organic synthesis

Question 1

How can you purify an organic liquid?

Answer 1

1. Pour the distillate (somethng formed from distillation) of impure product into a separating funnel.
2. Wash with:
   a. NaHCO3 solution TO NEUTRALISE ANY ACID
   IMPURITIES, shake, and release pressure from
   CO2 produced.
   b. Saturated NaCl solution to seperate layers.
3. Allow layers to separate and discard aqueous layer (the organic layer will on top usually due to a lower density).
4. Run organic layer into clean dry conical flask.
5. Add drying agent (eg, anhydrous sodium sulphate or calcium
   chloride) to dry liquid. When dry, it should be clear. 6. Decant liquid into flask.
6. Redistill to collect pure product.

Question 2

What should you ensure with the drying agent for purifying organic liquids?

Answer 2

● Be insoluble in the organic liquid. ● Not react with the organic liquid.

Question 3

How can you purify an organic solid by recrystallisation? Why is each step used?

Answer 3

1. Dissolve impure compound in minimum volume of hot solvent.
   a. To ensure saturated solution. 2. Filter using filter paper quickly.
   a. Removing any insoluble impurities.
2. Cool filtered solution by inserting beaker in ice.
   a. Crystals will reform but soluble impurities will remain in solution. They are present in small quantities so not saturated.
3. Scratch with glass rod to initiate crystallisation.
4. Suction filtrate with a buchner flask to separate out crystals. 6. Wash the crystals with COLD solvent (otherwise it will
   dissolve in hot).
   a. To remove soluble impurities.
5. Dry the crystals between absorbent paper.

Question 4

By what 3 ways is yield lost under recrystallisation?

Answer 4

1. Crystals lost when filtering/washing.
2. Some product stays in solution afterwards. 3. Side reactions occurring.

Question 5

How is the solvent for recrystallisation chosen? Why? Otherwise what happens?

Answer 5

● A solvent in which the product (to be purified) is very soluble when solvent is hot and nearly soluble when cold.
● If not soluble enough, hot solvent won’t dissolve it all.

● If too soluble in cold solvent, most will remain in solution about cooling giving low yield.

Question 6

How can measuring melting point of a product indicate purity?

Answer 6

● A very pure sample will have a sharp melting point (as quoted in data books).
● One with impurities may have a lower melting point or may melt over a range of several degrees

Question 7

Give 2 ways melting point can be measured with a precaution

Answer 7

• Using an electronic melting point machine.
• Putting a capillary tube (with the product inside) into heating
oil with a thermometer.
● Heat slowly near melting point to record accurate
temperature when it JUST melts

Question 8

Describe the usual set up for determining melting point

Answer 8

● Thermometer and capillary tube strapped together.
● Heating oil with boiling point higher than sample and low
flammability.
● Constant stirring.

Question 9

What problems are there with producing chiral drugs? How is it solved?

Answer 9

Problems:
• Seperation is expensive.
• Can have different effects than intended.
Solutons:
• Use chiral synthesis.
• Use a chiral catalyst.