuric acid?
Cold, concentrated sulfuric acid
Why is a dipole induced in bromine during electrophilic addition?
Electrons in the double bound repel electrons in bromine, so they shift to one end of the molecule creating an induced dipole.
What is the intermediate formed in electrophilic addition of alkenes?
Carbocation
What is formed in electrophilic addition of alkenes using hydrogen halides?
Halogenoalkanes
Why are 2 different products produced when unsymmetrical alkenes undergo electrophilic addition?
The amount of each of the two products formed is determined by the stability of the carbocation intermediate.
The more alkyl groups bonded to the carbocation intermediate, the more stable it is.
This is because the alkyl groups push electrons towards the electron deficient carbocation to stabilise it.
What is the order of stability in primary, secondary and tertiary carbocation intermediates?
Primary are the least stable, then secondary, and tertiary are the most stable.
What are primary, secondary and tertiary carbocations?
Primary- one alkyl group attached to the carbocation
Secondary- two alkyl groups attached to the carbocation
Tertiary- three alkyl groups attached to the carbocation
Which type of carbocation intermediate is most likely to form the products in electrophilic addition?
Tertiary, as it is the most stable
What is the minor and major products of electrophilic addition?
Major- formed from the more stable carbocation, so more of it formed
Minor- formed from the less stable carbocation, so less of it formed
What is formed when sulfuric acid reacts with alkenes by electrophilic addition?
Alkyl hydrogen sulfates
What does sulfuric acid do in making an alcohol from an alkene?
Acts as a catalyst
How are alcohols made by the electrophilic addition of alkenes and sulfuric acid?
An alkyl hydrogen sulfate is formed.
This acts as the intermediate, and reacts with water by hydrolysis, to form an alcohol and reform sulfuric acid.
What are the conditions for making alcohol from alkenes using sulfuric acid?
1- cold concentrated sulfuric acid
2- add water and reflux
How does water react with alkyl hydrogen sulfates to form alcohols?
An OH from water is added to form an alcohol, and the remaining H is used to reform sulphuric acid.
What is a polymer?
A long chain molecule made up of many repeating units called monomers.
They can be natural or synthetic
What are the monomers used to form addition polymers?
Alkenes
When were polymers first developed and how have they developed since then?
First used in the 1800s and have been developed to revolutionise our standard living. New polymers are being synthesised all the time with new properties and for new uses.
What is an addition polymer and how are they formed?
When no other substances but the polymer are formed in the reaction.
Many monomers combine to form one large molecule.
The double bond is broken and opened to join to another molecule.
What are the conditions for addition polymerisation?
High temperature
High pressure
Catalyst
What are the properties of poly(alkenes)?
Saturated
Non-polar, and hence unreactive (except with halogens added eg. Polychloroethene)
Do not degrade well in landfill
Why is it important for their physical properties that poly(alkenes) are non polar?
They only have van-der-waals forces between chains
This means that the longer the chain and the closer they are to each other, the more van der waals forces there are.
This means shorter polymers with more branching are more flexible and weaker than those that are long and have little branching.
Why do poly(alkenes) with halogens have different properties?
Are polar so form stronger permanent dipole-dipole forces and so have different properties.
What is a plasticiser?
Added to polymers to change their properties
How do plasticisers change the properties of polymers?
They slide between the polymer chains and push them apart.
This weakens the intermolecular forces between the chains
Means chains can slide over each other more so makes the polymer more flexible/easily bendable.
What is PVC and what is is used for?
Polyvinylchloride or PVC - correctly named poly(chloroethene)
A polymer made from long closely packed polymer chains that are hard but brittle.
Used in drain pipes
Add plasticiser to make more flexible
Used fort electrical cable insulation and clothing
