Lesson summary
Complete PDF with figures and original page layoutPage 1 OCR A Level Chemistry A Your notes Organic Synthesis Contents Techniques Synthetic Routes Page 2 Techniques …
Part 1 of 7Distillation
- Purpose
- to separate compounds by boiling point when a reaction does not go to completion or other chemicals form
- The compound with the lowest boiling point distils first.
- An example is oxidising a primary alcohol to an aldehyde,
- which has a lower boiling point because it lacks hydrogen bonding and distils off as it forms.
1
Put the alcohol and acidified potassium dichromate in a pear-shaped or round-bottomed flask.
2
Add anti-bumping granules for smooth boiling.
3
Set up Quickfit apparatus with a still head and a condenser at the side, greasing the joints thinly with silicone grease for a good seal and easy dismantling.
4
Place the thermometer bulb where the vapour enters the condenser.
5
Pass a steady stream of water through the condenser jacket, in at the bottom and out at the top.
6
Heat with an electric heating mantle, which gives temperature control and avoids flames with flammable chemicals.
7
Collect the distillate in a receiving vessel within ±2 °C of the boiling point of the desired product.
- Discarding
- distillate below that range is discarded and a clean vessel used, and collection stops if the temperature rises above the range
Part 2 of 7Heating under reflux
Purpose
- Purpose organic reactions are often slow at room temperature, so reflux lets the mixture react fully without loss of reactants, products or solvent
- In distillation the product is separated off, but in reflux all chemicals stay in the vessel.
Uses
- A carboxylic acid from a primary alcohol with acidified potassium dichromate.
- A ketone from a secondary alcohol.
- An ester from an alcohol and acid with an acid catalyst, e.g. ethyl ethanoate after about 15 minutes of reflux.
1
Put the reaction mixture in a pear-shaped or round-bottomed flask with anti-bumping granules.
2
Heat in a heating mantle or water bath.
3
Set up Quickfit apparatus with the condenser clamped vertically, joints greased.
4
Pass a steady stream of cold water in at the bottom and out at the top.
5
Let the mixture boil, then stop heating and let it cool to room temperature.
Part 3 of 7Practical safety and choices
Heating
- Heating use an electric heating mantle or water bath rather than a Bunsen burner
Hazards
- Hazards handle corrosive, irritant, flammable and toxic liquids carefully
Apparatus
- Apparatus choose and set up the equipment to suit the reaction
Part 4 of 7Purification with a separating funnel
Layers
- Layers water often forms an aqueous layer with the organic product in the organic layer
- To identify the organic layer, add water and see which layer grows.
- The organic layer is the bottom layer only if it is denser than the aqueous layer;
- the question will give information on this.
Neutralising
- Neutralising add sodium carbonate solution to remove acidic impurities, opening the stopcock slowly to avoid losing product
1
Transfer the contents of the reaction vessel to a separating funnel and add a stopper.
2
Invert the funnel and open the stopcock to release pressure, repeating 15–20 times.
3
Allow the two layers to separate.
4
Open the stopcock to drain the lower layer, then run the other layer into a clean beaker.
Part 5 of 7Drying and redistillation
Drying agent
- Drying agent an anhydrous inorganic salt that readily hydrates, used to remove traces of water
- Anhydrous calcium chloride dries hydrocarbons.
- Anhydrous calcium or magnesium sulfate are general-purpose drying agents.
Using a drying agent
- Add a spatula of drying agent to the organic product and swirl, adding a lid if the product has a low boiling point.
- If the agent clumps, water remains, so add more until some stays dispersed as a fine powder.
- Decant or filter the dry liquid, which should look clear.
Redistillation
- Redistillation used when traces of other liquids with similar boiling points remain
- The apparatus is cleaned, dried and set up again.
- A narrower boiling point range for the distillate gives a purer liquid.
Part 6 of 7Aliphatic functional group interconversions
Aliphatic functional group interconversions
Key groups
- Key groups alkanes, alkenes, haloalkanes, alcohols, carbonyls and carboxylic acids
- (aldehydes and ketones)
Reactions linking them
- Alkane to haloalkane by free radical substitution.
- Alkene to alkane by hydrogenation and to haloalkane by hydrohalogenation.
- Alkene to alcohol by hydration, and alcohol to alkene by elimination.
- Haloalkane to alcohol by hydrolysis and alcohol to haloalkane by substitution.
- Alcohol to aldehyde, ketone or carboxylic acid by oxidation.
Part 7 of 7Designing a reaction pathway
1
Draw the structures of the target molecule and the starting molecule.
2
Check whether they have the same number of carbons; to lengthen the chain, add a nitrile group by nucleophilic substitution.
3
List all compounds that can be made from the starting molecule and all that can be converted into the target.
4
Match the groups they have in common and work out the reagents and conditions.
- Level
- at AS only a two-step route is required; A Level needs multistage synthesis
- 1(reactant2intermediate3product)
- Chloroethane to ethanoic acid goes by hydrolysis to ethanol, then oxidation under reflux.
- at AS only a two-step route is required; A Level needs multistage synthesis
Key terms
| Term | Meaning |
|---|---|
| Distillation | Separating liquids by boiling point |
| Reflux | Heating with a vertical condenser so vapours condense back into the flask |
| Drying agent | An anhydrous salt that removes traces of water from an organic liquid |
| Target molecule | The product a synthesis is designed to make |
| Redistillation | A second distillation to obtain a purer liquid |
Avoid these mistakes
1
Distilling when a reflux is needed to fully oxidise to a carboxylic acid
2
Assuming the organic layer is always the top layer — it depends on density
3
Adding a stopper to a separating funnel and not releasing pressure
4
Heating flammable organic liquids with a Bunsen burner — use a heating mantle or water bath