What is reflux and how does it work?
Reflux is the technique used when you want to heat volatile liquids as it allows strong heating without losing volatile reactants and products.
The volatile compounds evaporate and condense when they hit the cold condenser (Liebig) and return to the round bottomed flask.
What is distillation and how does it work?
Distillation is used when we want to separate substances with different boiling points.
If the compound has a lower boiling point than the starting mixture you heat to the temperature it boils at and collect it in the separate vessel.
The compound with the higher boiling point remains in the round bottom flask.
Why is distillation useful? Give an example
It is useful when you want to separate a compound before it reacts any further.
For example, oxidising primary alcohols can produce aldehydes however if you don’t remove the aldehyde, then it will oxidise further into a carboxylic acid. Therefore to obtain the aldehyde, you use distillation
What is redistillation and how does it work?
It is used when we want to purify volatile substances which can be purified further using separation.
First the separating funnel is inverted and shaken many times.
The stopper is opened to release pressure.
The immiscible liquids separate with the aqueous layer at the bottom and the organic layer at the top.
The stopper is removed allowing the aqueous layer to be removed.
Then add anhydrous CaCl2 which will remove any aqueous substances remaining as it is a dehydrating agent.
Filter the solid drying agent to remove
What is reduced pressure filtration and how does it work?
A vacuum is used to help separate the liquid and solid components thoroughly.
Place a filter paper disc in the Buchner funnel and dampen slightly to make a seal.
Pour the reaction mixture into the Buchner funnel with the vacuum line on.
The vacuum creates a reduced pressure in the flask and pulls the liquid through. The solid is left in the Buchner funnel.
The solid can be recrystallised to purify further.
What is recrystallisation and how does it work?
A method to purify solids and the solvent chosen is very important.
1. Add just enough hot solvent to allow the impure solid to dissolve this means you will have a saturated solution
2. Allow the solution to cool down slowly…crystals will start to form
3. Your impurities will remain dissolved in solution as there is a smaller quantity of them, it takes a longer time for them to crystallise
4. Reduce pressure filter them to get your solid purified crystals. Wash with very cold solvent and then dry them off.
Why should you chose your solvent for recrsytallisation carefully?
The process of the separation of the desired compound from impurities relies on solubility differences at different temperatures.
You want your impure solid to dissolve fully in hot solvent but virtually insoluble in cold solvent. If not your substance won’t dissolve in hot solvent and you can’t filter the purified solid if it is soluble in cold solvent.
It means that impurities are also easier to remove.
How can the purity of a compound be determined by measuring the melting point?
- Add the sample of the solid product into a capillary tube and place into the heating element of the melting point apparatus
- Slowly increase the temperature until the substance starts to melt
- There is a temperature range from when the solid just starts to melt to when it fully melts
- Compare the melting points against data book values
- If your substance contains impurities the melting point will be lower and the temperature range the substance melts at will be larger.
What does thin layer chromatography (TLC) allow us to do?
Separate and identify compounds
What are the steps to TLC?
- TLC uses a stationary phase of silica or alumina mounted onto a glass/metal plate. A pencil base line is drawn and drops of mixtures added
- Place the plate in a solvent - the base line must be above the solvent level
- Leave until solvent has moved up to near the top of the plate. Remove, mark the solvent front and allow to dry
- It works by the mixture spots dissolving in the solvent. Some chemicals in the mixture may not dissolve as much and stick to the stationary phase quickly. We are then left with a chromatogram
- We can identify the chemicals using the positions on the chromatogram
What can we use to see colourless compounds in TLC?
Iodine
Fluorescent dyes
UV light
How can we use fluorescent dyes and UV light to see colourless compounds in TLC?
Add the fluorescent dye to the silica/alumina and shine a UV light on to it.
The colourless spots on the chromatogram will block any glow from the dye.
You can then draw around them to mark where they are.
How can we use iodine to see colourless compounds in TLC?
Place the chromatogram in a sealed jar with a few iodine crystals.
The iodine vapour sticks to the chemicals on the plate dying them purple.
What is the iodine vapour known as when identifying colourless compounds in TLC?
Locating agent
What is the equation for calculating Rf values?
Rf = distance travelled by spot / distance travelled by solvent
Measure the distance travelled by solvent from the base line to the solvent front.
What is column chromatography and how does it work?
Ideal for separating and purifying larger quantities of a mixture.
A burette or another glass column is packed with silica or alumina (stationary phase).
The mixture and solvent (mobile phase) is run through the column. The solvent is run through continuously.
The different compounds in the mixture run through the column at different rates. This means they come out of the bottom of the column at different times.
Each pure substance can be collected separately.
What is IR spectroscopy?
Uses infrared radiation to increase the vibrational energy of covalent bonds in a sample.
What is the frequency of infrared absorbed by a covalent bond dependent on?
The atoms that are either side of the bond.
The position of the bond in the molecule.
What is mass spectroscopy?
Used to find the relative atomic mass of a compound (Mr).
What is the M+ peak?
The molecular ion peak.
This is the same as the relative molecular mass of the molecule.
What is m/z?
The mass of a fragment divided by charge. Most fragments charge is +1 which is the same as the fragment mass.
What is fragmentation?
When molecules break up when they are bombarded with high energy electrons. Fragmentation can be used to determine molecular structure.
It produces a positive fragment and a radical, only the positive charged fragment is detected.
How do we combine mass spec and infrared to help identify an unknown compound?
- Functional groups
Use infrared to identify any functional groups in your unknown sample. - Structure
Use mass spec to identify the structure from fragment patterns.
What is green chemistry?
A way in which resources are used sustainably by protecting the Earth’s resources and the environment.
