Summary
Summary Purification of drugs
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Drug design and mechanisms of drug action - 1st semester Includes bullet points, key diagrams and images
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Purification Extraction, recrystallization, distillation, chromatography and solid phase
extraction
Extraction
Liquid Extraction
Performed in work up stage
Different components of mixture have different solubility’s
Immiscible solvents to extract compound from one solvent
Organic and aqueous solvent used
Should readily dissolve substance, not react with the substance, not react/be
miscible in water and have a low boiling point
Organic solvents are diethyl ether, ethyl acetate and denser than water
dichloromethane and chloroform
Washing
Use a solvent which the substance is soluble in but water used most commonly
Acid-Base Extraction
Can separate different molecules by converting 1 of them into the salt form so
that they are soluble in the solvent.
Acidic solvent removes base and vice versa and can use weak acid to remove a
weak base (using pKa)
Recrystallisation
Dissolve crude material in minimum hot solvent and cool slowly to form crystals
Based on idea that hot solution can dissolve more material than cool
Only useful up 10-15% impurities
Can remove: insoluble material, small quantities of unreacted/by-products and
coloured by-products
Method: dissolution, hot filtration as it cools impurities stay in solution and pure
product precipitates out
, Distillation
Used for liquid mixtures with different boiling points
Simple distillation (<200oC) at atmospheric pressure
Fractional distillation used to separate liquids with similar boiling points
(<25oC)
Vacuum distillation (>200oC) to lower the pressure and boiling point
Chromatography
Physical separation where substance is distributed between 2 phases; where 1
phase is stationary and the other is the mobile phase (moving in a definite
direction)
Measure drug stability, breakdown products and impurities
Thin layer chromatography (TLC), flash column chromatography and high
performance liquid chromatography (HPLC)
Separation by adsorption in TLC and HPLC but also partitioning, ion exchange
and chiral
Ionic, dipolar and non-polar properties all determine whether the analyte is
soluble in the mobile phase or the stationary phase
Distribution coefficient Kx=C(stat)/C(mob) where C is concentration of
compound in that phase
Definitions
Analyte- substance being analysed
Mobile phase- phase that move in a definite direction so the sample and solvent
move through the column
Stationary phase- substance fixed in place
Eluent- fluid entering column
Eluate- fluid leaving the column
Elution- passing liquid through the column
Stationary Phase
Normal phase is silica (SiO2) or alumina (Al2O3) more polar compounds are more
strongly retained by hydrogen bonding. More polar solvents increase elution
Reverse phase is modified silica. Less polar compounds are more strongly
retained by hydrophobic interactions. Less polar solvents increase elution
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