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alkenes

  1. alkenes are
    hydrocarbons containing a carbon carbon double bond , the atoms around the carbon carbon double bond adopt a planar arrangement and the bond angle is 120
  2. e-z stereoisomerism 
    in double bonds the first bond involves an overlap of atomic orbitals directly in between the nuclei of two atoms . this is known as a
    σ bond . all single covalent bonds are σ bonds
  3. the second bond however cannot bond in the same place instead
    two p orbitals overlap and above and below the internuclear axis this is known as a pi bond
  4. since these orbitals overlap in more than one place
    it is not possible to rotate a pi bond about its axis without breaking the bounds . thus there is restricted rotation about the double bond
  5. if both carbon atoms on either side of the bond are attached to different groups then two different structures arise which can be interconverted . this is known as
    EZ isomerism
  6. stereoisomers are
    molecules with the same molecular formula and the same arrangement of covalent bonds but with different spatial orientations of the group
  7. EZ isomers also known as geometrical isomers are stereoisomers with
    different spatial orientations around the carbon carbon double bond
  8. EZ isomerism is caused by
    a restricted rotation about a carbon carbon double bond
  9. EZ isomerism arises when
    the carbon atoms on both sides of the bond are attached two different groups
  10. in cases where both C atoms are attached to one hydrogen atom , it is possible to distinguish between the isomers by a simple prefix
    • if both hydrogen atoms are on the same side of the molecule the prefix Z is used 
    • if the hydrogen atoms are on different sides of the molecule the prefix E is used
  11. it is only possible to name geometrical isomers using E and Z prefixes if
    both carbon atoms are attached to a hydrogen atom
  12. note tat molecules which show geometrical isomerism always have two specific structural features
    • there is a carbon carbon double bond 
    • both the carbon atoms are attached to two different groups
  13. the presence of the C=C bond gives alkenes a
    number of chemical properties that are not seen in alkanes
  14. since the alkenes contain pi bonds
    it is possible to break the pi bond and form σ bonds with other species without forcing any atoms on the molecule to break off
  15. as a result alkenes (unlike alkanes) are capable of
    undergoing addition reactions
  16. molecules which contain pi bonds are said to be
    unsaturated
  17. molecules which don't contain pi bonds are said to be
    saturated
  18. alkenes are
    unsaturated and hence undergo addition
  19. addition is
    the combination of two or molecules to form a single molecule
  20. addition reactions are generally ...... than substitution reactions
    faster
  21. this is because
    only weak pi bonds are broken rather than stronger σ bonds
  22. the pi bond in an alkene is an area of
    high electron density
  23. it can thus
    attract electrophiles and undergo heterolytic fission
  24. heterolytic fission is the
    breaking of a covalent bond which results in both electrons going to the same atom . this is in contrast t alkanes which can only react with free radicals and undergo homiletic fission
  25. an electrophile is a species
    which can accept a pair of electrons from a species  with a high electron density
  26. the fact that alkenes can react by heterolytic mechanisms , however doesn't mean
    that the pi bond will not undergo homiletic fission as well . on the contrary since the bond is non polar it is very likely to undergo homiletic fission
  27. alkenes can thus react in two ways
    • by free radical addition 
    • electrophilic addition
  28. the ability of alkenes to undergo addition and their ability to react with electrophiles as well as free radicals means that they are much more
    reactive than alkanes
  29. in the presence of electrophiles the C=C pi bond tends to
    undergo electrophilic addition
  30. possible electrophiles are
    hydrogen halides and halogens . alkenes also undergo electrophilic addition reaction with H2SO4
  31. alkene --> polyalkene
    • conditions : high pressure 
    • type of reaction : addition polymerisation 
    • equation : Image Upload 2
  32. alkene --> haloalkane
    • reagent : H-X 
    • conditions : room temperature
    • mechanism : electrophilic addition 
    • equation/ example : Image Upload 4
  33. alkene --> dihaloalkane
    • reagent : X2 in water or in an organic solvent 
    • conditions : room temperature 
    • mechanism : electrophilic addition
    • equation/example : Image Upload 6
  34. alkene --> alkylhydrogensulphate
    • reagent : conc sulphuric acid 
    • conditions : cold 
    • mechanism : electrophilic addition 
    • equation/example : Image Upload 8
  35. alkene --> alcohol
    • reagent : steam
    • conditions : 3000C , 60atm , H3
    • type of reaction : hydration 
    • equation : Image Upload 10
Author
ghoran
ID
275449
Card Set
alkenes
Description
revision
Updated

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