reaction work-up and separation

	> Home > About Us > Privacy Policy > Contact us

Setting up a reaction is one of the easiest things in organic synthesis, but the purification could be always difficult, frustrated, and most likely time consuming. This page is a quick reference of reaction work up and compound purification.

Reaction Work up

n-BuoH or CH2Cl2/i-Propanol(4/1) could be used to extract polar compounds from aqueous solution. Distillated water (1 equiv.) could added into BuoH to form low boiling point (92oC) mixture that n-BuOH solution could be easily concentrated.

Crystallization

Classification and Separation Methods

Adsorption Chromatography
"Separation is based mainly on differences between the adsorption affinities of the sample components for the surface of an active solid." Common adsorbents are silica gel, aluminium oxide, polyamide, and active carbon.

1. Silica Gel is currently the most popular adsorbent in chromatography. The common chemical formula is SiO2/xH2O for normal phase column. The general way of packing a flash column is described in lot of online publications, like this one.
The siloxane structure of Silica Gel.

The silanol group is the function group in silica gel. It has stronger affinity with polar chemicals than with non-polar chemicals. The pH of silica gel is slightly lower than pH 7, but should not be bigger than pH 5 unless you will separate acidic compounds. The acid-base interaction between silanol group and strong acidic or basic chemicals is the reason that acids or bases are difficult to be separated under normal condition, in silica gel TLC, the spots of acids or bases are always have tails. The silica gel could be modified by adding acid, base, AgNO3, B(OH)3, NaB(OH)4, Na2SO3, FeCl3, CuSO4 to satisfy different chemical separation. The alkylation of hydroxyl group in silanol affords reverse phase silica gel, such as ODS (Octadecyl Silane). Both normal phase and reverse phase silica gel are used in open columns. The best combination of eluting solution in normal phase should be hexane/ethyl acetate/methanol, and methylene chloride/methanol/H2O (6/4/1) combination could be very useful in polar chemicals separation.

Solvents used in Normal phase silica gel	Solvents used in reverse phase silica gel
Hexane Petroleum ether, Methylene chloride, Chloroform, Diethyl ether, Ethyl acetate, Acetone, Methanol, Water.	Water, Acetonitrile, Methanol, Acetone.

2. Aluminium oxide is also very popular in separation. It has three types, basic aluminium oxide for basic chemicals' separation; neutral aluminium oxide for the separation of chemicals not stable under basic and acidic conditions, like lactone; acidic aluminium oxide for acidic chemicals' separation.

Ion-Exchange Chromatography
"Separation is based mainly on differences in the ion exchange affinities of the sample components." This type of adsorbent is used a lot as acid or base scavenger.

Exclusion chromatography
"Separation is based mainly on exclusion effects, such as differences in molecular size and/or shape or in charge. The term Size-Exclusion Chromatography may also be used when separation is based on molecular size. The terms Gel Filtration and Gel-Permeation Chromatography (GPC) were used earlier to describe this process when the stationary phase is a swollen gel. The term Ion-Exclusion Chromatography is specifically used for the separation of ions in an aqueous phase."

Sephadex Gel.

Partition Chromatography
"Separation is based mainly on differences between the solubilities of the sample components in the stationary phase (gas chromatography), or on differences between the solubilities of the components in the mobile and stationary phases (liquid chromatography)."

Affinity Chromatography
"This expression characterizes the particular variant of chromatography in which the unique biological specificity of the analyte and ligand interaction is utilized for the
separation."

Electrophoresis
Migration of charged particles (ions) in an electric field.

Side-Product Structure Identification