Carboxylic Acids and Derivatives

• • Why would you not use an acid chloride when manufacturing?
  • Acid chlorides are more reactive but they are difficult to handle
  • need to be used in water free conditions
  • produce corrosive HCl
• acid anhydrides are used more often:

  • cheaper to produce
  • less corrosive
  • not as moisture sensitive
  • react to produce carboxylic acids - safer than HCl
• undergo nucleophilic addition elimination
• • saponification - boiling lipids with KOH to make salts of fatty acids and glycerol
  • mixture is used as soap
• • carbon chain of fatty acid salt is hydrophobic and attracts oil
  • the O- K+ end is hydrophilic and attracts water
• • biodiesel can be made from lipids
  • lipid is broken down into three fatty acids and glycerol by reaction with potassium hydroxide
  • the fatty acids are converted into three new esters using methanol
• biodiesel is renewable and carbon neutral
• • undergo nucleophilic addition elimination
  • reagent - acyl chloride or acid anhydride
  • alcohol, amines, water or ammonia can be reacted with the acid
• water reacts to produce carboxylic acid

  alcohol reacts to produce an ester

  amines react to produce N-substituted amides

  ammonia reacts to produce amides
• during nucleophilic addition elimination, if amines or ammonia are being reacted, the acid reacts with a second molecule of the amine/ammonia to form an ammonium salt of the acid
• What are the steps involved in purifying aspirin?

  1. Shake sample with solvent and warm to dissolve.
  2. Filter the hot solution using a Buchner funnel and pump.
  3. Allow the solution to cool and crystallize.
  4. Filter the cold solution using a Buchner funnel and pump, keeping the residue.
  5. Wash the residue with a small amount of cold solvent.
  6. Dry the product on a watch glass at room temperature or in an oven.
• Glycerol structure:

  A) OH

  B) OH

  C) OH

  
• fatty acid structure:

  A) saturated

  B) unsaturated

  
• saponification example:

  A) glycerol

  B) soap

  
• if acid anhydrides are used during nucleophilic addition elimination, carboxylic acids are also produced
• • carboxylic acids are weak acids so they partially dissociate
  • they react with carbonates to form a salt, carbon dioxide and water
• • carboxylic acids react with alcohols in the presence of a sulfuric acid catalyst to make esters
  • alcohol also react with acid anhydrides to form esters
• uses of esters:

  • perfumes and food flavourings
  • solvents - they are polar so other polar compounds dissolve easily in them. they have low boiling points and evaporate easily so are valuable in making glue
  • plasticisers - make plastics more flexible during polymerisation
• esters can be hydrolysed with:

  • dilute acids (HCl or H2SO4) under reflux to form carboxylic acid and alcohol
  • dilute base (NaOH) under reflux to form carboxylate ion and alcohol
• glycerol + fatty acid --> ester which makes fats and oils
• vegetable oils:

  • unsaturated hydrocarbon chain
  • not straight so can't pack closely together
  • lower van der waal forces
  • lower melting points
  • liquid at room temperature
• saturated hydrocarbon chains:

  • straight chain so can pack closely together
  • higher van der waal forces
  • higher melting points
  • solid at room temperature
  • e.g. butter
• making aspirin:

  A) ethanoic anhydride

  B) salicylic acid

  C) aspirin

  D) ethanoic acid

  
• reflux apparatus:

  A) water out

  B) water in

  C) Liebig condenser

  
• Buchner funnel diagram:

  A) filter paper

  B) porous plate

  C) rubber bung

  
• aspirin is an ester made by reacting ethanoic anhydride or ethanoyl chloride with salicylic acid
• reflux is used when you want to heat volatile liquids
• • reflux allows strong heating without losing the volatile reactants and products
  • volatile compounds evaporate and condense, so they fall back into the flask
  • the Liebig condenser has cold water running through the wall: when hot evaporating substances hit the cold condenser they turn into a liquid and return to the round-bottom flask to react further
  • no naked flame is used in reflux as the liquids involved are flammable
• distillation - separates substances with different boiling points:

  • useful if you want to remove a chemical before it reacts any further
  • e.g. oxidation of primary alcohols to aldehydes - aldehyde is distilled before it can oxidise further to a carboxylic acid
• filtration - separating solids from liquids:

  • place a filter paper disc in the Buchner funnel and dampen slightly to create a seal
  • pour reaction mixture into funnel and turn vacuum on
  • vacuum creates a reduced pressure in the flask and pulls the liquid through the porous plate of the funnel, leaving the solid behind
• recrystallisation - purifying solids:

  • add enough hot solvent to dissolve the impure solid
  • this gives a saturated solution of impure product
  • allow solution to cool and wait until crystals start to form
  • impurities remain dissolved in the solution - as they are in smaller quantities they take longer to crystallise
  • filter to obtain purified solid crystals - wash these with cold solvent and leave to dry
• the solvent in recrystallisation is very important:

  • the impure solid has to dissolve fully in the hot solvent but must be insoluble in cold solvent
  • if it cant do these things, the substance wont dissolve in the hot solvent and purified solid cant be filtered if it is soluble in cold solvent
• • measuring the melting or boiling points of a substance helps to detect impurities
  • sample is impure if it boils/melts over a range of temperatures or if the boiling/melting point is higher than data book value
• distillation helps to determine boiling point:

  1. heat sample and measure temperature it distils at
  2. this is the boiling point
  3. compare this to a data book value
• melting point helps to detect impurities:

  1. add solid sample into a capillary tube and place into heating element of melting point apparatus
  2. slowly increase the temperature until sample starts to melt
  3. there is a range of temperatures from when the substance starts to melt to when it has fully melted
  4. compare the melting point to the data book value
• distillation apparatus:

  A) thermometer

  B) condenser

  C) water in

  D) water out

  
• how would you carry out an investigation to show that a solvent is suitable for recrystallisation?

  the product should dissolve in the hot solvent but is insoluble in the cold solvent
• the yield of aspirin might be less than 100% because some product could be lost during crystallisation
• the yield of aspirin might be higher than expected because some of the water might not have dried
• give two ways of maximising the yield of propanal obtained by distillation

  • keep temperature of reaction mixture below the boiling point of the alcohol
  • cool the distillate