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
