4.1.3 - alkenes Flashcards
what is the general formula of alkenes?
CnH2n
what is the functional group in an alkene?
carbon-carbon double bond
what is a sigma bond?
end on end overlap of orbitals directly between atoms
what is a pi bond?
sideways overlap of p-orbitals to give a pi bond above and below a sigma bond
how many sigma and pi bonds does a double bond contain?
1 pi, 1 sigmah
how many sigma and pi bonds does a triple bond contain?
2 pi, 1 sigma
state and explain the shape around the carbon in a carbon-carbon double bond.
120 - trigonal planar. all electron pairs repel. lone pairs repel more than bonding pairs. around the c atom, there are 3 regions of electron density and no lone pairs.
why cant C-C double bonds rotate?
p-orbitals must stay in the same position to overlap and form a pi bond
what is stereoisomerism?
stereoisomers have the same structural formulas but different arrangements of atoms in space.
why does stereoisomerism occur?
C-C double bond restricts rotation
what is “Z-isomerism”?
when both priority groups are on the “same side”
what is “E-isomerism”?
when the priority groups are on different “sides”
what are the Cahn-ingold-prelog rules?
help to assign E or Z isomerism to molecules with different atoms on either side of the bond.
assign a priority group to each side of the double bond (the atom with the highest atomic number on each side is the priority group)
what is cis/trans isomerism?
a special case of E/Z isomerism in which two of the substituent groups either side of the double bond are the same e.g. pent-2-ene, where there’s a hydrogen on both sides of the double bond
why are alkenes more reactive than alkanes?
pi bonds are weaker then sigma bonds. pi bonds can break and electrons can be donated to form another bond.
what type of reactions are alkenes involved in and what is the % atom economy of these reactions
addition reactions
100% atom economy, only 1 product is formed
What are the reagents and reaction conditions for the synthesising a dihaloalkane from an alkene (halogenation)?
X2 (e.g. Br2)
20 degrees celsius
What are the reagents and reaction conditions for synthesising a haloalkane from an alkene (halogenation)?
HX (eg HBr)
20 degrees celsius
that is the test for alkenes and what is the positive result? why does this happen?
add bromine water to the solution and shake.
if a double bond is present, halogenation will occur and the bromine will be used up.
bromine is orange, so when the bromine is used up, there is a colour change from orange to colourless.
What are the reagents and reaction conditions for synthesising an alkane from an alkene (hydrogenation)?
Ni catalyst
150 degrees celsius
H2
What are the reagents and reaction conditions for synthesising an alcohol from an alkene (hydration)?
H3PO4 catalyst
60-70 atm
300 degrees celsius
water (steam)
what is a nucleophile?
electron pair donor
what is an electrophile?
electron pair acceptor
what do curly arrows represent in a mechanism?
movement of a pair of electrons
describe in words the 4 steps in electrophilic addition where the electrophile is X-Y
- 2 electrons from the c-c double bond attack the partially positive X creating a new bond
- X-Y bond breaks by heterolytic fission
- a positively charged carbocation intermediate is formed
- Y- acts as a nucleophile and attacks the positively charged carbocation intermediate donating its lone pair of electrons, forming a new bond.
how do unsymmetrical alkenes form a mixture of products when reacting with a hydrogen halide of water?
they form a mixture of carbocation intermediates, based on which carbon reacts with the hydrogen and which with the X- or OH-
what is Markownikoff’s rule?
the major product of the hydration/halogenation of an unsymmetrical alkene is the one that forms the most stable carbocation intermediate
primary carbocation intermediates are the least stable (only bonded to 1 other alkyl group) and tertiary carbocation intermediates are the most stable (bonded to 3 other alkyl groups)
what is a monomer?
single repeating unit (usually an alkene in addition polymers)
what is a polymer?
long chain of repeating units
explain the reactivity of polymers
polymers are saturated compounds so are chemically inert and are non-biodegradable
give a pro and con of polymers.
polymers are chemically inert and so are non-biodegradable.
pro - this means they’re long lasting and reusable
con - it means they’re difficult to dispose of
give 3 methods of disposing of waste polymers
burying plastic waste
recycling plastics
burning waste plastic
when can waste plastic be buries (put in landfill)
when too difficult to separate from other waste
when not in sufficient quantities to make separation financially worthwhile
when too difficult to technically recycle
explain the recycling of plastice
we can sort plastics into different types
- eg polypropene can be melted and remoulded
- some can be cracked into monomers,
which can be used as organic feedstock to make more plastics/chemicals
overall process involved sorting and processing
explain the combustion as a method of disposal of waste plastic.
waste plastics ban be burned, and the heat is used to generate electricity
toxic gases can be released i.e. PVC (polyvinyl chloride) releases HCl gas
passing through scrubbers containing NaCHO3 neutralises and removes HCl
what are the benefits of biodegradable/photodegradable polymers
renewable material will not run out
carbon neutral for plant-based polymers
plant-based polymers save energy
