Organic techniques

Question 1

Preparation and purification techniques of organic molecules

Answer 1

Refluxing
Purification by washing
Solvent extraction
Recrystallisation
Drying
Distillation + steam distillation
Melting temp determination
Boiling temperature determination

Question 2

Why is reflux done?

Answer 2

Using a condenser so when flammable and volatile compounds are heated, they are prevented from escaping due to evaporation
Heating to increase rate of reaction
Allows complete reaction as reactants fall back into flask due to condensing to continue to react

Question 3

Reflux apparatus

Answer 3

A vertical Liebig condenser: water enters at bottom and leaves at top against gravity
Attached to a round bottom/ pear shaped flask with anti bumping granules being heated at the bottom

Question 4

Water in vs out in reflux?

Answer 4

Water in at bottom
Water out at top

Question 5

How does reflux work?

Answer 5

Due to heat, the mixture is continuously boiled, evaporated then condensed (due to cold water in condenser) and recycled back into flask, will not escape and can lead to complete reaction

Question 6

Why use heating mantles in reflux?

Answer 6

Because this avoids naked flames which can ignite the flammable organic compounds

Question 7

Why must antibumping granules be used in reflux?

Answer 7

Prevent vigorous, uneven boiling by making/ providing a surface for small bubbles to form instead of large bubbles

Question 8

Why must the condenser not be sealed in reflux?

Answer 8

Build up of gas pressure could lead the apparatus to explode

Question 9

Why is distillation done?

Answer 9

To separate substances which have different boiling points in a mixture
Or distill a product to separate it from the reaction mixture = obtain a pure organic product

Question 10

Distillation apparatus

Answer 10

Heating pear shaped/ round bottom flask with anti bumping granules
Thermometer at T junction/ still head
Slanted Liebig condenser, water in at bottom and water out at top
Delivery to a flask
NOT A CLOSED SYSTEM

Question 11

What does the thermometer do in distillation?

Answer 11

Shows the boiling point of what is evaporating at that time

Question 12

Boiling points when distilling an impure substance

Answer 12

When the thermometer changes, a different liquid is about to be collected as there is a change in bp

Question 13

How does distillation work?

Answer 13

Gently heats a mixture which evaporates substances out of flask in order of boiling point
Thermometer tells us the bp of what is being produced

Question 14

Determining purity with boiling point

Answer 14

Use distillation apparatus to gently heat organic product
Read temp at which it is distilled (bp)
Look up bp in data booklets and compare this value: if the bp is higher it has impurities and boils at a range of temps

Question 15

Purification by washing

Answer 15

Obtain reaction mixture in separating funnel/conical flask and add sodium hydrogencarbonate or sodium chloride/ calcium chloride

Question 16

Purification by washing: why is sodium hydrogen carbonate added?

Answer 16

Neutralises any remaining acid in the reaction mixture
In making an ester: allows detection of ester by fruity smell, because presence of acid masked smell so neutralising acid allows detection

Question 17

Purification by washing: why is sodium chloride added?

Answer 17

To help separate organic layer from aqueous layer in a reaction mixture

Question 18

When is purification by washing done?

Answer 18

Esterification: neutralises H+ acid catalyst
Halogenation of an alcohol: neutralises conc HCl reactant

Question 19

What type of method is purification by washing?

Answer 19

Purification method to neutralise aqueous acid in reaction mixture
Then help the separation of aqueous and organic layer in a separating funnel

Question 20

Solvent extraction uses what apparatus?

Answer 20

A separating funnel

Question 21

Separating funnel drawing

Answer 21

Cone pointing down with a stopper at the top
Small channel from bottom of the cone with a tap to control opening

Question 22

Steps of solvent extraction

Answer 22

Pour mixture into separating funnel
Add water/wash with sodium hydrogen carbonate (washing method)
Shake funnel + allow to settle
Separates into 2 distinct layers: aqueous layer with impurities and organic layer
Run the tap to decant lower aqueous layer

Question 23

How do we ensure we know what layer is the aqueous layer in a mixture of products?

Answer 23

Wash through with water/sodium hydrogen carbonate/sodium chloride solution
See which layer increases in volume

Question 24

Theory behind solvent extraction

Answer 24

Separating different layers based on their differences in density
Aqueous high density layer on the bottom
Organic low density layer at the top

Question 25

Immiscible liquids, what they are, why they form and how can they be separated?

Answer 25

Liquids that do not mix when shaken together (due to differences in intermolecular forces, relative differences in solubility) SO can be separated by solvent extraction following washing

Question 26

Drying

Answer 26

Using a drying agent to remove any traces of water from an organic product

Question 27

When do we remove traces of water with anhydrous salt

Answer 27

If using separation to remove impurities, the product may still contain trace amounts of water

Question 28

How to remove trace amounts of water with anhydrous salt: drying method?

Answer 28

Add an anhydrous salt which acts as a drying agent by binding to water and becoming hydrated Add more until the salt is no longer lumpy AND ORGANIC LAYER IS CLEAR Filter to remove the newly hydrated salt and decant off organic liquid

Question 29

What acts as the drying agent?

Answer 29

Anhydrous salt Eg anhydrous calcium chloride

Question 30

What must the drying agent be?

Answer 30

*be insoluble in the organic liquid * not react with the organic liquid

Question 31

When do we know the organic liquid is fully dried?

Answer 31

Is clear

Question 32

Why does water enter the condenser from the bottom and leave against the force of gravity?

Answer 32

For efficient cooling of the condenser Prevents backflow of water

Question 33

Steam distillation

Answer 33

Pass a vapour (from the top) into apparatus which condenses due to Liebig condenser and distils off as a liquid

Question 34

Why use steam distillation?

Answer 34

If a product has a high boiling point

Question 35

Purification by Recrystallisation steps

Answer 35

Hot filtration Cool filtrate Suction filtration Wash and dry crystals

Question 36

How does hot filtration work in recrystallisation?

Answer 36

Dissolve impure solid compound in MINIMUM volume of hot solvent

Question 37

What should the solvent be in recrystallisation?

Answer 37

That dissolves crude product and impurities when hot but when cold, the crude product does not dissolve well and recrystallises out

Question 38

Why use minimum volumes of solvent in recrystallisation?

Answer 38

So solution has high concentration of dissolved crude product so recrystallisation out of solution is more likely

Question 39

What does hot filtration remove in recrystallisation?

Answer 39

Insoluble impurities that are not dissolved by solvent

Question 40

Why should the solution be kept hot in hot filtration?

Answer 40

So crude product does not recrystallise out of solution then get filtered out Thus decreases % yield

Question 41

How does cooling the filtrate work in recrystallisation?

Answer 41

Obtain hot filtrate from hot filtration and cool in a beaker of ice water Causes crude product to recrystallise out of solution

Question 42

Suction filtration in recrystallisation

Answer 42

Pour cold filtrate through Buchner funnel So soluble impurities in solution pass through but separates residue containing pure crude crystals

Question 43

What happens during cold suction filtration?

Answer 43

Wash through with cold solvent to remove soluble impurities into filter

Question 44

Final step of recrystallisation

Answer 44

Dry crude product (residue from cold filtration

Question 45

Apparatus for recrystallisation

Answer 45

Buchner funnel + Buchner flask

Question 46

What happens if crystals arent dried properly?

Answer 46

Mass of crude product weighed to be higher than reality because some solvent present

Question 47

Why use a buchner funnel/flask?

Answer 47

Reduces pressure and speeds up filtration

Question 48

Loss of yield in recrystallisation?

Answer 48

* Crystals lost when filtering or washing * Some product stays in solution after recrystallisation * other side reactions occurring

Question 49

When is purification by recrystallisation occur?

Answer 49

2,4-DNPH test - obtain pure hydrazone product Obtaining pure product of aspirin Obtain pure product of nitrobenzene in nitration

Question 50

Apparatus for melting point test

Answer 50

Thiele tube - has an arm at bottom that is heated Filled with oil And thermometer attached to capillary tube containing sample

Question 51

How does melting point test work?

Answer 51

Note the temperature (range) the substance in capillary tube melts at

Question 52

Why do we complete a melting point test?

Answer 52

Comparing an experimentally determined melting point value with one quoted in a data source will verify the degree of purity Eg if it melts over a range of temperatures or sharply

Question 53

Pure products melting point

Answer 53

Melts sharply at one temperature

Question 54

Impure products melting point

Answer 54

melting point will be lowered sample will melt over a range of several degrees Celsius

Question 55

Why use ice water to cool a mixture down instead of just ice?

Answer 55

For more effective cooling because ice water provides a larger surface area For more contact with organic product