[16.1] practical techniques in organic chemistry

Question 1

draw a diagram for a reflux setup

Answer 1

• condenser
• water out (top)
• water in (bottom)
• flask (pear or round-bottomed)
• heat ⬆

Question 2

draw a diagram for a distillation setup

Answer 2

• thermometer
• liebig condenser
• flask (pear or round-bottomed)
• water out
• water in
• beaker
• heat ⬆

Question 3

draw a diagram for a separating funnel

Answer 3

• separating funnel
• beaker

Question 4

describe how to purify an organic product

Answer 4

1. ensure that the tap of the separating funnel is closed
2. pour the mixture of liquids into the separating funnel
3. place a stopper in to the top of the funnel
4. invert to mix the contents
5. allow the layers to settle
6. add some water to see which layer increases in volume - this is the aqueous layer
7. place a conical flask under the separating funnel
8. remove the stopper and open the tap until the whole of the lower layer has left the funnel

Question 5

what are some common inorganic salts used to dry organic liquids?

Answer 5

• anhydrous calcium chloride (CaCl₂) - for drying hydrocarbons
• anhydrous calcium sulfate (CaSO₄) - for general drying
• anhydrous magnesium sulfate (MgSO₄) - for general drying

Question 6

describe the procedure for drying an organic liquid

Answer 6

1. add the organic liquid to a conical flask
2. use a spatula to add some of the drying agent to the liquid and gently swirl the contents to mix them together
3. place a stopper on the flask to prevent the product from evaporating away. leave for about 10 minutes
4. decant the liquid from the solid into another flask. if the liquid is dry it should be clear

Question 7

redistillation

Answer 7

• sometimes organic liquids have boiling points that are relatively close together, so your prepared sample may still contain some organic impurities
• a second distillation is carried out, but only collecting the product with the boiling point of the compound you are trying to make

Question 8

why should you never seal the end of the condenser in reflux?

Answer 8

the build up of gas pressure could cause the apparatus to explode

Question 9

why are anti-bumping granules added to the flask in both distillation and reflux?

Answer 9

to prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles

Question 10

why are electric heaters often used to heat organic chemicals?

Answer 10

organic chemicals are normally highly flammable and could set on fire with a naked flame

Question 11

what is reflux used for?

Answer 11

• heating organic reaction mixtures for long periods
• the condenser prevents organic vapours from escaping by condensing them back to liquids

Question 12

what is distillation used for?

Answer 12

• separating an organic product from its reacting mixture
• you need to collect the distillate at the approximate boiling point range of the desired liquid

Question 13

what are the conditions of distillation of a primary alcohol?

Answer 13

• use a limited amount of dichromate and warm gently and distil out the aldehyde as it forms
• this prevents further oxidation to a carboxylic acid

Question 14

what are the conditions of reflux of a primary alcohol?

Answer 14

• use an excess of dichromate and heat under reflux
• distil off product after the reaction has finished using distillation set up

Question 15

what would be observed in the reflux or distillation of primary alcohols?

Answer 15

organic dichromate solution changes to green colour of Cr³⁺ ions