Isomerism and carbonyl compounds

• What are optical isomers

  Non superimposable mirror images with a chiral carbon atom
• What form of isomers are optical isomers

  Stereoisomers: same structural formulas but different arrangements of atoms in space
• What is a chiral carbon

  A carbon with four different groups attached, these can be arranged in two different ways, called enantiomers
• How to draw an enantiomer

  • Tetrehedral shape with a dotted line and the image flipped on the opposite side
• How to differentiate enantiomers

  • Rotate plane polarised light in equal and opposite directions
• What is a racemic mixture

  Equal amounts of each enantiomer in the mixture
• Do racemic mixtures rotate plane polarised light

  No, the enantiomers are both rotating in equal and opposite directions, therefore they cancel out
• Why are racemic mixtures formed

  Equal chance of a nucleophilic attack on either side of the planar carbonyl on the asymmetric molecule to form each enantiomer
• Problems behind racemic mixtures

  • Only one enantiomer may be optically active, and it is very expensive and difficult to split a racemic mixture
  • Both enantiomers may be active but one enantiomer may be harmful to the body
• What is used to reduce aldehydes and ketones

  Reducing agents such as NaBH4 solution

  e.g: CH3CH2COH + 2[H] -> CH3CH2CH2OH
• Mechanism arrows to reduce an aldehyde to a primary alcohol

  • Lone pair on the H- to the delta positive carbon
  • Double C-O bond to the O
  • Lone pair on the O to the H+

  
• Mechanism name for KCN and carbonyl group and product

  • Nucleophilic addition
  • Hydroxynitire
• Risks of KCN

  • KCN is an irritant and very dangerous if ingested or inhailed
  • When KCN reacts with moisture it can form the toxic gas HCN
• Carboxylic acid features

  • Weak acids
  • React with carbonates to form carbon dioxide, salt and water
  • Dissociate partially to form a carboxylate and a H+
• Esterification

  • Reacting alcohols with carboxylic acids or acid anhydrides and a sulfuric acid catalyst to make esters and water or a carboxylic acid respectively

  
• Uses of esters

  • Perfumes and food flavourings
  • Solvents as they are polar with low boiling points
  • Plasticisers
• Ester acid hydrolysis

  • Using sulfuric acid or hydrochloric acid under reflux
  • Ethanoic acid + Ethanol

  
• Ester base hydrolysis

  • Use sodium hydroxide under reflux
  • Ethanoate + ethanol

  
• Glycerol + fatty acid chains

  Makes an ester of fats and or oils

  • Oil = unsaturated fatty acid chains, cant pack together closely and weak vanderwaals, therefore liquids.
  • Fats = Saturated fatty acids
• Soap production through hydrolysis

  Glycerol

  
• How do we make biodiesel

  • Reaction of oils with methanol and a KOH catalyst
  • Glycerol + Methyl ester

  
• Ethanoyl chloride + water

  -> Ethanoic acid + HCl
• Ethanoyl chloride + Ammonia

  -> Ethanamide + HCl
• Ethanoyl chloride + methanol

  -> Methyl ethanoate + HCl
• Ethanoyl chloride + Primary amine

  -> N-methyl ethanamide + HCl
• Ethanoic anhydride + water

  -> Ethanoic acid + Ethanoic acid
• Ethanoic anhydride + Ammonia

  -> Ethanamide + Ethanoic acid
• Ethanoic anhydride + Alcohol

  -> Methyl ethanoate + Ethanoic acid
• Ethanoic anhydride + Primary amine

  -> N-methyl ethanamide + Ethanoic acid
• Why do we use acid anhydrides in industry opposed to acyl chlorides

  • Slower, safer reactions
  • Less expensive
  • Does not produce harmful HCl gas
• Name of the mechanism between ethanol and propanoyl chloride and the arrows

  • Nucleophilic addition elimination
  • Lone pair on the oxygen to the delta positive carbon
  • C-O double bond to the O
  • Lone pair on the O to reform the C-O double bond
  • C-Cl bond to the Cl
  • O-H bond to the O, removing hydrogen and the positive charge

  
• Recrystallisation

  1. Add just enough hot solvent to allow the impure solid to dissolve, to make a saturated solution
  2. Allow crystals to cool down slowly, crystals start to form as they become insoluble
  3. Impurities remain dissolved in solution as there is a smaller quantity of them, it takes longer for them to crystallise
  4. Filter to get your solid purified crystals, wash with very cold solvent and dry off
• Purity and boiling point

  • Set up distillation
  • Gently heat until it distils using a thermometer
  • Compare boiling point with data book
  • Larger range and higher boiling point implies impurities
• Purity and melting points

  • Add a sample of the solid product into a capillary tube and place into the heating element of the melting point apparatus
  • Gradual temperature increase until substance starts to melt
  • Compare value to a data book value
  • Larger range and lower melting point implies impurities