Alkenes

• Alkenes


  Unsaturated Hydrocarbons
• General formula for non-cyclic alkenes

  C_n H_2n
• Alkenes


  • C=C
  • Can polymerise
  • Decolourises bromine water
  • More reactive than alkanes due to double bond – means they are more useful
• Spatial arrangement around C=C

  Trigonal planar, bond angle 120°
• Suffix for alkene names

  • ene, name from end nearest the double bond
• Sp2 hybridisation in alkenes

  • The atoms affected are the carbons in the carbon-carbon double bond
  • Sp2 hybridisation is the blending of the s orbital and 2 p orbitals, leading to three sp2 hybrid orbitals, with one electron in each
  • The fourth electron stays in an unhybridised p orbital

  
• Alkene sp2 hybridisation


  • The first sp2 orbital overlaps with the sp2 orbital of another carbon to form a covalent bond, and the other two for each carbon each overlap with a hydrogen orbital
  • The singular p orbitals for the two carbons both expand above and below the sigma bond until they too overlap, forming a new region of space wherein the pair of electrons can be either above or below the sigma bond at any one time
  • The new region is called a pi bond as they are formed from the sideways (rather than end to end) overlap of orbitals

  
• Sigma and pi bonds

  The sigma bond is much stronger than a pi bond
• Lack of rotation in C=C double bonds


  Because the Pi bond locks the two carbon atoms in space, so no rotation can occur
• Structural isomerism


  Structural isomers have the same molecular formula, but different structural formula
• Types of structural isomerism

  • Chain Isomerism – The carbon chain is arranged differently
  • Positional Isomerism – The functional group is attached to a different carbon atom, this also occurs in alkenes with 4 or more carbon atoms
  • Functional Group Isomerism – Contains different functional group, different homologous series
• Stereoisomerism


  Stereoisomers- species with the same structural formulae, but different spatial arrangements of atoms or groups
• Forms of stereoisomerism


  • Cis-Trans Isomerism
  • Optical Isomerism
• E/Z system for stereoisomer naming

  • E= Entgegen (opposite, one below the bond, one above)
  • Z= Zusammen (together, so same side of double bond)
• Determining E/Z isomerism


  1. First we establish the priority group (the priority group is the heavier one with the higher atomic number, and (if all else fails) is bonded to higher priority atoms, so basically anything that isn't hydrogen)

  2. Then we work out where the priority groups are in relation to each other
• E/Z isomerism is possible if the groups on the carbons are different, and not possible if they are the same
• Cis/trans isomers


  E/Z isomers can be called cis/trans (cis=Z, trans= E) if on each carbon of the double bond you have a hydrogen and non-hydrogen group
• Reaction of alkenes


  • The pi bond in the double bond breaks so into two sigma bonds so that new atoms can become part of the compound
  • This scenario is an electrophilic addition reaction

  
• Hydrogenation of alkenes


  1. The pi bond in the double bond is broken, making two single bonds which allow the hydrogen atoms to become part of the compound

  2. For this reaction to occur, a nickel catalyst and temperature of 150 degrees is needed

  3. The number of double bonds and moles of hydrogen must be the same in order for all double bonds to be broken
• In naturally occurring molecules, the hydrogen atoms are on the same side of the double bond
• Polyunsaturated species


  Species with more than one double bond
• The more saturated


  The higher the melting point, so hydrogen may be added to polyunsaturated fats to 'harden' them
• In the hydrogenation process, a cis fat can stay cis or become trans
• Trans fatty acids are linked to heart disease as they raise low-density-lipoprotein levels
• Reaction of alkenes with halogens

  1. These reactions happen at room temperature

  2. When bromine water reacts with alkenes, it goes from orange to decolourized

  
• Reactions of alkenes with hydrogen halides

  There are no specific conditions for this reaction
• Alkene hydration reaction


  1. This react occurs at over 100 degrees, and Phosphoric acid (H3PO4) or sulphuric acid (H2SO4) can be used to catalyse it

  2. In the reaction water vapour is split into a hydrogen and a hydroxide before bonding into the compound

  3. For hydroxide, the line should always be shown joining to the O, not the H

  4. Symmetrical alkenes produce only one product, but non-symmetrical can produce two
• Electrophilic addition mechanism


  • Electrophile- electron pair acceptor
  • The reaction is called an addition reaction because the halide is added across the double bond, and because the two reactants make one product
  • Halogen atoms are non-polar, as both atoms in the diatomic molecule have the same electronegativities, meaning that the electron pair is exactly in the middle
  • The double bond in alkenes contain twice the number of electrons as single bonds
  • Due to this increased electron density, the electrons in the double bond will repel the electrons in the single bond of the halogen, pushing the majority of the density to one side and inducing creating a dipole (the closest atom becomes electron deficient, the other becoming electron-rich)
  • The side of the diatomic molecule with the delta positive charge will be the one closest to the double bond, and as opposites attract, the electrons will leave the weaker pi bond (destroying it) to join with the delta positive side of the molecule
  • The new electrons at the previously delta positive side repel the shared pair of electrons and the delta negative side, breaking the bond and leading to two halide ions
  • This is heterolytic fission, as one side gets both the electrons from the originally shared pair
  • The previously delta positive halide then joins onto the compound, and because one of the carbons is now positively charged (as it has in effect lost one of its electrons that was originally in the pi bond to form the bond holding the first halide in place), and because the compound contains carbon, the overall compound is called a Carbocation(carbon containing cation)
  • The halide ion that was originally the delta negative part of the molecule has become a negative ion as due to the heterolytic fission it has gained one electron that originally belonged to the delta positive side
  • This negative halide ion is now attracted to the positive carbocation, and bonds to become part of the new halogenoalkane molecule

  
• Electrophilic addition does not happen with alkanes as they do not have a double bond, so there is not a great enough electron density to convert the halogen molecule into a dipole

  
• If the reaction occurs with a polar molecule, the dipole is pre-existing, not created, and the more electronegative element will be delta negative
• Markovnikov's Rule


  • When electrophilic addition occurs, if the alkene involved is symmetrical, then only one product can be produced
  • If it is asymmetrical, then two products are possible
  • However, the production of both products is not equal
  • One product is more likely to be produced (this is called the major product), and so consequently the other (the minor product) is less often produced

  
• Carbocation stability


  • A primary carbocation is where the carbocation involved only has one carbon directly attached to it
  • Secondary is when it's directly attached to two, tertiary is when it's directly attached to three
  • The more carbons directly bonded to the carbocation, the more alkyl groups (bits that aren't the carbocation so look like branches) will be present
  • Alkyl groups (regardless of whether they are ethyl, methyl ..etc) are electron-releasing, so release electron density towards the positive charge of the carbocation, spreading the positive charge around the carbocation, and make it slightly more stable because the charge is spread out
  • The more alkyl groups there are, the greater this effect is, so tertiary carbocations are more stable than secondary, which are more stable than primary
• Markovnikov's rule


  The more stable carbocation will always be the major product as a result of its stability
• Determining the major product using Markovnikov's rule

  When trying to work out which product will be major, the hydrogen (if there is one) prefers to go to the carbon with the most hydrogens attached
• Addition polymerisation


  Formation of a very long molecular chain (addition polymer) by repeated addition reactions of many unsaturated alkene molecules (monomers)
• Addition polymerisation


  1. When alkenes are exposed to a high temperature and pressure with a catalyst present, it breaks their double bond

  2. If there are lots of alkene monomers, when the double bonds are broken, they join together in a long chain to form a polymer(typically 2000-20,000 monomers long)

  
• Repeat unit


  The shortest repeating unit of the polymer
• Naming polymers


  The name of the polymer formed from monomers is the name of the monomer with 'poly' in front
• Always make your monomer look like an ethene molecule (so put the double bond in the middle, rest condensed to fit on the four branches available)

  
• When asked to draw more than one repeat unit, show that many monomers joined together without the brackets or n