Practical techniques in organic chem

• To carry out an organic preparation you need to use Quickfit apparatus. Basic Quikfit apparatus set contains:

  • round-bottom or pear-shaped flask
  • receiver
  • screw-tap adaptor
  • condenser
  • still head
• Heating under reflux is used to prepare an organic liquid without boiling off the solvent, reactants, or products. To heat under reflux, you will need the following pieces of apparatus:

  • round-bottom or pear-shaped flask
  • condenser
  • rubber tubing
  • stand and clamp
  • heat source
  • bunsen burner allows the reaction to be carried out at a fixed temp
  • water bath can be used if the reaction can be carried out below 100 degrees c
  • for heating flammable liquids, a heating mantle can be used so that there is no naked flame present - provides added level of safety should any apparatus leak or crack
• When heating under reflux, the flask is clamped by its neck so before fitting the condenser you need to add the reaction mixture and the anti-bumping granules to the flask.

  Anti-bumping granules are added to the liquid before the flask is heated so that the contents will boil smoothly.

  If they are not used, large bubbles form at the bottom of the liquid and make the glassware vibrate or jump violently
• Before attaching the condenser to the flask, apply a thin layer of grease to the ground-glass joint on the condenser. Place the condenser carefully into the flask and gently rotate the condenser back and forth to provide a good seal and ensure that the apparatus comes appart easily at the end of the experiment
• During reflux the condenser is kept in the upright position. Condensers should be clamped only loosely as the glass outer jacket is very fragile and is easily broken. Never put a stopper in the top of the condenser. Otherwise you would have a closed system and pressure would build up inside as the heated air expanded. This could result in the apparatus exploding!
• When using a condenser for reflux, rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink. Water always enters the condenser at the bottom and leaves at the top to ensure that the outer jacket is full
• Heating under reflux enables a liquid to be continually boiled whilst the reaction takes place. This prevents volatile components from escaping and the flask from boiling dry. The vapour from the mixture rises up the inner tube of the condenser until it meets the outer jacket containing could water. The vapour then condenses and drips back into the flask.
• 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 present in the flask needs to be purified to remove any by-products and remaining reactants. Distillation is a common method used to separate a pure liquid from its impurities
• To carry out distillation you need:

  • round-bottom ore pear-shaped flask
  • condenser
  • rubber tubing
  • heat source
  • stand and clamp
  • screw-cap adaptor
  • receiver adaptor
  • still head
  • thermometer
• In a distillation, the flask is clamped by its neck and the still head is connected to the flask.

  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.

  A second clamp is placed round the receiver adaptor at the point at which it is attached to the condenser. This removes the need to clamp the condenser, as it will be supported sufficiently at both ends.
• As with reflux, in distillation, rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink. Water always enters the condenser at the lowest point.

  For distillation, this is closest to the receiver adaptor
• In distillation, a flask is used to collect the distillate so that the distillation apparatus is not completely upright
• In distillation, the different liquids in the mixture will have different boiling points. The liquid with the lowest boiling point is the most volatile and will boil first .

  The vapour moves out of the flask up into the other parts of the apparatus, leaving behind the less volatile components of the mixture. When the vapours reach the cold condenser, they condense and become liquid. This liquid then drips into the collecting flask
• When preparing samples of organic liquids, water may be obtained along with the product.

  If this has happened you will see two liquid layers inside your collection flask, one organic layer and one aqueous or water layer.

  An easy way to identify the organic layer is to add water to the mixture. The layer that gets bigger is the aqueous layer.
• The two layers can be separated using a separating funnel:

  1. ensure the tap of separating funnel is closed
  2. pour the mixture into the funnel, place a stopper and invert to mix contents
  3. allow layers to settle
  4. add some water to see which layer increase in volume - this is the aqueous layer
  5. place conical flask under separating funnel, remove stopper and open tap until the whole of the lower layer has left the funnel
  6. place a second conical flask under funnel to collect the other layer
  7. you will now have one conical containing organic layer and another containing aqueous layer. Label them.
• In preparation using acids, your impure product may contain acid impurities. These can be removed by adding aqueous sodium carbonate and shaking the mixture in the separating funnel.

  Any acid will react with sodium carbonate releasing CO2 gas. The tap needs to be slowly opened, holding the stoppered separating funnel upside down, to release any gas pressure that may build up.

  Finally the aqueous sodium carbonate layer is remover and the organic layer washed with water before running both layers off into two separate flasks
• Traces of water are removed from organic product by adding a drying agent to it. A drying agent is an anhydrous inorganic salt that readily takes up water to become hydrated.

  • E.g. anhydrous calcium chloride used for drying hydrocarbons
  • anhydrous calcium sulfate used for general drying
  • anhydrous magnesium sulfate used for general drying
• Procedure for drying an organic liquid:

  1. add organic liquid to a conical 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 10 mins approx
  4. If the solid has all stuck together in a lump, there is still some water present. Add more drying 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.
• Sometimes organic liquids have boiling points that are relatively close together, so your prepared sample may still contain some organic impurities. The distillation apparatus is cleaned and dried and set up again so that a second distillation can be carried out.

  This time, only collect the product with the boiling point of the compound you are trying to make. The narrower the boiling range, the purer the product. You now will have separated your product from any impurities