3.3.4 - alkenes Flashcards
what is the general formula of an alkene
CnH2n
what elements do alkenes contain
only carbon and hydrogen atoms, making them hydrocarbons
what bonds do alkenes contain
alkenes contain at least one C=C double covalent bond
are alkenes saturated or unsaturtaed
alkenes are unsaturated
why are alkenes unsaturated
molecules with C=C double bonds are unsaturated as they can make more bonds with extra atoms in addition reactions
what feature does the C=C bond in an alkene have
as there are 2 pairs of electrons in the C=C double bond, it has a really high electron density
are alkenes reactive or unreactive?
alkenes are pretty reactive
are alkenes saturated with hydrogen?
alkenes aren’t saturated with hydrogen
how can double covalent bonds be formed?
by sharing 4 electrons between a pair of atoms
what shape do alkenes have?
alkenes have a flat planar shape
what is the reasoning for the shape of alkenes?
as there can be no rotation around the double bond, unlike with single bonds
why is there no rotation around the double bonds in alkenes?
as in addition to the single bond between the carbons, there is a p orbital (containing a single electron) on each carbon that overlap to form a pie orbital, preventing rotation of the bonds
what is sometimes called, when the rotation of the double bonds in alkenes is prevented?
this is sometimes called restricted rotation
why are alkenes reactive?
because of the double bonds which have a greater bond enthalpy, but they are an electron rich area of the molecule that can easily be attacked by positively charged electrophiles
why are alkenes more reactive than alkanes
carbon-carbon double bond gives the molecule an electron rich area that attracts positively charged electrophiles
what are electrophiles?
electron pair acceptors
electrophilic addition reaction:
step one
positively charged electrophile is attracted towards double bond in the alkene
electrophilic addition reaction:
step two
electrophile accepts a pair of electrons from the double bond, forming a positive ion (carbocation)
electrophilic addition reaction:
step three
carbocation reacts with a negative ion to form a bond
what two types of products can from after an electrophilic addition reaction
major or minor products
what are major products
product that forms most of the time
what are minor products?
products that form less often
secondary carbocations
secondary carbocation is more stable as it has 2 alkyl groups, this carbocation will form most of the time
primary carbocations
primary carbocation is less stable as it has only 1 alkyl group, this carbocation forms less often
how do you test for unsaturation
shake an alkene with orange bromine water
what happens when you shake an alkene with orange bromine water
solution quickly turns from orange to colourless
what is the reasoning for the result in the test for unsaturation?
bromine is added across the double bond to form a dibromoalkane, happens by electrophilic addition
for electrophilic addition with hydrogen halides, what are the charges for the hydrogen halide
the hydrogen has a slightly positive charge, the halide has a slightly negative charge
what are polymers
long chain molecules formed when lots of small molecules (monomers) join together
what 2 types of polymers are there
polymers can be natural or synthetic
give an example of a natural polymer
DNA
give an example of a synthetic polymer
polythene
what sort of things can polymers be used for?
plastic bags, non stick pans, car tyres, fabric
describe addition polymerisation
double bonds in alkenes can open up and join together to make long chains called polymers (addition polymerisation)
what is an addition polymer
addition polymer is a polymer made from monomers, containing C=C double bonds
what happens when alkenes polymerise
when alkenes polymerise, the double bond is broken and links the alkenes to form a backbone of carbons all joined by single bonds
how to name polymers
find the name of the monomer and add poly in front
what happens to alkene monomers once they form polymers
once they form polymers they become saturated (only single bonds in carbon chain)
reactivity of addition polymers
addition polymers are very unreactive - polyalkenes are chemically inert
what is the reason for the reactivity of addition polymers
main carbon chain of polyalkenes is usually non polar
what bonds do monomers within a polymer have
monomers within a polymer chain have strong covalent bonds
intermolecular forces between polymer chains
intermolecular forces between polymer chains are much weaker which affects the properties of the polymer
intermolecular forces of longer chains
longer chains with fewer branches have stronger intermolecular forces
how does the intermolecular forces in longer chains affect the polymer material’s properties
makes these polymer materials stronger and more rigid
polyethene
polymer with few/no branches e.g. polyethene can pack closely together so polymer chains are attracted to each other by van der Waals forces, making a strong/rigid material
what is polychloroethene also known as
polyvinyl chloride or PVC
what is PVC made from?
PVC is an addition polymer formed from chloroethene monomers
covalent bonds on polychloroethene
covalent bonds between the chlorine and carbon atoms are polar, chlorine is more electronegative
polychloroethene: permanent dipole-dipole forces
slightly negative charges on the chlorine atoms and slightly positive charges on carbon atoms means there are permanent dipole-dipole forces between the polymer chains
properties of PVC
PVC is a hard but brittle material
what is PVC used to make
it is used to make drain pipes and window frames
why add plasticisers to polymers?
adding plasticisers to polymers modifies their properties
how does adding a plasticiser to a polymer affect its properties
adding a plasticiser to a polymer makes it more bendy
how do plasticisers alter properties of polymers
plasticiser molecules get between the polymer chains and push them apart, reduces strength of intermolecular forces between the chains so the chains can slide around more, becoming more flexible
give an example of a polymer containing plasticisers?
plasticised PVC
how is plasticised PVC different to normal PVC
plasticised PVC is much more flexible than rigid PVC
what is plasticised PVC used for
to make electrical cable insulation, flooring tiles and clothing
