Organic Techniques

Question 1

Describe how to purify an organic liquid

Answer 1

1. Put the distillate of the impure product into a separating funnel
2. Wash product by adding sodium hydrogencarbonate, shaking and releasing the pressure from CO2 produced
3. Allow the layers to separate in the funnel and then run & discard the aqueous layer
4. Run the organic layer into a clean, dry conical flask and add 3 spatula loads of drying agent (e.g. anhydrous sodium sulfate or calcium chloride)
5. Decant the liquid into the distilation flask
6. Distill to collect pure product

Question 2

In a separating funnel, will the liquid with a higher density be the top layer or bottom layer?

Answer 2

The liquid with the higher density will be the bottom layer

Question 3

Describe how to recrystallise an organic compound

Answer 3

1. Dissolve the impure compound in a minimum volume of hot solvent
2. Hot filter solution through filter paper quickly
3. Cool the filtered solution and leave to crystallise by inserting beaker in ice
4. Filter under reduced pressure with a buchner flask to separate out crystals
5. Wash the crystals with cold solvent
6. Dry the crystals between filter papers

Question 4

During recrystallisation, why dissolve the organic compound in a minimum volume of hot solvent?

Answer 4

• produces a saturated solution
• ensures a high yield

Question 5

In recrystallisation, what should be true of the solvent?

Answer 5

• high solubility at high temperature
• low solubility at low temperature

Question 6

In recrystallisation, why should you filter the solution when still hot?

Answer 6

removes insoluble impurities before crystals start to form

Question 7

In recrystallisation why should you cool the filtrate in ice?

Answer 7

Ice to increase yield

Question 8

In recrystallisation, why might the yield be greater than 100%

Answer 8

Sample was still wet (when recording dry mass)

Question 9

In recrystallisation, why might the yield be less than 100%

Answer 9

Product left in glassware

Question 10

What would you expect when measuring the melting point of pure organic crystals?

Answer 10

• solid collapses to liquid at one temperature
• sharp, specific melting point

Question 11

What would you expect when measuring the melting point of impure organic crystals?

Answer 11

• lower melting point
• melts over a range