C28 - Organic Synthesis Flashcards
What apparatus is necessary to carry out filtration under reduced pressure?
Büchner flask Büchner funnel Pressure tubing Filter paper Access to a filter or vacuum pump
What’s filtration under reduced pressure?
A method for separating solid product from a solvent of liquid reaction mixture.
By what method is filtration under reduced pressure carried out?
1) Connect one end of the pressure tubing to the vacuum outlet or filter pump, whilst attaching the other end of the rubber tubing to the Büchner flask.
2) Fit the Büchner funnel to the Büchner flask tightly (e.g. with a rubber bung).
3) Switch on the vacuum pump or tap, to which the filter is attached.
4) Check for good suction by placing your hand across the top of the funnel.
5) Place a piece of filter paper inside the Büchner funnel and wet with the same solvent used in preparing the solid.
6) Slowly pour the reaction mixture from a beaker into the centre of the filter paper.
7) Rinse our the beaker with the solvent so all of the solid crystals collect in the Büchner funnel.
8) Rinse the crystals and leave them under suction for them to dry.
Why is recrystallisation carried out?
Solid product obtained after filtration (under reduced pressure) will contain impurities.
Recrystallisation removes any impurities.
What is the method for the recrystallisation of a solid?
1) Pour a quantity of the chosen solvent into a conical flask. (If the solvent is flammable, warm in a water bath; if it is water, warm using a Bunsen).
2) Tip the impure sample into a second conical flask or beaker.
3) Slowly add the solvent to the impure sample until it dissolves in the solvent. (Add the minimum volume of solvent needed to dissolve the solid).
4) Once the solid has dissolved, allow the solution to cool. Crystals of the desired product should form in the conical flask or beaker. When no more crystals form, filter the crystals under reduced pressure to obtain the dry crystalline solid.
How can chemists determine how pure a solid compound is?
By determining its melting point.
A pure organic substance usually has a sharp melting range of one or two degrees (from start to stop of melting).
If impure, it has a wide range and melts at a lower temperature than the pure compound.
What’s the method for determining the melting point of an organic solid compound to identify purity, using electrically heated melting point apparatus?
1) Before taking the melting point, the solid should be completely dry and free flowing.
2) Take a glass capillary tube. Hold one end of the capillary tube in the hot flame of the Bunsen burner and rotate until the end is sealed.
3) Allow the tube to cool and fill with the crystals being sampled.
4) Place the tube into a sample hole and a 0-300°C thermometer into the melting point apparatus and heat the sample.
5) Once it’s seen melting, record the rough melting point.
6) Prepare a second and repeat, this time heating slowly to obtain a more accurate melting point reading.
What’s the method for determining the melting point of an organic solid compound to identify purity, using an oil bath or a Thiele tube?
1) Before taking the melting point, the solid should be completely dry and free flowing.
2) Take a glass capillary tube. Hold one end of the capillary tube in the hot flame of the Bunsen burner and rotate until the end is sealed.
3) Allow the tube to cool and fill with the crystals being sampled.
4) Set up the oil bath or Thiele tube (b shaped).
5) Attach the capillary tube containing the sample to a thermometer using a rubber band.
6) Insert the thermometer through a hole in the cork if using a Thiele tube or clamp the thermometer if using an oil bath. The end of the thermometer and the end of the capillary tube should dip into the oil.
7) Using a micro burner, slowly heat the side arm of the Thiele tube or oil bath whilst observing this solid. When the solid starts to melt, remove the heat and record the temperature which all of the solid has melted.
8) Repeat a second time to ensure you obtain an accurate value.
What happens when nitriles are reduced?
Nitriles can be reduced to amines by reacting with hydrogen in the presence of a nickel catalyst.
What conditions are necessary for the reduction of nitriles?
Hydrogen and a nickel catalyst
How are nitriles hydrolysed?
They’re heated with dilute aqueous acid, forming carboxylic acids (and ammonium salts).
