REQUIRED PRACTICAL 5 - distilling an organic product Flashcards
How do we prepare cyclohexene?
By the dehydration of cyclohexanol and distilling cyclohexene from reaction mixture
What is distillation?
Technique to separate chemicals which have different boiling points
What happens in step 1 - distillation? for preparation of cyclohexene
- Add H2SO4 to flask containing cyclohexanol
- Add anti-bumping granules
- Connect rest of distillation equipment and turn water on (water goes in bottom of condenser and comes out at the top)
- Gently heat to 83 degrees via water bath or electric heater
- Warm gas rises and goes into condenser
- Cool water surrounding condenser condenses gas back into a liquid which is collected as an impure product
Whats the purpose of antibumping granules?
To promote production of small bubbles
Why do we use a water bath or electric heater instead of a bunsen burner?
As alcohol is flammable so it’s a safety method
What happens in step 2 - separation? in preparation of cyclohexene
- Product from distillation contains impurities
- Transfer liquid into separating funnel and add water to dissolve water soluble impurities ad create and aqueous solution
- Mixture allowed to settle
- Drain off aqueous layer at bottom leaving behind impure cyclohexene
Function of separating funnel in step 2 of preparation of cyclohexene
Used to separate a miscible and immiscible liquids
Miscible meaning
Liquid dissolves in water
Immiscible meaning
Liquid dissolves in an organic solid
What happens in step 3 - purification? in preparation of cyclohexene
- Put impure cyclohexene into round bottomed flask 12. Add anhydrous CaCl2 (drying agent - removes traces of water) and stopper flask 13. Let mixture dry for 20 minutes with occasional swirling 14. Distill one more time as cyclohexene may still have impurities Only collect mixture which comes over 83 degrees as it will be pure cyclohexene
How do you prepare ethanal?
By oxidation of ethanol and distilling ethanal from reaction mixture
Method of preparing ethanal
- Add sulphuric acid and potassium dichromate (VI) to a round bottomed flask or pear shaped flask 2. Slowly add ethanol dropwise and add anti-bumping granules 3. Heat flask of water gently and slowly distill off approximately 5cm3 of liquid distillate into a test tube which is immersed in ice cold water in a beaker 4. Keep test tube cool to avoid loss of volatile ethanal 5. Test for ethanol
What is potassium dichromate (VI)?
A drying agent
Distillation equipment: ice water bath
Cools collection flask so reduced evaporation of condensed ethanol
Distillation equipment: heating bath/source
Used for flammable products (e.g. alcohols, ketones and aldehydes) preventing flammable liquids from igniting. The boiling point of the liquid should be above the boiling point of the sample in the round bottomed flask.
Distillation equipment: condenser
Water in at bottom and out at top ensures gases condense into liquids. Ensure water is turned on. Maintains cool water supply along whole length of condenser
Separating alcohol from water via fractional distillation: fractionating column filled with glass beads?
Allows vapours to condense and fall back into round bottomed flask. Water and ethanol are separated in this section. Water vapours condense back into the liquid mixture
Separating alcohol from water via fractional distillation: thermometer
Observe and check boiling point stays below that of the alcohol so that only the alcohol vapour passes into the condenser
Reflux description from mark schemes
- Mixture of liquid heated to its boiling point for a prolonged time - Evaporation - vapour formed which escapes from liquid mixture - Condensation - vapour changed back into liquid and returns to liquid mixture - Any ethanal and ethanol that initially evaporate can then be oxidised
What happens when a secondary alcohol is reflected using acidified potassium dichromate (VI)?
Creates a Ketone
What happens when a primary alcohol is reflected using acidified potassium dichromate (VI)?
Creates carboxylic acid
What and how much acid do you add to alcohol before the potassium dichromate in reflux?
25cm3 of H2SO4
A limitation of a water bath in reflux (as it prevents overheating)
Will limit reaction temperature to 100 degrees
What speed should water be through a condenser?
Slow, it makes little difference and faster usually leads to flooding
Why does the top need to be open in reflux?
Allows vapours to escape and prevent a build up of pressure
What do you need to ensure in reflux with an open top?
To reduce the rate of heating to ensure maximum yield of product
Why do you use a gauze over a bunsen burner?
Allows mixture to boil more smoothly. Good idea to use anti-bumping granules too (to promote small bubbles)
Distillation set up diagram
Would be a flask at recieving end containing impure product
Seperating funnel diagram
Rather than kerosene oil, impure cyclohexene
The water layer is aqueous containing water soluble impurities
Remove the stopper from the top to run substances out of seperating funnel
Reflux equipment diagram
Can be round bottomed flask too
