*Colour by Design

• Dyes must absorb light in the visible spectrum by having at least one chromophore and a conjugated system (alternating double and single bonds)
• A chromophore is an atom or chemical group which is responsible for the colour of a compound
• Some functional groups can modify the chromophore by altering the colour of the compound due to the lone pairs creating an extended area of delocalisation
• Dyes attatch themselves to fibres by intermolecular forces, covalent and ionic bonds. The fibres have specific groups to allow this bonding to occur
• Solubility of dyes can be altered by the functional groups attatched to it
• Unsaturated molecules have lower melting points due to them not being closely packed together so the id-id forces between the molecules are less
• An addition reaction forms a single product
• A substitution reaction is one functional group being replaced by another functional group
• Hydrolysis is the breaking of a molecule with the use of water
• Condensation reactions are when two small molecules react to form a larger molecule and one small molecule is produced
• Elimination is when a small molecule is removed from a larger molecule
• Aromatic compounds contain benzene rings as part of their structure
• Benzene has a ring of delocalised electrons that form pi bonds due to the p-orbital electrons being delocalised
• Benene is very stable due to the delocalised ring structure of the molecule
• Benzene structure is backed up by the hydrogenation value being lower than expected so it was different to the structure of cyclohexatriene
• All bond lengths in benzene were found to be the same which wouldn't be the case if they were alternating double and single bonds as doube bonds are shorter than single bonds
• Benzene doesn't undergo electrophilic addition and only electrophilic substitution as the electrophile partially destroy the electron ring and substitiute the functional group for the hydrogen
• Benzene halogenation can occur when iron (III) bromide is used as a catalyst to generate the Br+ electrophile
• Nitration of benzene occurs in the presence of a NO2+ ion and concentrated sulfuric acid catalyst with nitric acid
• Sulfonation of benzene occurs when sulfur trioxide and fuming sulfuric acid are added to benzene
• Acylation of benzene can occur if an aliminium chloride catalyst is present with an acyl chloride to form the nucleophile
• Acylation of benzene realeses white misty fumes of HCl and also H from benzene reacts with AlCl4- to form AlCl
• Alkylation of benzene is the substitution of an alkyl group using an AlCl3 catalyst and forms HCl
• To make an azo dye a diazonium salt needs to be produced by reacting phenylamine and nitrous acid under 5ºc (+HCl)
• Nitrous acid is formed by:

  • NaNO2 + HCl -> HNO2 + NaCl
• In the coupling reaction the dye is produced which involves the diazonium salt, coupling agent (phenol) and NaOH
• Cotton and linen have OH groups so can form hydrogen bonds
• Wool, silk and nylon can form ionic bonds between acid and OH group
• Aldehydes can be heated under reflux with acidified potassium dichromate and H2SO4 to produce carboxylic acids
• Test for aldehydes is adding Tollen's reagent and heating to form a silver mirror if aldehyde is present
• Test for ketones is adding Fehlings solution and it remaining blue
• Hydroxynitriles are formed by HCN reacting with an aldehyde and extends the carbon chain by one in nucleophilic addition
• CN mechanism:

  1. C=O donates pair of electrons to O breaking double bond forming C-O bond
  2. CN donates pair of electrons to C forming a bond C-CN
  3. O- bonds with H+ ion to form C-OH bond
• KCN is often used as opposed to HCN as HCN is much more toxic and hard to store
• Things to consider when designing a synthetic pathway:

  • Product yield
  • Reaction set up, including: Catalyst, reagents and conditions
  • Process involved (batch or continous)
  • Hazards
  • Cost
  • Formation of Isomers - some enzymes won't work on enantiomers so specific enanatiomer must be produced
• Dyes are coloured due to the conjugation as the electrons are more readily promoted to higher energy levels that are in the visible light spectrum due to conjugation reducing energy levels
• When a specific wavelength is absorbed by a molecule the colour we see are the wavelengths reflected from the molecule - the complementary colour