Purifying An Organic Liquid (6.4 3) Flashcards
Organic product could be contaminated with …
Unreacted starting materials
Other organic products
The inorganic reagents used or in organic products formed from them
Water
Techniques use (liquid) - technique depends on whether it is a gas liquid or solid
Simple distillation
Fractional distillation
Solvent extraction
Drying
Laboratory apparatus
Pieces of glassware connected by tubing corks and bungs inexpensive and easy to use - if a gas leaks like carbon dioxide is not a major problem
Organic products leak
More of a problem flammable and toxic ,may attack corks and bungs so increase leaks and contamination
Solution for apparatus
Apparatus made only or mostly of of glass fitted tightly together using ground glass joints
Eight pieces of apparatus
Need to add from of.209
Heating
Using a buses burner but often an electric heating mantle or hot water bath or hot oil bath is preferred for safety reasons
Simple distillation
Heating flask connected to condenser
Liquid with lowest boiling point evaporated or boils off first pass into condenser first . Collected in receiver separately from any other liquid
Thermometer if temp remains steady as vapour passes into condenser one compound . If temp begins to rise different compound
Add figure c from page 209
Advantages of simple distillation
Easier to set up and quicker
Disadvantage simple distillation
Does not separate liquids as well as fractional distillation
Only used of boiling temps very different ideally more than 25’c
Fractional distillation
Fractionating column between heating flask and still head
Column filled with glass beads or pieces of broken glass which act as surfaces on which the vapour leaving can condense and then evaporated again as more hot vapour passes up the column
Effectively vapour undergoes several repeat distillations as it passes up the column which provides better separation
Add fig D
Advantage and disadvantages
Takes longer than simple
Best used when difference in boiling temperature is small and where there are several compounds needing to be separating from the mixture
Solvent extraction
Using a solvent to remove desired organic product from other substances in the reaction mixture
Solvent added should be
Immiscible (not mix) with solvent containing desired organic product
Desired organic product more soluble in solvent added than the reaction mixture
Solvent extraction process
Place reaction mixture in separating funnel and then add chosen solvent (should form separate layer )
Place stopper in neck on funnel and gently shake contents in funnel for a while
Allow the contents to settle into two layers
Remove the stopper and open tap to allow lower layer to drain from flask . Pour upper layer into separte flask
Most of desired organic product would have moved to added solvent
How to make solvent extraction more efficient
Use solvent in small proportions with same total volume removes more of the desired product
Final step of solvent extraction
Simple or fractional distillation now used to separated organic product from solvent used
Drying principles
Prepared using inorganic reagents which are often in aqueous solution
A liquid organic product may be partially or completely dissolved in water so water may be an impurity that may need to be removed by a drying agent
Important feature of a drying agent
Does not react with organic liquid
Most common anhydrous metal salts form hydrated salts when come in contact with water absorb water as a form of crystallisation.
Anhydrous calcium chloride can also be sued from some but does react with others and is soluble in alcohols
Drying process
Drying agent is added to organic liquid the mixture is swirled or shaken then left for a period of time
Before sue a drying agent is powdery but after absorbing water it looks more crystalline
If more drying agent is added it will remain powdery - indication it is drying
The drying agent is ringed by recantation (pouring the organic liquid off the solid drying agent ) or filitraton
