4.1.3 - Alkenes
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What are alkenes?
Unsaturated hydrocarbons that contain at least one C=C bond made up of a pi bond and a sigma bond
What is the general formula of alkenes?
Cn H2n
How is a pi bond formed?
Electrons in the adjacent p orbitals overlap above and below the carbon atoms. They can only be made after a sigma bond is formed
What bond restricts the rotation of carbon atoms?
pi bond
What is the angle and shape of a double bond?
Trigonal planar 120
Are they more or less reactive than alkanes? Why?
More reactive due to high electron density of double bond and the fact that the pi bond is slightly easier to break
What intermolecular forces of attraction do alkenes have?
Only london forces due to non polar bonds
Are they soluble in water?
No, non-polar bonds
Write an equation for the complete combustion of pent-2-ene
CH3CH=CHCH2CH3 + 7.5 O2 -> 5CO2 + 5H2O
What are the types of isomers that can be formed using alkenes
E/Z isomers - due to the restricted rotation Cis-trans isomers - if two of the same substituents at=re attached to each carbon
What is an electrophile?
Species that are electron pair acceptors
What is the most stable type of carbocation intermediate? Why?
Alkyl groups have a positive inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms i.e. A tertiary carbocation
Major products will be formed from which kinds of carbocations?
Tertiary (or the most stable available)
What conditions are needed for the electrophilic addition of H2O to an alkene? What is this type of reaction called?
Steam in the presence of an acid catalyst, using phosphoric acid Reaction is called hydration
What are the product (s) of the hydration reaction?
An alcohol
Draw a mechanism for the addition of water to ethene
mechanism
What conditions are needed for the electrophillic addition of a hydrogen halide to an alkene?
Hydrogen halide gases must be at room temperature
Draw a mechanism for the reaction of HBr and ethene
mechanism
What is the reaction called when a halogen is added to alkene
Halogenation
How does a molecule with a non-polar bond react as if it is an electrophile?
C=C double bond with a high electron density induces a temporary dipole in the halogen molecule Delta + atom attracted to double bond
Draw a mechanism for the reaction between bromine and ethene
Mechanisms
How can an alkene be converted into alkane? What is the reaction called and what are the required conditions?
Alkene + hydrogen = alkane Hydrogenation Conditions = 150 degrees celcius and nickel catalyst
What is an addition polymer?
Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
What are monomers? What form do they usually take?
Molecules which combine to form a polymer Usually have a C=C bond which breaks to leave a repeating pattern