Section 5 ORGANIC

• What is an optical isomer?


  Type of stereoisomer (same structural formula, dif spacial arrangement).

  Has a chiral carbon.
• What is a chiral carbon?


  A carbon atom with 4 dif groups attached.

  You can arrange the groups in 2 dif ways around the C atom, so 2 dif enantiomers can be made.
• What are enantiomers?


  Non-superimposable mirror images.

  If molecules can be superimposed they are achiral.
• What is plane polarising light?


  If normal light passes through a polarising filter it becomes polarised and vibrates light in the same plane.

  In an optically active mixture of enantiomers, the molecules will rotate the light in opposite directions.
• What is a racemate?


  Equal mixture of enantiomers.

  Aren't optically active as enantiomers cancel out.

  If you react 2 achiral things you get a racemate.
• How would you distinguish between separate samples of 2 enantiomers?


  Use plane polarised light to rotate the plane of each enantiomer in opposite directions.
• Why does a reaction produce a racemic mixture?


  The planar carbonyl group can be attacked from either side with equal probability.
• Why might using plane polarised light not distinguish isomers?


  Product might be a racemic mixture with equal enantiomers.

  Racemic mixture is inactive.

  Product from isomer might be achiral.
• Oxidising aldehydes/ketones?


  Aldehydes can be easily oxidised to carboxylic acids due to the H atom attached to the carbonyl group.

  Ketones can't be easily oxidised as it requires breaking a C=C bond.
• Tollen's reagent?


  (colourless solution of silver nitrate dissolved in aq ammonia).

  If heated with aldehyde, a silver mirror forms.

  Aldehyde is oxidised.

  Diaminesilver ions are reduced producing silver and ammonia.

  eg Ag(Nh3)2^+ + e- > Ag + 2NH
• Fehling's solution?


  (blue solution of complexed copper (II) ions dissolved in NaOH)

  If aldehyde is heated, copper (II) ions are reduced to brick red precipitate of copper (I) oxide.

  eg Cu^2+ + e- > Cu^+
• Reducing aldehydes and ketones?


  Reducing agent NaBH4 reduces them into their primary/secondary alcohols.

  eg CH3CH2COCH3 + 2[H] > CH3CH2CH(OH)CH3
• Nucleophilic addition reactions of aldehydes/ketones?


  C=O bond is polar, the positive C attracts the lone pair on H- ion.

  H- ion attacks C and donates its lone pair of e- forming a bond with it.

  H- ion causes C=O bond to break forcing e- onto the O atom.

  O2 donates its lone pairs to a H+ ion.

  Produces an alcohol.

  
• How do you produce a hydroxynitrile?


  Nucleophilic addition reaction.

  Aldehyde/ketone + KCN > hydroxynitrile + K^+ ions.

  Dilute acid catalyst.

  KCN dissociates in water forming K^+ ions and CN^- ions.

  
• Producing racemic mixture from hydroxynitriles?


  Double bonds are planar so nucleophile cane attack from either side, producing different enantiomers.

  Or attack equally from both sides to produce racemate if bond is planar.
• Risks of producing hydroxynitriles?


  KCN is toxic so do reaction in fume cupboard.

  Organic compound is flammable so heat in water bath.
• Dissociation of carboxylic acids?


  COOH- acids are weak acids and only partially dissociate in water to form a carboxylate ion and H+ ion.

  
• Carboxylic acid + carbonate reactions?


  Carboxylic acid + carbonates > Salt + CO2 + H2O

  eg HCOOH + NaHCO3 > HCOONa + CO2 + H2O

  Carbonate= CO3^2- or HCO3^-
• What is an esterification reaction?


  Heating a carboxylic acid + alcohol > ester + water with a strong acid catalyst.

  Acid catalyst= concentrated H2SO4, HCL, H3PO4

  
• Useful properties of esters?


  Perfumes/food flavourings (sweet smell).

  Polar liquids (glues).

  Evaporate easily from mixtures.

  Plasticisers (make plastics flexible).
• Acid hydrolysis of esters?


  Splits ester into acid + alcohol.

  Reflux ester with dilute acid eg H2SO4, HCl.

  
• Base hydrolysis of esters?


  Splits ester into carboxylate ion + alcohol.

  Reflux ester with dilute alkali eg NaOH.

  
• What are fats and oils?

  They are esters of glycerol and fatty acids (long chained COOH-)

  Fats have saturated fatty acid chains so fit closer and have stronger VDW forces. They have higher boiling points and solid at room temp.

  
• Hydrolysis of fats/oils?


  Base hydrolysis, NaOH catalyst.

  OH- ions from catalyst react with fat/oil > carboxylate ion + alcohol.

  Alcohol formed= glycerol.

  Carboxylate ion combines with Na+ ions to form a sodium salt (soap).

  
• Converting a salt back into its fatty acid?


  Add HCl.

  H+ ions displace Na+ ions forming a carboxylic acid + free Na+ ion.
• How to make biodiesel?

  Methanol + fat/oil > methyl ester.

  Strong alkali catalyst eg NaOH.

  
• Is biodiesel carbon neutral?


  When crops grow they absorb same amount of CO2 produced when they are burned.

  However it doesn't take into account fossil fuel energy to make fertilizer, harvest and convert the oil.
• What are acid anhydrides?


  Compounds made from 2 identical COOH- molecules.

  
• Reactions of acid anhydrides?


  + water > 2x carboxylic acid

  + alcohol > ester + carboxylic acid

  + ammonia > amide + carboxylic acid

  + amine > N-substituted amide + carboxylic acid
• How to make aspirin?


  Aspirin is an ester.

  Salicylic acid + ethanoic anhydride > aspirin + ethanoic acid

  
• Why is ethanoic anhydride better than ethanoyl chloride?


  It is cheaper, less corrosive, doesn't produce dangerous HCl fumes (corrosive) and reacts slower with H2O.
• What are acyl chlorides?


  CnH2n-1

  Ends in -oyl chloride.
• Reactions of acyl chlorides?


  + water > carboxylic acid + HCl

  + alcohol > ester + HCl

  + ammonia > amide + HCl

  + amide > N-substituted amide + HCl

  Cl is eliminated producing white misty fumes of HCl.
• Nucleophilic addition-elimination reactions?


  Nucleophile adds onto acyl chloride, displacing a Cl- ion.

  Hydrogen leaves to create acyl chloride derivative.

  
• Reflux reactions?


  Used for continuous boiling/condensing of mixture.

  Allows mixture to be heated without losing any reactants/products.

  Anti-bumping granules prevent vigorous boiling by forming smaller bubbles.
• Separating funnel?


  Used to purify organic layer from high density liquid.

  1. Put impure product in funnel.
  2. Wash product using saturated NaCl solution.
  3. Allow layers to separate in funnel.
  4. Discard aq layer.
  5. Run organic liquid into conical flask and add drying agents.
  6. Distill to collect pure product.
• What are drying agents?


  Anhydrous salts eg MgSO4, CaCl2.

  They bind to any water present, becoming hydrated.
• Recrystallisation?


  Used to purify organic solid

  1. Dissolve impure compound in low volume of hot solvent. (hot= dissolves both compound and impurities. Low volume= Obtain saturated solution).
  2. Hot filter through filter paper. (filter= Removes insoluble impurities. Heat = prevents crystals forming during filtration).
  3. Cool solution in ice beaker. (ice= increases crystal yield).
  4. Suction filtrate with buchner flask to separate crystals. (water pump connect= reduces pressure and speeds up filtration).
  5. Wash and dry crystals. (removes soluble impurities and removes excess water).
• Testing purity?


  Impurities have lower melting point but greater melting range.

  1. Pack solid into glass capillary tube.
  2. Place in heating element.
  3. Increase temp until near melting point.
  4. Compare temps when solid begins to melt until melted completely (melting range).