RP 10 - Preparation of Organic Solids & Liquids Flashcards
What are the main steps in producing a pure organic solid?
- Synthesis of the compound (usually using reflux, distillation etc.)
- Filtration (usually vacuum filtration)
- Purification (recrystallisation)
How do you use lab equipment to head under reflux?
Quickfit apparatus is used to heat a substance under reflux
- The substance is boiled in a pear-shaped or round-bottomed flask
- As it evaporates, it’s cooled by the water in the liebig condenser and so condenses back into a liquid and drips back down into the flask to be heated again
Why’s heating under reflux used in the preparation of organic solids & liquids?
• Allows heating for a long period of time
• Prevents the flask from boiling dry
• Prevents volatile reactants/products escaping
• Ensures even heating
What does a diagram of Quickfit apparatus set up for heating under reflux look like?
’Round bottom flask with reactants and anti-bumping granules’ connected to a ‘Liebig Condensor’ with ‘Cold Water In’ at the bottom and ‘Water Out’ at the top on the same side of the condensor. The top of the condensor is also bunged off
Why are anti-bumping granules used when heating under reflux and distillation?
To allow smooth boiling - they prevent splashing up the sides of the flask due to bubbles caused by vapours
How do you use laboratory equipment to filter under reduced pressure?
Using a Buchner funnel and Buchner flask, connected by rubber tubing to the vacuum source
• The funnel contains a layer of filter paper
• Pour the substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask
• The solid will remain on the paper
What does a diagram of filtration under reduced pressure look like?
*There’s a ‘Buchner flask’ (looks like a conical flask with an opening valve on the side just below its neck) with a liquid in it. Attached to its side valve is a ‘Laboratory tubing’ with an arrow pointing out of it labelled ‘Vaccum pulled’. On top of the Buchner flask is a ‘Buchner funnel’ (looks like a normal filter funnel but with this glass casing on its outside that’s shaped like a cylinder on its top, then narrows with the funnel and finally is a wider tubing around the narrow part of the funnel that fits perfectly within the neck of flask). There’s an arrow going into the funnel labelled ‘Substances to be filtered goes in here’. The top of the funnel inside of the glass casing of the Büchner funnel is also labelled ‘Porous surface with filter paper on top’.
How do you purify a solid product?
• By crystallisation:
1. Add minimum amount of warm solvent to the impure sample until it has dissolved
2. Allow to cool, crystals should form
3. When no more form you can filter under reduced pressure to obtain a dry crystalline solid
How do you determine the melting point of a substance and why can this information be useful?
- Place a small sample of the solid in a capillary tube
- Melt using a melting apparatus available, measuring the temperature with a thermometer
- A pure substance will usually melt at a single temperature (or a very small range) but an impure substance, will melt over a range of temperatures (usually lower than that of the pure substance)
- Record the starting and ending points of the melting, when the first crystal can be seen to melt, and when the last crystal becomes a liquid respectively
- You can then compare the melting point to known values to identify the substance
How do you purify a liquid product?
- Use a separating funnel to isolate the organic layer from the aqueous layer
- Add anhydrous MgSO4 to the organic product to get rid of all water
- Purify the liquid by distillation, which separates the substances by boiling point
How do you calculate the percentage yield of a product?
= (Actual Yield / Theoretical Yield) x 100
