Colour by design

• conjugated system


  alternating single and double bonds allowing overlap of p orbitals
• delocalised electrons


  Electrons that are shared between more than two atoms.
• transparent objects


  all wavelengths of visible light transmitted
• opaque objects


  all wavelengths of visible light reflected
• complementary colours


  colours opposite each other on the colour wheel

  
• excited state


  electrons in a higher than normal energy level
• coloured compounds


  electrons absorb light from visible region and become excited
• colourless compounds


  electrons absorb uv light and are excited
• chromophore


  the part of the molecule responsible for light absorption and colour
• factors affecting colour

  when there is a smaller gap between the ground and excited state in a conjugated system
• more delocalised electrons...


  smaller gap between ground and excited state , longer wavelength of light e.g usually in a system with more than 5 pi bonds in visible range
• delocalised model of benzene


  6 p electrons delocalise and spread out evenly and shared between all 6 carbon atoms

  120° with uniform density around molecule

  all bonds the same length

  form charge cloud above and below plane of benzene

  
• Kekule model


  asymmetrical hexagonal planar , 120° with alternating bond lengths

  
• why is the kekule model not accepted


  benzene is more stable then kekule model due to delocalisation
• evidence used to disprove kekule model


  thermochemical data
• thermochemical data


  when cyclohexene is hydrogenated the enthalpy change is -120kjmol and you would expect the kekule model to be -360kJmol but its actually a lot less endothermic at -208kjmol
• why does benzene undergo addition reactions


  electrons take away electrons from the delocalised ring so addition is less stable
• why does benzene undergo electrophilic substitution


  the hydrogen atom is swapped for something else so the delocalised electron stability is preserved
• arenes


  aromatic compounds
• phenyl group


  group of atoms with C5H6 formula

  
• how to check conjugated systems are aromatic


  planar , 4n+2 delocalised p electrons and cyclic
• compounds derived from benzene


  nitrobenzene, benzoic acid , benzene sulfonic acid , phenyl amine and phenol
• why are phenyl groups important


  act as feedstocks in the chemical industry
• feedstocks


  reactants that go into a chemical process
• non systematic named compounds derived from benzene


  benzealdehyde , benzylalcohol , benzene 1,4 carboxylic acid
• esters derived from phenol and benzoic acid


  phenyl ethanoate , methyl benzoate
• dye


  a soluble coloured organic molecule that is able to bind to a substrate e.d a fibre and cause a colour change
• bromination


  substituting a bromine group to the benzene ring

  
• conditions needed for bromination


  benzene and liquid Br under catalyst presence e.g Iron bromide and heat
• bromination equation


  H+ + FeBr4- -----> HBr + FeBr3
• chlorination


  substituting chlorine into the benzene ring

  
• conditions for chlorination


  aluminum chloride , room temperature , anhydrous conditions
• why are anhydrous conditions needed for chlorination


  AlCl3 reacts violently with H20
• equation for chlorination


  Cl2 + AlCl3 -------> AlCl4- + Cl+
• Freidel- Crafts Reaction


  acylation and alkylation
• alkylation


  A reaction that introduces an alkyl group into a molecule

  
• conditions needed for alkylation


  reflux
• example of alkylation


  benzene + chloroethane---->ethyl benzene + HCl
• acylation


  addition of an acyl group to a molecule - benzene + an acyl chloride or acid anyhydride

  
• conditions needed for acylation


  anhydrous conditions