experimental techniques Flashcards
what is reflux used for
allows strong heating without losing volatile reactants and products
how does reflux work
- volatile compounds evaporate and condense and fall back into the flask
- Liebig condenser has cold water running through the wall
when hot evaporating substances hit the cold condenser they turn back into a liquid and return back to the round bottom flask to react further
heating is done with an electric heater → mantle as we are using flammable liquids
what is distillation used for
used when we want to separate substances with different bpts
distillation is useful when you want to extract a chemical before it reacts any further
how does distillation work
gently heating the mixture will result in the compounds separating out in order of bpt
what happens if your compound has a lower boiling point than the starting mixture in distillation
- you heat to the temperature of the boiling point of your compound you want to separate
- collect product in a separate vessel
what happens if your compound has a higher bpt than your starting mixture in distillation
- heat to the temp of the bpt of the compound you want to separate
- compound will remain in round bottom flask
what is steam distillation used for
used when we want to separate substances with high bpts or decompose when heated
when is steam distillation used
if the product is immiscible with water then steam distillation is used to separate compounds that couldn’t be done under standard conditions
how does steam distillation work
- if the substance were trying to collect is less volatile then the substances were trying to separate from it then the desired product will evaporate out of the flask with the steam
- the product is then condensed and collected in a separate flask
the product can be separated using a separating funnel (if product is partially miscible then you have to use solvent extraction)
how does steam distillation work
- if the substance were trying to collect is less volatile then the substances were trying to separate from it then the desired product will evaporate out of the flask with the steam
- the product is then condensed and collected in a separate flask
the product can be separated using a separating funnel (if product is partially miscible then you have to use solvent extraction)
what are the two types of filtration
gravity filtration
vacuum filtration
what is gravity filtration used for
- used to separate solids from liquids
- used if we want to keep liquid and dispose of solid
how is gravity filtration carried out
- place fluted filter paper in the funnel and dampen to make a seal
- pour the reaction mixture into the funnel slowly to stop overflowing
- gravity will pull through the liquid component into the vessel below which can then be purified if needed
what is vacuum filtration used for
- used to separate liquids from solids
- a vacuum is used to help separate the liquid and solid thoroughly
how does vacuum filtration work
- place a filter paper disc in the buchner funnel and dampen slightly to make a seal
- pour the reaction mixture into the bucner funnel with the vacuum line on
- vacuum creates a reduced pressure in the flask and pulls the liquid through → solid left in buchner funnel which can be recrystallized to purify it
what is recrystallisation used for
used to purify solids
how do you carry out recrystillisation
1) add just enough hot solvent to allow the impure solid to dissolve → this will mean you have a saturated solution of your impure product
2) allow the solution to cool down slowly → crystals ill start to form
3) impurities will remain dissolved in solution as there is a small quantity of them → is takes a lot longer for them to crystallise
4) filter to get the slid purified crystals and then wash with very cold solvent and dry them off
what does the solvent do in recrystallisation
you want impure solid to dissolve fully in hot solvent but insoluble when its cold
- if not the substance wont dissolve in hot solvent and you cant filter the purified solvent if it isn’t soluble in cold solvent
what can boiling points and melting points be used for
help detect impurities and identify compounds
how does using boiling points to identify substances work
- gently heat the sample → measure the temp at which it distils using the thermometer → this is the bpt
- compare this bpt to the data booklet value
- the sample contains impurities if the bpt is higher than what is recorded in the data book as the sample will oil over a range of temps
how does identifying a compound using melting points work
1) add a sample of the solid product into a capillary tube and place into the heating element of the mtp apparatus
2) slowly increase the temp until the substance starts to melt
3) there is a temp range from when the solid just started to melt to when it fully melts
4) compare the mtp against data book value
5) if substance contains impurities the mtp will be lower and the temp range the substance melts at will be larger
what is the separation technique used for
separation techniques are used to remove impurities that are dissolved in water
how does separation work
- add the products from distillation into a separating funnel and add water to dissolve soluble impurities to create an aqueous solution
- after allowing the solution to settle→ two layers will form
- top layer→ impure
- bottom layer→ aqueous layer containing water-soluble impurities
- drain the aqueous layer off
how does purification by washing work
- removal of impurities my washing
- wash the product with another liquid to remove impurities
how does purification by drying work
- take impure product from the separating funnel and add to a round bottom flask
- can add a dehydrating agent such as anhydrous CaCl2 → remove aqueous substances still remaining
- filter the solid drying agent to remove, using fluted filter paper (to inc surface area)
how do you add carbon atoms
friedel crafts alkylation
(nitrile produced using haloalkane and KCN)
->hydrolysis of a nitrile
grignards reagents
how do you prepare a grignard reagent
bromoalkane + magnesium -> R-MgBr
conditions - ethoxyethane
reflux
what are the properties of grignard reagent
act as nucleophiles because alkyl chain is slightly negitive, MgBr is slightly positive so is removed
grignard + water
alkane + Mg(OH)Br
grignard + CO2
grignards + carbonyl
aldehyde + grignard
ketone + grignard
what are the 2 methods of removing carbon atoms
Hoffman degradation
hunsdieker reaction
what is the general equation for Hoffman degradation
amide (R- CONH2) + Br2 + 4KOH —> R-NH2 (amine) + K2CO3 + 2KBr + 2H2O
what is the general equation for Hunsdieker reaction
R- COO-Ag+ + Br2 –> R-Br + CO2 + AgBr
silver salt haloalkane
