Most people who are new recruits to the research labs learned their basic skills for carrying out reactions in an undergraduate laboratory. Inevitably, most of the organic chemistry undertaken in these lab classes involves following recipes, which have been well tried and tested. Therefore the conditions and the time taken for the reactions to reach completion arc well established, and work-up can be carried out after a preset time. Unfortunately, the idea that you can guess the time it takes for a reaction to reach completion is a very bad habit to carry over into a research environment.
Every reaction you carry out should be monitored, and one of the first things you should do before starting any reaction is to decide on a suitable method for monitoring its progress. Even if you are following a literature procedure, reaction monitoring is still essential and it will usually save you time as well as giving you confidence about what is happening. Carrying out a reaction without monitoring its progress is like trying to thread a needle with your eyes closed!
The simplest and most universal method of reaction monitoring is thin layer chromatography (tic) and this will be discussed first of all, but it is not always the best or only method, and sometimes you may have to use a little ingenuity to find an appropriate reaction monitoring technique.
1 Thin Layer Chromatography (TLC)
Tic is a simple, but extremely powerful analytical tool. However it may take a little time before your expertise reaches a consistently high level since a certain element of intui-tion is always involved in choosing the appropriate solvent system, spotting the correct amount of sample, etc. Once you have gained experience and confidcnce in the use of tic, you will find it extremely useful for a variety of purposes.
1. 1 The Main Uses of TLC
Tic is normally the simplest and quickest way to monitor a reaction and the reaction mixture should be chromatographed against starting materials (and a co-spot). This allows you to follow how the reaction is progressing, and to assess when is the best time to work it up. In all cases a record of the tic should be made in your lab book.
Tic can be used to indicate the identity of a compound, by comparing the unknown sample, with a known material. In general each substance is spotted separately and also together (co-spot). Caution should be applied as co-running on tic is not definitive proof of identity. Of course, substances that do not co-run are definitely not the same.
TLC usually gives a good indication of the purity of a substance. Diastereoisomers can usually (but not always), be distinguished.
For flash chromatography, tic is first used to determine the solvent system and quantity of silica required, and secondly to monitor the column fractions.