16 - Organic Synthesis Flashcards
When do we use distillation
there are times that a reaction does not go to completion or there are other chemicals produced as well as the desired product
Which compound will distill first
lowest boiling point will distil first
When do we use distillation - example
produce an aldehyde from a primary alcohol
Distillation process - producing an aldehyde
- primary alcohol + acidified potassium dichromate solution is placed into a pear-shaped or round bottomed flask
- Anti-bumping granules are added
- Quickfit apparatus is then set up, including a still head and condenser connected to the side
- thermometer can be used, with the thermometer bulb sitting where the vapours will pass into the condenser
- A steady and constant stream of water passes through the condenser in = enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser
- The reaction mixture is heated until it boils using a heating mantle
- The distillate which forms in the condenser drips directly into a receiving vessel
Why do we use anti bumping granules
to promote smooth boiling
Why are the joints of the apparatus often have thin layer of silicon grease on
to give a better seal as well as to make it easier to disassemble the equipment afterwards
Why do we use electrical heating mantles
temperature can be controlled, and because you are using chemicals which are flammable
Advantages of heating under reflux
• allows the mixture to react as fully as possible without the loss of any reactants, products or solvent
Reflux process
• reaction mixture is placed into a pear-shaped or round bottomed flask
• Anti-bumping granules are added
• The flask is placed in a heating mantle or it can be immersed in a water bath for heating
• Quickfit apparatus is then set up with the condenser clamped vertically in place
• A steady and constant stream of water passes through the condenser
• The water is heated and the reaction mixture allowed to boil
• The heated is stopped and the mixture allowed to cool back to room temperature
Why do we heat reactions?
To overcome activation energy/increase rate of reaction
What is the advantage of heating under reflux?
Enables liquid to be continuously heated, prevents volatile components evaporating and the flask boiling dry.
Advantages of ant bumping granules
Ensures contents boil smoothly
• Prevents large bubbles forming and thus prevents the glass from bumping violently
3 steps to purify organic solvents
• Separating funnel
• Drying agents
• Redistill
If a substance needs to be neutralised before purified what should we do
adding sodium carbonate solution to the reaction vessel or separating funnel
How to tell which layers which in the separating funnel
adding water and seeing which layer increases in volume
Method for separating funnel
- Ensure that the tap is closed
- Pour mixture into the funnel, place a stopper in top, invert + mix contents.
- Allow the layer to settle
- Add some water – the layer that increases is the aqueous layer.
- Place a conical flask under the separating funnel, remove the stopper, open the tap until the whole of the lower layer has left the funnel.
- Collect the second layer into a different conical flask
Method for removing acid impurities
• Add aqueous sodium carbonate and shake in funnel
• Hold upside down
• Any acid will react with sodium carbonate, releasing carbon dioxide
- Slowly open tap to release gas
Purpose of drying agents
• May be water left in the product
• Removed by adding a drying agent
What are drying agents
• Anhydrous inorganic salt
Examples of drying agents
• Anhydrous calcium chloride, anhydrous calcium sulfate, anhydrous magnesium sulfate.
Process of using drying agent
spatula of drying agent is added into the organic product and swirled
Why would we add a lid when using drying agents
• If the organic product has a low boiling point, a lid / stopped can be added to reduce the potential evaporation of any product
What happens if the drying agent clumps together r
then there is still water in the organic liquid
When do we stop adding drying agent
until some remains dispersed in the organic liquid as a fine powder
I’d the organic liquid is dry…
it should appear clear
How to get rid of the drying agent after
Filter / decant
Why would we Redistill
organic liquids have boiling points that are close together, the prepared sample may contain some organic impurities.
Process of redistillation
• Use clean, dry distillation apparatus to collect the product with the boiling point of the compound you are trying to make.
• Use narrower range
What’s different about redistilling and distilling
Redistilling = use narrower temp range
Synthetic routes
Aliphatic reaction pathways
Memorise this diagram
How to go from an alkane to a haloalkane
Free radical substitution
Reagents + conditions to go from alkane to haloalkane
Halogen + UV Light
How to go from an alkene to a alkane
Electrophillic addition / hydrogenation
Reagents + conditions for alkene to alkane
Hydrogen + nickel catalyst
How to go from a alkene to a haloalkane
Electrophilic addition
Reagents + conditions for alkene to haloalkane
Hydrogen halide + room temp
How to go from haloalkane to alcohol
Nucleophilic substitution
Reagents + conditions from haloalkane to alcohol
Aqueous sodium hydroxide + heating under reflux
How to go from alcohol to haloalkane
Nucleophilic substitution
Reagents + conditions from alcohol to haloalkane
Sodium halide + Sulfuric acid + heating under reflux
How to go from alkene to alcohol
Hydration
Reagents + conditions from alkene to alcohol
Steam + phosphoric acid
How to go from alcohol to alkene
Dehydration / elimination
Reagents + conditions from alcohol to alkene
Concentrated acid + reflux
How to go from secondary alcohol to ketone
Oxidation
Reagents + conditions from alcohol to ketone
Secondary alcohol
Acidified potassium dichromate + heat under reflux
How to go from alcohol to aldehyde
Oxidation
Reagents + conditions from alcohols to aldehyde
Acidified potassium dichromate + distillation
How to go from alcohol to carboxylic acid
Oxidation
Reagents + conditions from alcohol to carboxylic acid
Acidified potassium dichromate + heat under reflux
What undergoes hydrolysis
Haloalkane to alcohol
what colour are dichromate ions
orange
what colour are chromium 3+ ions
green
