PAG 6 - Synthesis of Organic Solid (6.2.5a, 6.3.1a)

Question 1

During the organic synthesis of a solid, state the procedures used to

I. Separate a solid product from a liquid/solvent

II. Purify a sample

III. Check purity of a sample

Answer 1

I. Filtration under reduced pressure

II. Recrystallisation

III. TLC; Melting pt. Determination;

Question 2

What equipment is required for filtration under reduced pressure?

Answer 2

• Buchner flask
• Buchner funnel
• Pressure tubing
• Filter paper

Question 3

Describe the practical procedure for filtration under reduced pressure

Answer 3

1. Attach the tubing out of the Buchner flask to the suction pump connected to the tap
   
   1. Tap your hand against the end of the pipe to check if there is suction
2. Place filter paper in the Buchner funnel and add crystals on top
   
   1. Wet filter paper with same solvent used when preparing the solid
3. Turn the tap on to begin suction, rinse the crystals with more solvent repeatedly

Question 4

Describe the process of recrystallisation

Answer 4

• Even after filtration the products may be contaminated

1. ​Gently warm a solvent to boiling
2. Keep adding the solvent to impure sample until it dissolves
   
   • Do slowly as you want to dissolve the impure sample in the minimum amount of solvent​
   • Minimum amount to ensure solution is saturated for max amount of crystals upon cooling
3. Allow solution to cool until crystals appear
4. Once no more crystals from, filter under reduced pressure to obtain dry crystalline solid
5. Wash with an ice cold solvent and dry - to prevent crystals from dissolving again and to remove any soluble impurities

Question 5

How do you determine melting point of crystals?

Answer 5

1. Use a capillary tube with one open and one sealed end
2. Tap the open end into the sample to move it up the tube
3. Place in an electronic melting point meter
   
   1. Place capillary tube in sample hole
   2. Begin to heat, and watch through the magnifying glass to see when the crystals melt
   3. Record the temperature
4. Compare to known values