Chemical reactions gone wrong, I’m sure we all have experienced this issue, I know I have. You add your reagents in the proper amounts with a suitable solvent and perform your reaction only to find your by-product yield was greater than your product; by a lot. So, what do you do to isolate what little product you created with maximum yield and purity without breaking the proverbial bank on a big flash column and the solvent required for the purification?
For most chemists purifying organic reaction mixtures, normal-phase flash chromatography is the go-to technique. Why not, it is quick, relatively efficient, and can provide relatively high loading capacities if the separation is properly optimized. However, most of these same chemists rely on only two sets of solvents to perform these purifications…
Automated flash chromatography systems have helped synthetic chemists speed up their synthetic research. One major advancement with these systems over the past 15 or so years has been the addition of photo-diode array ultraviolet (PDA-UV) UV detectors with which chemists can detect and fractionate using one, two, or multiple wavelengths. Enabling detection and fractionation with multiple wavelengths increases the likelihood that target and by-product compounds will be isolated with increased purity.
Chemists using silica columns for normal-phase flash chromatography typically equilibrate their columns prior to loading their samples. Companies manufacturing automated flash purification systems often have the equilibration volume and flow rate pre-programmed and tied to a column size. Some of these flash system companies allow equilibration volume to be edited while others have the volume fixed. Is one of these options better than the others? In this post I discuss how equilibration volume impacts flash chromatography results.
For chemists isolating their synthesized product in maximum yield and purity is a primary goal. Sometimes the crude reaction mixture stays in solution, sometimes it does not. In these cases, is it better to just redissolve in a strong solvent, say DMSO, or to filter, wash, and then purify? After all, the precipitating material may be unreacted starting material and could potentially complicate the subsequent flash purification step. On the other hand, it may be product crystallizing on its own and worthy of your attempt to isolate it without further work-up other than filtration.
Purifying your synthetic product efficiently in high yield with minimal impurities is every chemist’s goal. At discovery-scale, flash chromatography is the go-to purification technique as it is relatively simple and effective.
For most chemists, flash purification is a means to an end. In other words, it is a tool needed to purify and isolate one compound from a mixture of compounds so that the next reaction can occur with reduced by-product formation. Other than choosing between normal- or reversed-phase, there typically is not much thought put into cartridge selection, especially not related to stationary phase media porosity.
For most small molecules, this approach makes sense, but for larger molecules and very lipophilic compounds, factoring for media porosity should be included. In this post, I will discuss the impact media porosity can have on chromatographic performance.
For most organic and medicinal chemists, normal-phase flash chromatography is used to purify and isolate many types of organic compounds, most with some polar functional groups which help them retain on silica. However, some compound mixtures are water insoluble such as lipids, carotenoids, terpenes, tocopherols, polyaromatic and other hydrocarbons with minimal polar functionality. These lipophilic compounds do not retain well on silica and do not dissolve readily in water making them really difficult to separate.
In this post I will talk about a technique called non-aqueous reversed-phase chromatography that can be very effective at separating and purifying very lipophilic compounds.
Wouldn’t it be nice if your reactions only created your desired product? Of course the answer is yes, but that is not the reality of synthetic chemistry. Because our chemical reactions yield multiple components, they need work-up and purification to isolate the desired compound.