In this instalment of relearning organic chemistry we’re going to take a look at chromatography, used for separation and purity control. It’s an important tool of organic chemistry and you’re likely to come across references to it in posts on DF, so apart from it being one of the systematic analysis steps, you’ll wont to have a working knowledge of the technique to be involved in chemistry discussions.
Chromatographic isolation (column)
Column chromatography involves a cylinder packed with a suitable absorbent substance.
The differing content of a mixture passes down the column at different rates, presenting as a series of bands moving down in sequence. Each of the constituent band is collected separately.
The fractions are then identified by experiment such as, observation of colour reaction or physical properties.
The column “packing” material and the solvent are governed by the mixture to be separated.
Most of the time the column is packed with Alumina or silica gel.
The solution in the column is referred to as the “elute” a fancy French word meaning to wash out or cleanse.
The slowest solvent or the lowest elution fraction would be the saturated petroleum’s Petroleum ether, Pentane, Hexane with a succession of solvents of increased elution, Benzene, Chloroform, Ether and Methanol /Ethanol.
A common example of the process would be a mixture as a solute first absorbed by the column packing then the elute collected in factions of Hexane,>> Hexane-benzene with gradually increasing Benzene ratio then Benzene,>>Benzene-ether with gradually increasing Ether ratio finishing with Ether,>>Ether-methanol. This is just one of many variations.
Chromatographic isolation (vapour-phase)
Vapour-phase chromatography (VPC) is not something the I’ve ever had any contact with so I’m reluctant to give any instructions on it. If you wont information on VPC just check it out at Wikipedia.
CONTINUED in About Organic Chemistry 1.5b