REQUIRED PRACTICAL 5 - distilling an organic product

Question 1

How do we prepare cyclohexene?

Answer 1

By the dehydration of cyclohexanol and distilling cyclohexene from reaction mixture

Question 2

What is distillation?

Answer 2

Technique to separate chemicals which have different boiling points

Question 3

What happens in step 1 - distillation? for preparation of cyclohexene

Answer 3

1. Add H2SO4 to flask containing cyclohexanol
2. Add anti-bumping granules
3. Connect rest of distillation equipment and turn water on (water goes in bottom of condenser and comes out at the top)
4. Gently heat to 83 degrees via water bath or electric heater
5. Warm gas rises and goes into condenser
6. Cool water surrounding condenser condenses gas back into a liquid which is collected as an impure product

Question 4

Whats the purpose of antibumping granules?

Answer 4

To promote production of small bubbles

Question 5

Why do we use a water bath or electric heater instead of a bunsen burner?

Answer 5

As alcohol is flammable so it’s a safety method

Question 6

What happens in step 2 - separation? in preparation of cyclohexene

Answer 6

7. Product from distillation contains impurities
8. Transfer liquid into separating funnel and add water to dissolve water soluble impurities ad create and aqueous solution
9. Mixture allowed to settle
10. Drain off aqueous layer at bottom leaving behind impure cyclohexene

Question 7

Function of separating funnel in step 2 of preparation of cyclohexene

Answer 7

Used to separate a miscible and immiscible liquids

Question 8

Miscible meaning

Answer 8

Liquid dissolves in water

Question 9

Immiscible meaning

Answer 9

Liquid dissolves in an organic solid

Question 10

What happens in step 3 - purification? in preparation of cyclohexene

Answer 10

11. 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

Question 11

How do you prepare ethanal?

Answer 11

By oxidation of ethanol and distilling ethanal from reaction mixture

Question 12

Method of preparing ethanal

Answer 12

1. 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

Question 13

What is potassium dichromate (VI)?

Answer 13

A drying agent

Question 14

Distillation equipment: ice water bath

Answer 14

Cools collection flask so reduced evaporation of condensed ethanol

Question 15

Distillation equipment: heating bath/source

Answer 15

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.

Question 16

Distillation equipment: condenser

Answer 16

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

Question 17

Separating alcohol from water via fractional distillation: fractionating column filled with glass beads?

Answer 17

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

Question 18

Separating alcohol from water via fractional distillation: thermometer

Answer 18

Observe and check boiling point stays below that of the alcohol so that only the alcohol vapour passes into the condenser

Question 19

Reflux description from mark schemes

Answer 19

• 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

Question 20

What happens when a secondary alcohol is reflected using acidified potassium dichromate (VI)?

Answer 20

Creates a Ketone

Question 21

What happens when a primary alcohol is reflected using acidified potassium dichromate (VI)?

Answer 21

Creates carboxylic acid

Question 22

What and how much acid do you add to alcohol before the potassium dichromate in reflux?

Answer 22

25cm3 of H2SO4

Question 23

A limitation of a water bath in reflux (as it prevents overheating)

Answer 23

Will limit reaction temperature to 100 degrees

Question 24

What speed should water be through a condenser?

Answer 24

Slow, it makes little difference and faster usually leads to flooding

Question 25

Why does the top need to be open in reflux?

Answer 25

Allows vapours to escape and prevent a build up of pressure

Question 26

What do you need to ensure in reflux with an open top?

Answer 26

To reduce the rate of heating to ensure maximum yield of product

Question 27

Why do you use a gauze over a bunsen burner?

Answer 27

Allows mixture to boil more smoothly. Good idea to use anti-bumping granules too (to promote small bubbles)

Question 28

Distillation set up diagram

Answer 28

Would be a flask at recieving end containing impure product

Question 29

Seperating funnel diagram

Answer 29

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

Question 30

Reflux equipment diagram

Answer 30

Can be round bottomed flask too