alkenes Flashcards
define alkene
unsaturated hydrocarbon, CnH2n
describe and draw the C=C bond
draw the formation of a sigma bond in alkenes
draw the formation of a pi bond in alkenes
what is the shape and bond angle around the C=C bond
planar, 120
define stereoisomer
Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms.
E-Z stereoisomers arise when:
(a) There is ____ around the C=C double bond.
(b) There are ____ different groups/atoms attached both ends of the
double bond.
restricted rotation, two
how do you name E/Z stereoisomers?
If the priority atom is on the same side of the double bond it is labelled Z from the german zusammen (The Zame Zide!)
If the priority atom is on the opposite side of the double bond it is labelled E from the german entgegen (The Epposite side)
what is a priority group?
The atom with the bigger atomic number is classed as the priority atom
what is cis-trans isomerism
two of the substituent groups are the same
Z-1,2-dichloroethene has a higher boiling point than E-1.2-dichloroethene, explain why?
Z-1,2-dichloroethene is polar. The polar C-Cl bonds are on the same side of the molecule. One side of the
molecule is slightly negative. The intermolecular forces are both London forces and permanent dipole-dipole attractions.
E-1,2-dichloroethene is non polar. The polar C-Cl bonds are on opposite sides of the molecule.
The dipoles cancel out. The intermolecular forces are is only London forces so lower boiling point.
describe the reaction of an alkene with hydrogen
alkene –> alkane
Reagent: hydrogen
Conditions: nickel catalyst
Type of reaction: Addition
draw ethene + hydrogen, what type of reaction is this?
addition
define electrophile
an electron pair acceptor
why do alkenes undergo electrophillic addition?
The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions
describe the reaction of alkenes with bromine/chlorine
Change in functional group: alkene –> dihaloalkane
Reagent: Bromine
Conditions: Room temperature (not in UV light)
Mechanism: Electrophilic Addition
Type of reagent: Electrophile, Br delta+ Type of Bond Fission: Heterolytic
draw the reaction of ethene with bromine, include intermediates
describe the reaction of hydrogen bromide with alkenes
Change in functional group: alkene –> haloalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile, H delta+
draw but-2-ene reacting with hydrogen bromide
what is Markownikoff’s rule?
In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it
In electrophilic addition to alkenes, the major product is formed via the more ____ carbocation intermediate.
the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are ____ and reduce the charge on the ion.
stable, electron releasing
what is order of stability for carbocations?
tertiary > secondary >primary
describe the reaction of alkenes with steam
alkene –> alcohol
Reagent : steam
Essential Conditions
High temperature 300 to 600°C
High pressure 70 atm
Catalyst of concentrated H3PO4
draw the polymer repeating unit for but-2-ene
