Alkenes Flashcards
Key points about alkenes
Alkenes are unsaturated hydrocarbons General formula is CnH2n. Alkenes contain a carbon- carbon double bond
somewhere in their structure.
Numbers need to be added to the name when positional isomers can
occur.
The arrangement of bonds around the
>C=C< is planar and has the bond angle 120o
The C=C bond in alkenes
C=C double covalent bond electron density. consists of one sigma (σ)
bond and one pi (π) bond.
π bonds are exposed and have high electron density.
They are therefore vulnerable to attack
by species which ‘like’ electrons: these
species are called electrophiles.
Stereoisomerism
Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms.
E-Z stereoisomers arise when:
(a) There is restricted rotation around the C=C double bond.
(b) There are two different groups/atoms attached both ends ont
the double bond
Z isomers
Cis isomers
Same end
(Top or bottom)
E isomers
Trans
Diagnal
Naming E-Z isomerism
First determine the priority groups on both sides of the double bond
Priority group: The atom with the bigger
atomic number is classed as the priority atom
If the priority atom is on the same side of the
double bond it is labelled Z from the german
If not its the E
Electrophilic addition reactions of alkenes
Definition Electrophile: an electron pair acceptor
The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions.
Ad dition reaction: a reaction where two
molecules react together to produce one
Reaction of bromine with alkenes
Change in functional group: alkene —> dihalogenoalkane
Reagent: Bromine
Conditions: Room temperature (not in UV light)
Mechanism: Electrophilic addition
Type of reagent: Electrophile, Br+
Reaction of bromine with alkenes
Change in functional group: alkene —> dihalogenoalkane
Reagent: Bromine
Conditions: Room temperature (not in UV light)
Mechanism: Electrophilic addition
Type of reagent: Electrophile, Br δ+
Further explanation of bromine with alkenes
As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This
induces a dipole. Br2 becomes polar and electrophilic (Brδ+).
The intermediate formed, which has a
positive charge on a carbon atom is called a
:Br -
carbocation
Reaction of hydrogen bromide with alkenes
Change in functional group: alkene —>halogenoalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophilic addition
Type of reagent: Electrophile, Hδ+
Further explanation of the reaction of hydrogen bromide with alkenes
HBr is a polar molecule because Br is more electronegative than H. The H δ + is attracted to the electron-rich pi bond.
This reaction can lead to two products when the alkene is unsymmetrical.
If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products
What is the major product in electrophilic addition
Electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.
This carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron releasing and reduce
the charge on the ion which
stabilises it.
What is the order of stability for carbocations
Thee order of stability for carbocations is
tertiary > secondary >primary
Reaction of sulfuric acid with alkenes step 1
Change in functional group
alkene —> alkyl hydrogensulfate
Reagents: concentrated H2SO4
Conditions: room temperature
Mechanism: Electrophilic addition
Type of reagent: Electrophile, H2SO4
CH2=CH2 + H2SO4 —> CH3CH2OSO2OH
Reaction of sulfuric acid with alkenes stage 2
Change in functional group
alkyl hydrogensulfate —> alcohol
Reagents: water
Conditions: warm mixture
Type of reaction: hydrolysis
CH3CH2OSO2OH + H2O —> CH3CH2OH + H2SO4
Direct industrial hydration of alkenes to form alcohols
Industrially alkenes are converted to alcohols in one step rather than the
two in the above sulfuric acid reaction. They are reacted with water in the
presence of an acid catalyst.
This reaction can be called hydration: a reaction where
water is added to a molecule
Essential conditions and equation for hydration of alcohols to form alcohols
High temperature 300 to 600°C
High pressure 70 atm
Catalyst: concentrated H3PO4
CH2=CH2 (g) + H2O (g) —> CH3CH2OH (l)
The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however,
as there are no waste products and so has a high atom economy. It would also mean separation of products
is easier (and cheaper) to carry out. See equilibrium chapter for more on the industrial conditions for this
reaction.
Testing for alkenes with bromine water
Bromine water decolourises in the presence of a double bond. This can be used as a test for the presence of an double bond in a molecule. It can be used quantitatively to show the presence of multiple double bonds in compounds like polyunsaturated oils.
Addition polymers
Addition polymers are formed from alkenes
This is called addition polymerisation
Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds
Polychloroethene
Poly(chloroethene) is a polymer that is water proof, an electrical insulator and doesn’t react with acids.
In its pure form it is a rigid plastic due to the strong intermolecular bonding between polymer chains prevents them moving over each other. In this un-plasticised form it is used make uPVC window frame coverings and guttering.
If a plasticiser is added the intermolecular forces are weakened which allows the chains to move more easily, resulting in more flexibility in the polymer. In this form PVC is used to make insulation on electrical wires, and waterproof clothing.
