My research involves synthesising dendrimers and dendrons, sometimes following literature procedures to make one that has been made before, sometimes combining literature procedures and basic chemical understanding to make something new to science. In either case (and it doesn’t really matter if you know what a dendrimer or dendron is), a key part of this process is working out what we’ve made. Now, I’m fortunate to work in a department with a whole range of instruments that can be used to identify the product(s) of a reaction. For things that have been made before, a couple of techniques are required to verify the identity and purity of what has been made. For things that have never been reported, every technique reasonably possible should be used to probe the structure of the molecule.
That all sounds fair enough for the final product of a reaction, but what is less obvious sometimes is the need to probe every intermediate step as well. Simply, a good chemist should not proceed with the next reaction step until they are sure they’ve made what they think they’ve made. It isn’t enough to look into a flask or vial and say ‘well it looks like I expected it to therefor its OK’. Similarly, it isn’t good enough to quickly scan an NMR spectrum (unless you’ve run the reaction so many times it is irreversibly imprinted on your brain stem) and think ‘yeah that looks OK, lets just get on with it’.
Do you know what you’ve made? Do you know exactly how much, and how pure? Do you know what residual solvents are stuck in the sample? If you don’t, go back to the lab bench and figure it out, then we’ll deal with the next step.
Of course, it isn’t always possible to characterise as you go along. If you end up waiting for the final step, then doing a whole set of analyses together, better get used to the idea that you’re making a leap of faith. Two or three weeks work could come crashing down on your shoulders because you assumed that your intermediate was the right one. Of course, you may have made something much more interesting…from a research point of view, but not what you intended.
The lesson that all chemists need to learn is characterise early, characterise often.