Alkenes

• alkenes

  unsaturated hydrocarbons

  CnH2n

  at least one double covalent bond

  they can undergo addition reaction

  double bond have high electron density and makes alkene fairly reactive

  cycloalkanes have 2 fewer hydrogens than their straight chain version of the same molecule
• sigma and pi bonds

  double bond contains both sigma and pi bonds
• sigma bonds

  when 2 s orbitals overlap they align horizontally to give a single covalent bond

  strong electrostatic attraction between nuclei and shared pair of electrons due to high electron density between nuclei

  strongest type of covalent bond

  have high bond enthalpy
• pi bonds

  parallel overlap of 2 p orbitals (shaped in figure 8)

  weaker as electron density is spread out above and below nuclei electrostatic attraction is weaker so pi bonds have lower bond enthalpy-which is why they are so reactive

  stick out a little and whole double bond has a high electron density meaning it is open to attack from electrophiles
• p orbitals

  dumb-bell shaped when they merge they form 2 oblong shapes

  one on top and one on bottom
• reactivity of alkenes

  double bond alkenes has high electron density
• alkenes

  have sigma and pi bond high electron density area that makes them reactive

  4 electrons are being shared in double bond

  good starting products for polymers and petrochemicals

  alkanes only have sigma
• sterioisomers

  have the same structural formula but a different arrangement of atoms in space

  c double bond c bonds and atoms bonded directly flat planar shape

  shape of c double bond CH2 is trigonal planar 120 degrees

  atoms cant rotate around c double bond c and is rigid -meaning you get a type of sterioisomer -e/z isomerism

  2 sterioisomers of each other they have the structural formula CH3CH2CHCHCH3 but we cant twist the double bond to turn one isomer into the other
• 2 isomers

  have restricted rotation around double bond

  you get e/z isomerism where you have 2 different atoms or groups of atoms on the same carbon
• e-entgagen

  opposite

  same groups opposite the double bond
• z-zusammen

  together same group on same side of double bond
• sterioisomers cip

  when deciding if isomer is e or z when we have 4 different groups around the double bond

  1) label carbons with double bond as carbon 1 and carbon 2

  2)calculate atomic number protons of first element directly bonded to c double bond c the atom with the highest atomic number is given higher priority

  you may have to check further down
• electrophilic addition

  alkenes are attracted by electrophiles due to double bond

  double bond=high density of electrons and is attacked by electrophiles -adds to molecule

  curly arrow starts from double bond
• electrophile

  electron pair acceptor

  deficient in electrons

  attracted to double bonds
• adding hydrogen

  reacting hydrogen gas with ethene at 150 degrees and nickel catalyst will make ethane
• addition of bromine (test for alkenes )

  decolouration of bromine water

  adding bromine water to an alkene causes a colour change from brown/orange to colourless

  bromine is the electrophile and adds to the alkene forming dibromoalkane (colourless)

  Br2 is polarised as electrons in double bond repels electrons in Br2

  electron pair in double bond is attracted to delta positive bromine and forms a bond this breaks the br-br bond

  a carbocation intermediate is formed and br ^-
• hydration of alkenes

  alcohols are produced by hydration of alkenes

  we use steam and acid catalyst to create an alcohol

  we can make ethanol by reacting steam and ethane with a phosphoric acid catalyst

  a temp of 300 degrees and pressure of 60 atm is needed

  this is a reversible reaction with an initial yeild of only 5%

  unreacted alkane is recycled through an overall yeid of 90-95% can be attained overall
• addition of hydrogen halides

  alkanes react with hydrogen halides to form halogenoalkanes

  the mechanism can apply to the other hydrogen halides and alkanes too

  an electron pair in the double bond is attracted to the delta positive hydrogen and forms a bond this breaks the H-Br bond

  a carbocation intermediate is formed and br- is attracted to C+

  bromoethane formed
• addition of hydrogen halides-unsymetrical

  reacting hydrogen halides with unsymmetrical alkene produces 2 different products

  the amount of the 2 products is determined by the stability of carbocation intermediate

  the more alkyl groups bonded to the carbocation the more stable the intermediate is

  this is because the alkyl groups push electrons towards positive carbocation stabilising it the more stable the carbocation the more likely it will form

  primary to tertiary = more stable

  
• Markownikoff's rule

  The major product from addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached
• addition polymers

  Alkenes are monomers which join to form addition polymers
• polymers

  made from monomer unit

  can be natural or synthetic

  charles goodyear discovered volcanized rubber

  he added chemicals to natural rubber to make harder wearing rubber suitable for tires
• natural polymers

  proteins and natural rubber
• synthetic polymers

  poly (ethene) and polypropene
• make polypropene

  need the monomer propene and add few of these together to get polypropene

  polyalkenes are saturated molecules normally non polar hence are unreactive -dont degrade well in landfill
• disposal of plastics (landfills)

  most polymers arent biodegradable so need to be disposed carefully
• landfill

  useful for disposing of plastics that are

  too difficult to recycle

  too difficult to separate from other materials

  there is not enough plastic to extract to make it economically viable

  landfills are not very sustainable as large amounts of land is needed it is becoming increasingly expensive to use land for wasted disposal and there is a need to reduce our reliance on landfill
• disposal of plastics (recycled)

  most polymers are not biodegradable so need to be disposed of carefully

  most plastics are made from crude oil which is now renewable source

  recycling means reducing dependency on crude oil for making plastics

  other plastics can be cracked (polymer chain broken up ) into monomers to be used as organic feedstock for plastics or other substances

  some plastics like poly propene can be remoulded into new objects
• disposal of plastics (incineration)

  most polymers are not biodegradable so need to be disposed of carefully
• incineration

  burning

  waste plastics could be used if plastics cant be recycled

  the energy from burning can be used to generate electricity
• burning plastics

  can release toxic fumes so these need to be monitored particularly chlorine based plastics like pvc which can produce harmful HCl gas when burned
• fuel gas scrubbers

  used to neutralise acidic gases produced like HCl

  they work by firing a base at the fuel gases
• biodegradable polymers

  decompose naturally

  under certain conditions by organisms

  made from both oil fractions and renewable sources

  more expensive than non biodegradable plastics
• biodegradable plastics

  for them to degrade you need a good supply of oxygen and moisture

  can be used in frost protective sheets for plants

  made from poly(ethene) and starch grains over time microorganisms break down the polymer meaning you dont need to dispose of the old sheeting
• photodegradable polymers

  degrade when exposed to sunlight