Organic techniques Flashcards
How are substances with low boiling and flash points heated in experiments?
Electrical heating is used
Prevents the substance catching fire as no naked flames
Why is refluxing done?
Prevents low boiling point solutions from evaporating or catching fire before they have time to react
Why do reflux reactions use a vertical Liebig condenser?
Allows the vapour which evaporated to condense
This gives the chemical time to react
What is the equipment set up in a reflux reaction?
Round bottom flask with lieberg condenser connected vertically
Anti-bumping granules in flask
Electrical source of heat
What are the electrical sources of heat commonly used?
Hot plates
Heating mantles
Electrically controlled water baths
What is the equipment set up in a distillation reaction?
Round bottom flask connected to a horizontally connected to a lieberg condenser
Thermometer at the intersection between the condenser and the flask
Flask collects the pure product
Electrical heating device used
What is the thermometer used for in distillation?
Tells you the boiling point of the product produced
Can compare to actual value to estimate if its a pure product
How can water soluble impurities be removed to form an organic product?
Separation in a separation funnel
How is a separation funnel used?
Add water and organic product with impurities to funnel
Invert the funnel 5 times and allow to settle
Organic and aqueous layer are immiscible so form two separate layers
Then separate layers by using tap
How can traces of water be removed from an organic sample?
Add anhydrous salt which acts as drying agent as binds to any water present
If lumpy add more salt and continue until a precipitate
Filter to remove the drying agent
What are examples of drying agents?
Magnesium sulphate (MgSO4) Calcium chloride (CaCl)
What is commonly used to measure the purity of a product?
Boiling or melting point
What will the boiling point of an impure product be compared to a pure one?
The boiling point of an impure sample will be higher
What is notable about the melting point of a pure sample?
It transitions from solid to liquid rapidly once at melting point
Called having a sharp melting point
What solute should be chosen for a recrystallisation of an impure solid?
Compound wanted is highly soluble when near b.p. and insoluble at room temp
How much solvent should be used in a recrystallisation?
Min amount to dissolve compound at b.p.
When is gravity filtration used?
Liquid is wanted and solid is not wanted
State the procedure for gravity filtration
Place fluted filter paper in funnel into a conical flask
Gently pour mix through
Rinse solid with pure sample of solvent in solution
Why do you wash the solid in gravity filtration with a pure sample?
Ensures that all soluble material has passed into conical flask
When is filtration under pressure done?
Usually done when the solid is wanted and the liquid is discarded
What piece of equipment is used for filtration under reduced pressure?
Büchner funnel
Conical flask
What is the procedure for filtration under reduced pressure?
Place filter paper on bottom of funnel
Wet paper with little solvent then turn vacuum on and pour mix into funnel
Rinse solid with a little solvent that mix was in
Wash off any original liquid from mix on crystals
Turn off and disconnect vacuum then leave solid to dry
Why do you wet the filter paper on a Büchner funnel before starting the vacuum?
Ensures it sticks to the bottom of the funnel and does not slip around when mixture poured in
How is the vacuum used in filtration under reduced pressure?
Flask is under reduced pressure
Liquid is sucked through the funnel into a flask
Why is the solid filtered under pressure washed with solvent from original mix?
Washes off any original liquid from mix on the crystals & soluble impurities
Leaves a more pure solid
How can the solid product of an organic reaction is solid, how can it be purified?
Recrystallisation
What is the procedure of recrystallisation?
Very hot solvent added to impure solid until it just dissolves
Filter solution while hot by gravity filtration
Solution left to cool slowly
Crystals of solid form and removed by filtration under reduced pressure
Solid washed with ice-cold solvent and dried
Why are solids dissolved in the minimum amount of hot solvent?
Gives a saturated solution of the impure product
Why are crystals in recrystallisation let to cool slowly?
Impurities stay in the solution
Present in smaller amounts than product so would take much longer to crystallise
What is the result of recrystallisation?
Crystals of the product which are much purer than the original solid
What must the solvent be for recrystallisation?
Only works if solid is very soluble in hot solvent
But solid must be nearly insoluble in cold solvent
What happens if the product is not soluble enough in the solvent for recrystallisation?
Will not dissolve in it at all and cannot be filtered by gravity
What happens if the product is too soluble in the solvent for recrystallisation?
Product stays in the solution after cooling - soluble in cold
Filtration would remove most of product, meaning very low yield
What method can be used to determine the purity of a liquid?
Measuring boiling point
How is the boiling point measured?
Distillation apparatus
Gently heat until it evaporates and read temp which is the boiling point
Then compare to value in data book
How will the boiling point of an impure liquid be different to the data books value?
Measured bp is higher than recorded value
Product boils over a range of temps rather than 1 temp
How can the purity of an organic solid be investigated?
Melting point
What is the melting point of a pure solid?
Specific melting point
Sharp
How are impure solid’s melting points affected?
Melting point is lowered compared to data book value
If very impure then melting occurs over wide range of temps
What is the procedure for a melting point?
Pack small sample of solid into glass capillary tube and place into a heating element
Increase temp under it turns from solid to liquid
Measure melting range
Compare to data booklet values
What is the melting range?
Range of temps from where solid begins to melt to where it has completely melted
