Organic Synthesis

Question 1

When purifying by recrystallising, why is the minimum volume of hot solvent used?

Answer 1

So that a saturated solution is created, so that as many crystals will fall out of solution as possible when it is cooled.

Question 2

Why is the solution filtered hot when purifying by recrystallisation?

Answer 2

To remove insoluble impurities and ensure that the crystals do not form in the filter paper.

Question 3

Why is the solution cooled in an ice bath when purifying by recrystallisation?

Answer 3

To ensure that as many crystals as possible fall out of solution — yield is higher

Question 4

Why are the crystals washed with cold water when purifying recrystallisation?

Answer 4

To remove soluble impurities.

Question 5

How would you separate the crystals from the reaction mixture when purifying by recrystallisation?

Answer 5

Filter under reduced pressure using a Buchner funnel.

Question 6

Why might percentage yield be below 100%?

Answer 6

Product is lost during filtration, drying and weighing — spills, not all transferred from one piece of apparatus to the other
Product is left dissolved in the solution — some does not crystallise. Some left on filter paper. Sample still wet.

Question 7

What are the three key steps to purify an organic solid?

Answer 7

1. filtration under reduced pressure
2. recrystallisation
3. measurement of melting points