C14 Flashcards
alkenes
unsaturated hydrocarbons- one or more C=C
- double bond makes them more reactive than alkanes because of the high electron density between the two carbon atoms
how are large quantities of alkenes produced
when crude oil is thermally cracked
what is ethene used for
the starting material for polymers e.g. polyethene, PVC, polystyrene and terylene fabric, aswell as antifreeze and paints
general formula of alkenes
CnH2n
why can a double bond not rotate
as well as a C-C bond, there is a p-orbital containing a single electron on each atom
- these two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond
- this is the pi orbital and its presence means the bond cannot rotate
—-> restricted rotation
what are the 3 types of isomers that alkenes form
chain isomers
position isomers
geometrical isomers
position isomerism
isomers where the functional group C=C is in different positions along the carbon chain
the longer the chain, the greater the number of isomers
geometrical isomers
a form of STEREOISOMERISM
two stereoisomers have the same structural formula but the bonds are arranged differently in the space
occurs only around C=C bonds
E-Z isomerism
two atoms/ groups with the greatest atomic number are on the same side of the C=C (Z) or on opposite sides (E)
physical properties of alkenes
VdW forces are the only intermolecular forces that act between alkene molecules
- this means that physical properties of alkenes are very similar to those of alkanes
- the mp and bp increase with the number of C atoms present
- alkenes are not soluble in water
how alkenes react
- the C=C makes alkenes more reactive than alkanes
- the C=C forms an electron rich area in the molecule, which can easily be attacked by positively charged reagents
—-> called electrophiles: electron pair acceptors - e.g. H+ ion
- as alkenes = unsaturated, they can undergo addition reactions
–> electrophillic addition
combustion of alkenes
alkenes will burn in air. however, they are not useful as fuels. this is because their reactivity makes them more useful for other purposes
electrophillic addition reaction mechanism
- electrophile = attracted to C=C
- electrophiles are (partially) positively charged and accept a pair of electrons from the double bond
- a positive ion (carbocation) is formed
- a negatively charged ion forms a bond with the carbocation
can occur with hydrogen halides/ halide molecules
asymmetrical alkenes and electrophillic addition
- when the C=C is not exactly in the middle of the alkene, there are two possible products, as the halide ion could bond to either C of the C=C
- alkyl groups haved a higher e- density than H
- they have a tendency to release e- in the positive inductive effect
- this stabilises the positive charge of the positive C+
- the more alkyl groups that are attached to C+ the more stable
- the product will tend to come from the more stable C+
test for a double bond
addition reaction = used to test for C=C bond
- few drops of bromine water
- orange —-> colourless
polymers
e.g. of natrually occuring
large molecules built from monomers
starch, protein, cellulose, DNA
addition polymers
- made from monomers with a C=C (alkenes)
- when monomers polymerise, the double bond opens and the monomers bond together to form a backbone of carbon atoms
modifying plastics
the properties of polymers materials can be considerably modified by the use of additives such as PLASTICISERS
- these are small molecules that get between the polymer chain, forcing them apart and allowing them to slide across each other
- e.g. PVC is rigid enough for use as drainpipes, but with addition of plasticiser, becomes flexible enough to make aprons
biodegradability of polyalkenes
- backbone of long chain saturated alkane
- alkanes have strong non-polar C-C and C-H
- they are very unreactive
- means they are not attaked by biological agents e.g. enzymes so are not biodegradable
- increasing problem where waste disposal = more difficult
LDPE
- made by polymerising ethene at high pressure and high temperature
1400 atm and 170 degrees C - via a free-radical mechanism
- produces a polymer with chain branching
—> a consequence of the random nature of free-radical reactions - the branched chains do not pack together well and the product is flexible, stretches, and has low density
- these properties make it suitable for packaging, sheeting and insulation for electric cables
HDPE
- made at temperatures and pressures little greater than room conditions
70 degrees C, 2 atm
Ziegler-Natta catalyst - less chain branching
- chains pack together less well - this makes the density of the plastic greater, and its melting temperature higher
solutions to pollution by plastics
- 2 types of recycling
- problems with recycling
mechanical recycling
- simplest form of recycling
- 1st step = separate different types of plastic
- 2nd step = wash plastics and sort
- 3rd step = ground up into small pellets to be melted and remoulded
feedstock recycling
- plastics = heated to a temperature that will break the polymer bonds and produce monomers
—> used to make new plastics
problems with recycling
e.g. thermoplastic polymers
- soften when heated, so can be melted + reused
- however this can only be done a limited amount of times because at heating some of the chains break and become shorter, degrading the plastics properties
why is there an attraction between a C=C bond and Br2
C=C is electron rich- has high e- density
so Br-Br = polarised
8+ Br attracted to C=C
