28. Organic Synthesis
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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
How do you form a primary alcohol from an aldehyde?
NaBH4 reduction: nucleophilic addition
How do you form a carboxylic acid from a primary alcohol?
K2CR2O7/H2SO4 oxidation + reflux
How do you form a ketone from a secondary alcohol?
K2CR2O7/H2SO4 oxidation + reflux
How do you form a secondary alcohol from a ketone?
NaBH4 reduction: nucleophilic addition
How do you form a hydroxynitrile from a ketone or an aldehyde?
NaCN(aq)/H+
How do you form a carboxylic acid from a nitrile or hydroxynitrile?
H2O/HCl + heat
How do you form an amine from a hydroxynitrile?
H2/Ni
How do you form a carboxylic acid from an aldehyde?
K2CR2O7/H2SO4 reflux