RQP 10: Preparation of a pure organic liquid

Question 1

Describe the general method for purifying an organic liquid

Answer 1

• put the distillate of the impure product into a separating funnel
• wash product by adding either:
  1. Sodium hydrogencarbonate solution, shaking and releasing the pressure form the CO2 released
  2. saturated sodium chloride solution
• Allow the layers to separate in the funnel, and then run and discard the aqeous layer
• Run the organic layer into a clean,dry conical flask and add three spatula loads of drying agent (eg. ahydrous sodium sulfate, or calcium chloride) to dry the organic liquid
• When dry the organic liquid should appear clear
• carefully decant the liquid into the distillation flask
• distil to collect pure product

Question 2

Why can sodium hydrogencarbonate be used to wash the impure product

Answer 2

Sodium hydrogencarbonate will netralise any remaining reactant acid

Question 3

Why can saturated sodium chloride solution be used to wash the impure product

Answer 3

Sodium chloride will help to separate the organic layer from the aqeous layer

Question 4

How do you know which layer is organic and which is aqeous

Answer 4

The layer with the lower density will be the upper layer. This is usually the organic layer.

Question 5

What two conditions make a suitable drying agent

Answer 5

The drying agent should:
- be insoluble in the organic liquid
- not react with the organic liquid

Question 6

Describe the detailed method for preparing and purifying cyclohexene from cyclohexanol

Answer 6

• Pour about 20cm^3 of cylcohexanol into a weighed 50cm^3 pear-shaped flask.
• Reweigh the flask and record the mass of cyclohexanol.
• Using a plastic graduated dropping pipette, carefully and with frequent shaking, add to the flask approximately 8cm^3 of concentrated phosphoric acid.
• Add a few anti-bumping granules to the flask and assemble the distillation apparatus, so that the contents of the flask may be distilled.
• Heat the flask gently, distilling over any liquid which boils below 100 degrees.
• Pour the distillate into a separating funnel and add 50cm^3 of saturated sodium chloride solution.
• Shake the mixture and allow the two layers to separate.
• Run off the lower layer into a beaker and then transfer the upper layer, which contains the curde cylcohexene, into small conical flask.
• Add a few lumps of anhydrous calcium chloride or anhydrous sodium sulfate or anhydrous magnesium sulfate to the crude cyclohexene to remove water.
• Stopper the flask, shake the contents and allow this to stand until the liquid becomes clear.
• decant the liquid into a clean, dry, wieghed sample container.
• reweigh the container, calculator the mass of dry cyclohexene produced.

Question 7

Describe the detailed method for making propyl ethanoate from ethanoic acid

Answer 7

1. Propan-1-ol (50cm^3) and ethanoic acid (50cm^3) are mixed thoroughly in a 250cm^3 round-bottomed flask.
2. Concentrated sulfuric acid (10cm^3) is adeed drop by drop to the mixture, keeping the contents of the flask well shaken and cooled in an ice water bath.
3. When the acid has all been added, a reflux condenser is fitted to the flask and the mixture gently boiled over an electric heating mantle for about 30 minutes.
4. The mixture is cooled, and the apparatus rearranged for distillation. The crude ester is distilled off.
5. The distillate is placed in a separating funnel and shaken with about half of its volume of 30% sodium carbonate solution, with the pressure being released at intervals.
6. The lower aqeous layer is then discarded.
7. The crude ester is shaken in a separating funnel with about half its volume of 50% calcium chloride solution, which removes unreacted alcohol. The lower layer is discarded.
8. The ester is run into a clean, dry flask containing some anhydrous calcium chloride and swirled.
9. The ester is filtered into a clean,dry flask, with a few anti-bumping granules, and distilled.
10. The fraction boiling between 100 degrees celsius and 103 degrees celsius is collected.

Question 8

Describe what precaution should be taken when using concentrated sulfuric acid as a catalyst for esterification and why

Answer 8

Add drop by drop and cool- this is because adding concentrated sulfuric aicd is an exothermic reaction and this precuation must be taken to prevent uncontrolled boiling over.

Question 9

How do you know which layer is organic

Answer 9

The upper layer is organic because it has a lower density than water

Question 10

Why are anti-bumping granules used

Answer 10

To prevent the vigorous, uneven boiling by making small bubbles form instead of large bubbles.

Question 11

Why is sodium carbonate added to the distillate after undertaking esterification

Answer 11

To react with any acid left in the distillate. This will produce CO2, so the pressure of gas will need to be released.

Question 12

How do you use a thermometer to get an accurate measurement for boiling point

Answer 12

The thermometer should be above the level of the surface of the boiling liquid and be measuring the temperature of the saturated vapour.