16 - organic synthesis

• What does a basic set of quickfit apparatus contain?


  1. Round-bottom or pear-shaped flask.

  2. Receiver.

  3. Screw-tap adaptor.

  4. Condenser.

  5. Still head.

  
• What apparatus do you need to heat under reflux?


  - Round-bottom or pear-shaped flask.

  - Condenser.

  - Rubber tubing.

  - Stand and clamp.

  - Heat source (bunsen burner, tripod and gauze or a heating mantel).

  
• Why do we heat under reflux for some reactions?


  Heating under reflux allows us to overcome the activation energy and increase the rate of reaction, without boiling off the solvent, reactants or products.
• What heating source can be used when heating flammable liquids?


  Heating mantle, so there us no nacked flame present. This provides and added level of safety should any of the apparatus leak or crack.

  
• How to set up a reflux experiment?


  1. The pear-shaped flask is clamped by the neck. Add the reaction mixture and anti-bumping granules to the flask.

  
  

  2. Place the condenser into the flask, making sure it is properly sealed.

  
  

  3. Rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink.

  
  

  4. Then the contents is heated under reflux.
• How does reflux work?


  Contents are boiled from the pear-shaped flask, the vapour rises up the inner tube of the condenser until it meets the outer jacket containing cold water. The vapour then condenses and drips back into the flask.
• What do anti-bumping granules do?


  They allow the contents of the flask to boul smoothly. Preventing large bubbles from forming.
• Why is a stopper never put in the top of a condenser during reflux?


  It would create a colsed system and pressure would build up inside as the heated air expanded which could result in the apparatus exploding.
• Where does water enter and exit a condenser during reflux?


  Water enters the consenser at the bottom and exits at the top.
• Why does water enter the condenser at the bottom and exit at the top?


  To make sure the outer jacket is always full. And so that the water stays the coolest it can.
• Why may we use distillation after reflux?


  Chemical reactions may not go to completion or may produce by-products as well as the desired product. Once reflux is complete, the crude liquid or solid product in the flask needs to be purified to remober any by-products or remaining reactants.

  
  

  Distillation is a common method used to separate a pure liquid from it's impurities.
• What apparatus do you need to carry out distillation?


  - Round bottom or pear-shaped flask.

  - Condenser.

  - Rubber tubing.

  - Heat source.

  - Stand and clamp.

  - Screw-cap adaptor.

  - Receiver adaptor.

  - Still head.

  - Thermometer.

  
• How to set up a distillation experiment?


  1. Flask is clamped by it's neck and the still head is connected to the flask.

  
  

  2. A second clamp is placed around the receiver adaptor at the point at which its attached to the condenser.

  
  

  3. Rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink.

  
  

  4. A flask is used to collect the distillate so the distillation apparatus is not completely airtight.
• How does distillation work?


  1. The flask is heated and the mixture in the flask will start to boil. The different liquids in the mixture will have different boiling points and the liquid with the lowest b.p. (most volatile) will boil first.

  
  

  2. The vapour moves out the flask through other parts of the apparatus. When the vapours reach the cold condenser, they condense and become a liquid. This liquid drips into the collecting flask.
• What does a still head look like?


  The still head adaptor is T shaped and has two ground-glass joints, one to fit the screw-cap adaptor and one to fit the condenser.

  
• Where does water enter the condenser in distillation?


  It enters the inlet of the condenser, which is always the lowest point of the condenser.
• What will you see if water is obtained along with an organic liquid?


  Two liquid layers inside your collection flask.
• How do you determine which layer is water when you have a mixture of water and organic liquid?


  Add some water to the mixture and the layer that gets bigger is the aqueous layer.
• What apparatus do you use to seperate a mixture of water and organic liquid?


  Seperating funnel.
• What are the steps in separting water from an organic liquid?


  1. Ensure that the tap of the separating funnel is closed.

  
  

  2. Pour the mixture of liquids into the separating funnel, place a stopper in the top of the funnrl, and invert to mix the contents.

  
  

  3. Allow the layers to settle.

  
  

  4. Add some water to see which layer increases in volume - this is the aqueous layer.

  
  

  5. Place a conical flask under the separating funnel, remove the stopper and open the tap until the whole of the lower layer has left the funnel.

  
  

  6. Place a second conical flask under the separating funnel to collect the other layer.
• What do you do to ensure no water is left in an organic product?


  Traces of water are removed by adding a drying agent.
• What are three common drying agents?


  1. Anhydrous calcium carbonate - drying hydrocarbons.

  2. Anhydrous calcium sulfate - general drying.

  3. Anhydrous magnesium sulfate - general drying.
• What's the procedure for drying an organic liquid?


  1. Add organic liquid to a concial flask.

  
  

  2. Using a spatula, add some of the drying agent to the liquid and gently swirl the contents to mix together.

  
  

  3. Place a stopper on the flask to prevent your product from evaporating away. Leave for about 10 min.

  
  

  4. If the solid has stuck together in a lump, there is still some water present. Add more drying agent until some solid is dispersed in the solution as a fine powder.

  
  

  5. Decant the liquid from the solid into another flask. If the liquid is dry it should be clear.
• Organic synthesis


  The preparation of complex molecules from simple starting materials.
• Alkene functional group


  C=C

  
• Alcohol functional group


  -OH

  
• Haloalkane functional group


  -Cl

  -Br

  -I

  
• Aldehyde functional group


  O=C-H

  
• Ketone functional group


  C=O on central carbon

  
• Carboxylic acid functional group


  O=C-OH

  
• Synthetic route: alkenes to alkanes


  Reagent: hydrogen and Ni catalyst.

  Condition: 150°C.

  Mechanism: electrophilic.

  Reaction type: addition/hydrogenation.
• Synthetic route: alkenes to haloalkanes


  Reagent: hydrogen halide.

  Conditions: room temperature.

  Mechanism: electrophilic.

  Reaction type: addition.
• Synthetic route: alkanes to haloalkanes


  Reagent: halogen.

  Condition: UV light.

  Mechanism: Free radical.

  Reaction type: substitution.
• Synthetic route: alkenes to alcohols


  Reagents: steam + H₂SO₄.

  Condition: heat.

  Mechanism: -

  Reaction type: hydration.
• Synthetic route: alcohols to alkenes


  Reagents: Al₂O₃ or concentrated acid.

  Condition: heat.

  Mechanism: elimination.

  Reaction type: dehydration/elimination.
• Synthetic route: haloalkanes to alcohols


  Reagent: NaOH(aq).

  Condition: heat under reflux.

  Mechanism: nucleophilic.

  Reaction type: substitution.
• Synthetic route: alcohols to haloalkanes


  Reagents: NaX + H₂SO₄.

  Condition: heat under reflux.

  Mechanism: nuleophilic.

  Reaction type: substitution.
• Synthetic route: alcohols to ketones


  Reagents: K₂Cr₂O₇ + H₂SO₄.

  Condition: heat under reflux.

  Mechanism: -

  Reaction type: oxidation.
• Synthetic route: alcohols to aldehydes


  Reagents: K₂Cr₂O₇ + H₂SO₄.

  Condition: distillation.

  Mechanism: -

  Reaction type: oxidation.
• Synthetic route: alcohols to carboxylic acids


  Reagents: K₂Cr₂O₇ + H₂SO₄.

  Condition: heat under reflux.

  Mechanism: -

  Reaction type: oxidation.