Practical techniques in organic chemistry

Preparing a sample of an organic liquid: Quick fit apparatus: To carry out an organic preparation you will need to use Quick fit apparatus. It contains: - round bottom or pear shaped flask - Receiver - Screw tap adaptor - Condenser - Still head Heating under reflux: Many organic reactions occur slowly at room temperature. It is common for organic reactions to be heated to overcome the activation energy and increase the rate of reaction. Heating under reflux is a common procedure used to prepare an organic liquid without boiling off the solvent, reactants or products. To heat under reflux, you need the following pieces of apparatus: - round bottom or pear shaped flask - Condenser - Rubber tubing - Stand and clamp - Heat source For heating flammable liquids, a heating mantle can be used so that there is no naked flame present. This provides an added level of safety should any apparatus leak or crack. Anti bumping granule are added to the liquid before the flask is heated so that the contents will boil smoothly. If the granules are not used, large bubbles form at the bottom of the liquid and make the glassware vibrate or jump violently. Apply a thin layer of grease to the ground glass point on the condenser. During reflux the condenser is kept in the upright position. Condensers would 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. Rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink. Water always enters 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 vapor from the mixture rises up the inner tube of the condenser until it meets the outer jacket containing the cold water. The vapor then condenses and drips back into the flask. Distillation: 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. You need: - Round bottom or pear shaped flask - Condenser - Rubber tubing - Heat source - Stand and clamp - Screw-cap adaptor - Receiver adaptor - Still head - Thermometer 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. You should grease the joints so that the apparatus comes apart easily after the experiment. A second clamp is places 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, rubber tubing is used to connect the inlet of the condenser to the tap and outlet to the sink. Water enters at the lowest point, for distillation this is the closest point to the receiver adaptor. A flask is used to collect the distillate so that the distillation apparatus is not completely airtight. 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 so the liquid with the lowest boiling point is the most volatile so will boil first. Purification: 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 the organic layer and the other aqueous or water layer. An easy way to identify which layer is which add some water and the aqueous layer will get bigger. Once the organic layer, has been identified, the two layers are separated using a separating funnel. 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 funnel, 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. 7. You will now have one conical flask containing the organic layer and another containing the aqueous layer. 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 present will react releasing carbon dioxide gas. The tap needs to be slowly opened, holding the stoppered separating funnel upside down, to release any gas that may build up. Finally, the aqueous sodium carbonate layer is removed and the organic layer washed with water before running both layers off into two separate flasks. Drying: There may be some water left in the organic product. Traces of water are removed by adding a drying agent to the organic fluid. A drying agent is an anhydrous salt that readily takes up water to become hydrates. The most common ones are: - Anhydrous calcium chloride – drying hydrocarbons - Anhydrous calcium sulfate- general drying - Anhydrous magnesium sulfate- general drying The procedure is: 1. Add the organic fluid 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 about 10 minutes. 4. If the solid has all 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. Redistillation: Sometimes organic liquids have boiling points that are relatively close together so your prepare 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.