28.2 Preparation of an Organic Solid

Question 1

What are the 3 stages of purifying an organic solid?

Answer 1

• Filtration under reduced pressure
• Recrystallisation
• Determining the melting point

Question 2

What does filtration under reduced pressure do?

Answer 2

• It separates the solid from the solvent or the liquid reaction mixture

Question 3

How does filtration under reduced pressure work?

Answer 3

• A Buchner flask is fitted with a Buchner funnel with filter paper
• Pressure is created through a vacuum pump that is connected to the flask through a side arm

Question 4

What does recrystallisation do?

Answer 4

• It removes impurities after filtration

Question 5

How does recrystallisation work?

Answer 5

• You choose a solvent that your solid will dissolve in when it’s hot, and the impurities will dissolve in when it’s cold
• Dissolve the impure solid in the minimum possible volume of hot solvent
• Cool the solution and filter the solid under reduced pressure
• Wash the crystals with cold solvent and dry them

Question 6

What can melting points determine about substances? Give 2 details.

Answer 6

• How pure they are
• Pure samples will have a sharp melting point close to the literature value (impurities lower the melting point, and causes the substance to melt over a range of temperatures)

Question 7

What are 2 methods for determining melting points?

Answer 7

• Using electrically heated melting point apparatus
• Using an oil bath/ Thiele tube