28. Organic Synthesis Flashcards
Describe how to purify organic crystals
dissolve crystals in a minimum volume of hot solvent
cool to recrystallise
filter under reduced pressure (Buchner flask)
wash residue with ice cold solvent
leave to dry to obtain dry crystalline solid
take melting point (should be sharp if pure)
compare to database of known melting points and identify
Describe how to purify organic liquids
place liquid in separating funnel and run off visible aqueous layer
(identify aqueous layer by adding distilled water and seeing which layer grows)
if liquid may contain acid impurity add Na2CO3 to neutralise
invert separating funnel several times and release pressure frequently
remove aqueous layer
add remaining organic layer to conical flask and add drying agent (MgSO4) until it stops clumping together
decant off dry organic liquid
redistill over a smaller temperature range
Why do we use reflux?
prevent loss of volatile substances and maximise yield
safer as flammable substances contained (use a heating mantle instead of a bunsen flame)
Why does purification by recrystallisation work?
the desired product and the impurities have different solubilities in the chosen solvent
What is the melting range?
difference between temperature at which solid starts to melt and temperature at which it has completely melted
What will be the features of a melting point of an impure sample?
will have a large melting range
will be lower
How do you use a melting point machine?
melt the end of a capillary tube to seal it
tap the open end into a sample of the dry crystals to obtain about 3mm depth
put capillary tube in melting point machine and observe when it is fully melted
repeat but be more cautious and use rough go to get more accurate reading
How do you form a primary alcohol from a haloalkane
NaOH (aq)
nucleophilic substitution
How do you form a haloalkane from an alkene
hydrogen halide
How do you form a primary alcohol from an alkene?
direct hydration w H3PO4/H2O
How do you form an alkene from a primary alcohol?
H2SO4
How do you form a haloalkane from a primary alcohol?
sodium halide/H2SO4
NaBr + H2SO4 => NaHSO4 + HBr
HBr reacts to form haloalkane
alcohol + NaBr + H2SO4 => bromoalkane + NaHSO4 + H2O
How do you form a nitrile from a haloalkane?
NaCN in ethanol
How do you form an amine from a nitrile?
H2/Ni
reduction
How do you form an amine from a haloalkane?
NH3/ethanol (solvent) to form ammonium salt
then add NaOH => amine + H2O + NaCl
How do you form an aldehyde from a primary alcohol?
K2CR2O7/H2SO4
oxidation + distil
