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Reactions of alkenes (Reaction with bromine/ halogen (Terms (Electrophile-…
Reactions of alkenes
Reaction with bromine/ halogen
Forms a dihalogenoalkane
Terms
Electrophile- species which are electron deficient and have a positive or slightly positive charge
Carbocation- a positive carobn
Two headed arrow- movement of 2 electrons
Heterolytic fission- unequal bond breaking
C2H4 + Br2 -> C2H4Br2 (1,2-dibromoethane)
Mechanism
Br- attacks/bonds to the carbocation
Heterolytic bond breaking of Br2 occurs- Br- is formed
E- pair from pi bond is used to form a new bond with the slightly positive Br (electrophile)
The pi bond polarises Br2 as it approaches (pi e- repels e- in Br2)
Test for C=C double bond- orange to colourless
Addition reaction
Oxidation with KMnO4 and H2SO4
C2H4 -> C2H6O2 (ethane 1,2 diol)
KMnO4 is the oxidising agent and it goes above the arrow (along with H2SO4)
Observation- purple to colourless
Forms a diol
Test for alkene double bond
Addition of H2 (hydrogenation)
Uses a nickel catalyst
Saturates alkenes by removing some of the double bonds
C2H4 + H2 -> C2H6
Uses- to make margarine from veg oils
Hydration
330 degrees and a high pressure
Uses a phosphoric acid catalyst (H3PO4)
C2H4 + H2O -> C2H5OH
Uses- industrial ethanol which is used as a solvent
Electrophilic addition of hydrogen halides (HBr)
C2H4 + HBr -> C2H5Br
Mechanism- the slightly positive hydrogen latches onto the pi bond and breaks the double bond. Heterolytic fission occurs and the Br get the 2 e- and becomes Br-. It is attracted to the carbcation and bonds to it forming a halogenoalkane
HBr is already slightly positive and slightly negative
Markovnikov's rule- when there are 2 possible products (due to uneven alkene) the H adds onto the C in the C=C with the most H attached to it. This leads to the formation of a more stable product
Combustion
Incomplete combustion
Not good fuels
Burns with a sooty flame