Organic chemistry syntheses often use polar, high boiling point solvents to facilitate high temperature reactions. However, these solvents also can complicate down-stream compound purification, either by evaporation, crystallization, or even flash chromatography.
The bane of organic synthesis for most chemists is purification rather than synthesis. Synthetic reaction mixtures are rarely devoid of impurities so some type of purification is necessary. Most often flash chromatography is used but for many chemists, it is less well understood than their chemical reaction and provides some level of anxiety.
In this post, I will summarize the five most important steps to creating a successful flash chromatography method and thus the anxiety associated with it.
Recently, I posted an article explaining why high performance TLC plates are not needed for method development for high-performance flash chromatography. Based on some excellent feedback, I see a need to discuss silica chemistry and its impact on chromatography.
For many chemists using generic linear gradients and even gradients based on TLC the purification results often are not selective enough to separate all of the compounds in their mix. This is especially true if your target has a closely eluting impurity. One method used to try and increase resolution is the use of an isocratic hold or gradient pause during purification.
In this post I examine the use of the isocratic hold to determine how well it works and when/if it should be inserted into a gradient method.
The products of organic synthesis are designed with specific functional groups in order to possess desired properties. Depending on the compound’s functionality, it can be neutral, acidic, or basic as determined by a compound measurement called pKa or acid dissociation constant. Compounds with low pKa are typically acidic while those with high pKa tend to be basic. Compounds with a pKa near 7 are deemed neutral.
The challenges organic, medicinal, and natural product chemists face are many: from designing reactions, to optimizing synthesis, work-up / extraction, and purification / isolation of the desired compound or compounds. Among those issues related to purification / isolation is the common problem of separating compounds with similar chemistry that either co-elute or separate poorly.
In this post I will discuss some tips on how to "resolve" this issue (yes, pun intended).
Usually, a 2-solvent or binary gradient will separate your desired compound from the by-products and impurities. Sometimes though, you can encounter a mixture in which some compounds co-elute and are not separable with any binary gradient you try.
I encountered this situation recently while trying to purify a lavender essential oil and have dedicated this post to how I solved the problem.
Are you observing more chromatographic peaks than you expect compared to TLC or other assessment data? Well, it could be that your method is separating some isomers or, it could be that there is an actual method issue.
In this post I will discuss what could cause a method issue and suggest some ideas as to how to fix it.
When it comes to the purification of polar, water-soluble compounds reversed-phase chromatography is the most commonly used approach. However, because of strong stationary phase – mobile phase repulsion forces, the use of highly aqueous (90-100% water) solvent systems has been shown to provide less retention than needed. This issue has led to the development of “aqueous compatible” reversed-phase media.
For most organic reaction mixture purifications the process is fairly straightforward. Use hexane/ethyl acetate or, for polar compounds, DCM/MeOH. But what do you do if this doesn't work and your compounds are basic organic amines?
In this post, I re-examine the options available to achieve an acceptable organic amine purification when typical separation methods are insufficient.
Organic and medicinal chemists frequently utilize flash chromatography to purify their reaction mixtures. Normal-phase flash chromatography is most often used but may not the best methodology, especially when the compounds are quite polar and/or ionizable.
For these molecules, reversed-phase flash chromatography is preferred but often is not used due to an uncertainty regarding the best solvent choices and the reversed-phase mechanism. In this post, I will discuss how organic solvent choice in reversed-phase chromatography can influence the chromatographic separation.
This, of course, is always one of the first questions an organic, medicinal, or peptide chemist has when starting the research process for a flash chromatography system. Here at Biotage, we receive this question hundreds and hundreds of times a year, likely within the first couple of minutes of any conversation.
Up to six compounds can be easily separated with an automated step-gradient optimizer embedded in modern flash chromatography systems.
Compounds precipitating during flash chromatography is at best an inconvenience when working up your crude reaction mixture. Precipitation during purification typically happens in the column or in the tubing exiting the column.
In this post, I will propose a strategy that can minimize and perhaps prevent this issue from occurring.
For chemists needing to purify natural product extracts or synthesis reaction mixtures flash chromatography is typically the tool of choice. In previous posts I have discussed various ways to optimize the purification to obtain the highest purity compounds with maximum load in minimal time.
Sometimes, though, chemistry gets in the way in the form of solubility issues. When this happens most often dry loading is recommended for these sample types. In this post I will show the impact various dry load sorbent options have on chromatography.
In this post I will delve into six key factors that impact your purification success in flash column chromatography.
Previously, I have discussed the use of TLC for solvent scouting and method development and optimization. I have have also talked about cartridge size, particle size, and surface area and their impact on flash purification. Here I integrate that information into the six factors below.