6.43 - Purifying an organic liquid Flashcards
What are the 4 main reasons as to why organic products are impure?
- Some starting materials are left unreacted
- Other organic products are produced
- Inorganic reagents were used, or inorganic products were formed from them
- Water is formed
What are the 4 main methods for purifying an organic liquid?
- Simple distillation
- Fractional distillation
- Solvent extraction
- Drying
How does simple distillation work?
The mixture is put into a pear shaped flask, and heated, and as the boiling points of the liquids are different, one evaporates off first, it can be collected separately to the other liquid, leaving a pure sample of the desired compound
What are the advantages of simple distillation?
- It is simple
- It is cheap
- It is easy to setup the apparatus
What is the main disadvantage with simple distillation?
It does not separate the liquids as well as fractional distillation - for it to work, the difference in boiling points should be at least 25K
How is fractional distillation different to simple distillation?
A fractionating column is added above the pear shaped flask
What does the fractionating column do?
Allows the vapour to condense back down into the flask
What does this mean in terms of the quality of separation?
It makes it better, as the vapour effectively evaporates several times
Does fractional distillation take less or more time than simple distillation? Why?
Longer, as there are multiple evaporations and condensations for each vapour particle
When is fractional distillation best used?
When the difference in boiling points is small, or when there are more than 2 liquids to separate
What is solvent extraction?
Using a solvent to remove the desired organic product from the mixture
What does the choice of solvent depend on?
- It must be immiscible (not mix) with the current solvent that the product is in
- The product must be much more soluble in the added solvent than the current solvent it is in
Summarise the process of solvent extraction:
- Place reaction mixture in separating funnel and add the solvent
- Place stopper in the neck of the funnel and gently shake for a while
- Allow the contents to separate into two layers again
- Drain the lower layer into a flask by opening the tap, and then put the other layer into a separate flask
Should the solvent be added all at once, or in smaller portions?
Smaller portions
How does drying work?
- Drying agent is added to mixture, and mixture is shaken
- Drying agent will turn crystalline
- If more drying agent is added, and remains powdery, then the liquid is dry
- Drying agent is then removed by decantation or by filtration
