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ALKENES (Reactivity of alkenes (the single pi bond is easy to break, the…
ALKENES
Reactivity of alkenes
the single pi bond is easy to break
the pi electrons act as a source of electrons, Lewis base
more reactive than alkanes
breaking of pi bond can be converted into 2 sigma bonds
Preparation of alkene
Dehydration of alcohols
ii. ionization
iii. proton abstraction
i. protonation of hydroxyl group
major product= determined by Saytzeff rule
Dehydrohalogenation of haloakenes /alkyl halides
NaOH/ KOH,reflux,ethanol
Products ;-alkene, allkyl halide
Dehalogenation of dihalides
NaI/ Zn+ acetic acid
Products;-alkenes,,
Reduction of alkynes
alkynes to cis-alkenes
alkynes to trans-alkenes
Structure of alkene
unsaturated hydrocarbon with C=C
has general formula of CnH2n
has 1 sigma bond and 1 pi bond
Physical properties of alkene
Boiling points and densities
boiling points increase with molecular weight
increased branching leads to greater volatility and lower boiling points
Solubilities
nonpolar
insoluble in water, but soluble in non polar solvents- hexane, ethers
Polarity
slightly more polar than alkanes
isomer of cis-alkene has dipole-dipole attractions
trans-distributed alkene have lower boiling point
Reactions of alkenes
Addition reaction
catalytic hydrogenation
Electrophilic additions
hydrogen halides
halogenations
hydrations
Oxidation of alkenes
combustion of alkenes
epoxidation
diol hydroxylation
ozonolysis
Stability of carbocations
1°carbocation<2° carbocation<3°carbocation