Alkenes Flashcards
General formula
CnH2n
Are there double bonds?
Yes
Describe bonding
Sigma
Pi
Single
Double
Sigma bonds - covalent bonds formed when electron orbitals overlap end on
Pi bonds - covalent bonds formed when electron orbitals overlap sideways
Single bonds = sigma
Double bonds = sigma and pi
Are sigma or pi bonds weaker and why
Pi bonds are weaker due to it being electron rich and making electrons more accessible and easier to react with
Name the 5 addition reactions of alkenes
Hydrogenation
Halogenation
Hydration
Hydrogen halides
Oxidation to diols
Hydrogenation
Conditions
Word and symbol equation example
Use
Nickel catalyst
Ethene + hydrogen —> ethane
C2H4 + H2 —> C2H6
Manufacture of margarine - vegetable oils with double bonds turned to single bonds making margarine
Halogenation
Conditions
Word and symbol equation example
Presence of Br, Cl, F or I
Ethene + bromine —> dibromoethane
C2H4 + Br2 —> C2H4Br2
Hydration
Conditions
Word and symbol equation example
Phosphoric acid used as a catalyst
Ethene + water (steam) —> ethanol
C2H4 + H2O —> C2H5OH
Hydrogen halides
Conditions
Word and symbol equation example
Done with bromine or chlorine
Ethene + hydrogen bromide —> bromoethane
C2H4 + HBr —> C2H5Br
Oxidation to diols
Type of reactions
Conditions
How the reaction works
Equation
Colour change
Addition & oxidation
Oxidising agent is potassium manganate in acidic conditions (dilute sulphuric acid)
Potassium manganate provides an oxygen atom and water provides 2 hydrogen atoms and an oxygen added across the double bonds
CH2=CH2 + [o] + H2O —> CH2OH-CH2OH
Purple to colourless
Mechanism of electrophilic addition explanation
High electron density makes alkene attractive to electron deficient molecules
Hydrogen bromide is polar as bromine is more electronegative
Hydrogen bromide and ethene react the molecule breaks and both electrons in the H-Br bond go to one atom (heterolytic fission
As hydrogen bonds to the C 2 ions are formed
Carbon becomes positively charged making it a carbocation and bromine is negative causing them to attest and form a covalent bond.
Electrophilic addition of halogens process
Very similar apart from the bromine wont have a polar bond
Electrons in the C=C pi bond repel the electrons in the Br-Br bond and induce polarity on the molecule
Bromine water test
Alkenes turn colourless in bromine water
If unsymmetrical molecules are attacking the C=C…
there are 2 possible products
Markovnikov rule definition
Is it major or minor
Why does this happen
The hydrogen becomes attached to the carbon with the most hydrogens attached to it already
- Major
A carbocation in which the positive charge can be spread over more atoms is more stable
