Required Practical 5

• describe how you prepare cyclohexene by the dehydration and distillation of cyclohexanol

  1. Acidify the potassium manganate(VII) solution combining a volume of a regular laboratory concentration with an equal volume of dilute sulfuric acid.

  2. Pour the mixture into a pear-shaped/round flask, with a still head containing a thermometer.

  3. Attached to a condenser with a ice cooled collecting vessel.

  4. Add a few anti-bumping granules.

  5. Heat the flask gently.

  6. Collect sample at boiling point of the desired product.

  7. The cooled collection vessel is essential to reduce evaporation of the product.
• what are some key points to remember when preparing cyclohexene?

  ● A water bath or electric heater should be used to heat the mixture if there are flammable substances present.

  ● Anti-bumping granules prevent large bubbles from forming and ensure that the liquid doesn't boil too vigorously as this would result in the mixture boiling over into the condenser and undesired impurities would contaminating the product.

  ● The condenser should be tilted slightly down, so any liquid can run into the collection flask.

  ● The bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point.

  ● The water must enter at the lowest point and leave at the highest point to go against gravity as this ensures that water fills the condenser (prevents backflow of water), maximising heat transfer for condensation (more efficient cooling).

  ● The collection flask must not be sealed to the condenser, the system should not be air tight because as it is heated the air inside the system expands. If it is air tight then the air cannot escape and may cause the apparatus to crack.
• how do you prepare ethanal by the oxidation and distillation of ethanol?

  1. Make the oxidising agent by dissolving potassium dichromate (VI) in dilute sulfuric acid. The concentration of the potassium dichromate(VI) should be approximately 1 g in every 10 cm3 of this dilute acid.

  2. Using a 25 cm3 measuring cylinder, carefully measure out 12 cm3 of the acidified potassium dichromate(VI) solution and pour this into a boiling tube.

  3. Cool the boiling tube in a beaker of cold water.

  4. Using a 10 cm3 measuring cylinder, carefully measure out 2 cm3 of ethanol.

  5. Using a teat pipette, slowly add the 2 cm3 of ethanol dropwise, to the oxidising agent in the cooled boiling tube, shaking the tube gently to mix the contents.

  6. Add a few anti-bumping granules to the boiling tube and attach to it a bung fitted with a right-angled glass delivery tube. Anti-bumping granules allow a more even heating of the mixture.

  7. Clamp the boiling tube at about 450 in a beaker of water so that the delivery tube goes to a test tube which is immersed in cold water in a beaker.

  8. Gently heat the beaker of water containing the ethanol to slowly distil off approximately 5 cm3 of liquid distillate.

  9. Use Tollen's reagent to test the distillate for ethanal. (Should produce a silver mirror).
• how do you test for tollens' silver mirror test?

  1. Add 5 drops of sodium hydroxide solution to 2 cm of silver nitrate solution in a new, clean test tube.

  2. Add just enough dilute ammonia solution to dissolve the brown precipitate completely.

  3. Using a beaker of hot water (50 - 60 °C), gently warm a 5 cm depth of this test reagent in a test tube.

  4. Add 10 drops of the distillate containing ethanal to the warmed Tollens' reagent in the test tube.

  5. Wait a few minutes and note any observations. (A silver mirror should form).
• Why are anti-bumping granules used in distillation?

  To allow smooth boiling- preventing bubbles caused by vapour to cause upwards splashing in the flask.
• How do you isolate a liquid product?

  You can use a separating funnel to isolate the organic layer from the aqueous layer. Shake the flask. Allow the layers to separate. Then open the tap and run off one layer into a separate container.