4.1.3 Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons with at least 1 C=C double bond.
General formula of alkenes
CnH2n
Reactivity of alkenes
High as the double bond is an area of high electron density
Nature of the C=C double bond
Formed of 1 sigma bond made from the direct overlap of two orbitals and 1 pi bond formed from the sideways overlap of two orbitals. This pi bond means that the double bond cannot rotate.
Geometry around C atom of ethene
Trigonal planar with H-C-H bond of 118 and H-C-C bond of 121. The double bond has higher electron density than the single bonds so repels slightly more giving a larger bond angle.
Method to identify alkene
Addition of bromine water. The bromine water is decolourised. It reacts with the alkene to form a saturated dibromoalkane.
Definition of stereoisomer
Compounds with the same structural formula but a different arrangement of atoms in space.
Reason for E/Z isomerism
For a molecule to have E/Z isomers it must have a C=C double bond as, C=C bond cannot rotate. Each C in the double bond must have 2 different groups attached.
How to identify E and Z isomers
Z has the priority groups on the Zame Zide.
E are Enemies on opposite sides
CIP rules
The atom with the higher Ar is given the higher priority e.g. Br above Cl. A longer chain has priority over a shorter chain.
Cis-trans isomerism
Special case of E/Z isomerism where H is attached to each C of the C=C. Cis for same side and trans for different sides.
Type of reactions alkenes undergo
Electrophilic Addition
Name of reactions with hydrogen, conditions and example
- Hydrogenation
- Nickel catalyst
3a. manufacture of margarine from unsaturated vegetable oils
3b. ethene + hydrogen –> ethane
Product of reaction with hydrogen halide
haloalkane
Product of reaction with halogen
di-substituted haloalkane
Conditions and reaction for forming ethanol from ethene
1) steam and phosphoric acid catalyst.
2) ethene + steam –> ethanol
Type of reaction, conditions for forming diols and equation for reaction on ethene
1) oxidation reaction
2) Cold, dilute and acidified potassium manganate
3) C2H4 + H2O + [O] –> CH2(OH)CH2(OH)
Electrophile definition
electron pair acceptor
Mechanism for reaction of propene with bromine
see notes
Mechanism for reaction of propene with hydrogen bromide
see notes
Markownikow’s rule
The major product in a reaction will always be the one formed via the more stable carbocation intermediate. Tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations.
Definition of addition polymers
When short chain monomers add together to form one larger polymer made of regularly repeating units.
Problems associated with raw materials for addition polymerisation
large amounts of energy and resources needed.
polymers are made from alkenes, which are obtained from non-renewable crude oil.
Extraction and cracking of crude oil are both high energy processes and more energy is needed for polymerisation.
Reason for low reactivity of addition polymers
Unreactive hydrocarbon chains due to multiple strong, non-polar covalent bonds in the chains.
Are addition polymers biodegradable?
Addition polymers are non-biodegradable due to strong, non-polar, covalent bonds.
How are addition polymers disposed of?
They mostly go to landfill.
Some can be recycled .
Some are used as organic feedstock for production of other plastics and organic chemicals.
Some are combusted to produce energy. Combustion can release toxic gases
