Knowing when it is time to replace your reversed-phase flash column is a question I am asked frequently along with…

Flash chromatography is the most commonly used purification tool for organic and medicinal chemists whose reaction scales typically range from milligrams to grams. The column size to be used for the purification of these reaction mixtures is selected either by using the 1% rule which states that for many reaction mixtures, a crude reaction mixture load equaling 1% of the column’s media weight often will provide the needed purity, assuming the right elution method is selected. While this strategy can work, often chemists either overload or underload the selected column resulting in low product purity which requires re-purification. In both situations, the chemists wastes time, solvent, and the cost of the column.

For my purification blog I often will synthesize compounds so I can show representative, real-world reaction product purification. In doing so, I decided I would also post on the impact of various synthesis variable. This post looks at the impact of reaction temperature time on an amide synthesis.

Though this is a purification blog I do, from time to time, like to address synthetic chemistry experimentation findings in the desire to assist you with your reactions, as this is the front-end of your synthesis workflow. So, in this post, I report on some findings of the effect of reaction temperature on the synthesis of an amide, 2-amino-N-benzylbenzamide, a potential antibacterial compound[1].

Flash chromatography competing with prep-HPLC? Why and when would flash outperform or even be equal to prep HPLC? Well, it depends on your purification needs and goals.

This is a question I asked myself while I have been studying synthesis variables to see what, if any, impact each variable has on reaction product yield and purity. For this post, I evaluated the order in which I added reactants and solvent.

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?

Think of orthogonal flash chromatography as 2D-chromatography where a reaction mixture or natural product extract is purified first with one methodology or solvent gradient then re-purified with a different method or solvent pair in order to remove co-eluting impurities. This is a technique practiced in medicinal chemistry, especially for final compound purification, when the final product is purified first with normal-phase flash followed by reversed-phase prep HPLC.

There are two general flash chromatography techniques...

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…

Chromatography is as much an art as it is a science. Between synthetic reaction products and natural products, the range of compounds requiring separation, purification, and isolation is broad and diverse creating challenges from time to time. Because of this diversity, not all chromatographic separations can be performed with a “neutral” solvent system – one without added pH modifiers or buffers.

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.

Flash chromatography is a purification technique used by chemists to isolate their targeted compound from by-products and impurities. Because the reaction mixture (or natural product extract) may have its best solubility in a solvent that is chromatographically “stronger” than the mobile phase, liquid sample loading can be problematic causing early eluting and/or broad peaks with poor purity. In those cases, a technique called dry loading is frequently used.

Flash chromatography is a purification technique used by chemists to isolate their targeted compound from by-products and impurities. Because the reaction mixture (or natural product extract) may have its best solubility in a solvent that is chromatographically “stronger” than the mobile phase, liquid sample loading can be problematic causing early-eluting, broad peaks which can reduce purification efficacy and product purity. In those cases, a technique called dry loading is a better alternative.

With all forms of chromatography there are limitations relating to sample load – both mass and volume. These are independent variables which, for the best results, should be investigated separately. In this post, I will address the impact of increasing solvent volume on flash chromatographic separations.

A good question I get asked frequently to which there is no specific value. When asked this question I answer, “it depends on your sample and how good your separation is”; not a satisfying response, but it is the truth.