After endless hours labouring away in the lab, the moment of truth inevitably comes when it’s time to calculate your percentage yield.
Ideally, you want to get a good yield and avoid wasting reagents, so you can keep costs down and operate more sustainably. If you are in an industry like pharmaceuticals, where many steps may be involved in the synthesis of an active molecule, then you’ll know how important a good yield can be.
When assessing whether or not you have a good yield, you should be realistic about what you can typically expect. Lower yields are characteristic of certain types of reactions.
If you’re not getting as a good a yield as you anticipated, then it may be helpful to go back to basics and consider what you can do to improve it. You may be missing something you can easily rectify.
Precisely what you need to do to improve your yield, will depend on the type of reaction you’re doing but there are a few general tips that may be helpful nonetheless. Take the time to think through each step of your reaction and consider what could be affecting your yield.
A checklist for maximising your yield
This simple checklist will help you make sure you’ve not forgotten to consider some basics which could be affecting how much product you get:
- Is your glassware sparkling clean? You don’t want something as simple as contaminated glassware affecting your reaction.
- Have you measured your reagents accurately? It’s easy for human error to creep in.
- Are you sure all your reagents are pure? We’ve certainly all, at least once, used starting materials which turned out to be old and have degraded.
- Are you choosing the best reaction vessel for each stage of your reaction? Maybe a slightly different type, or size, might work better.
- Are you controlling the temperature of your reaction effectively? Your reaction could be going too fast or slow, so try to keep the temperature optimal throughout. The equipment you choose could help with this. For controlling temperature precisely, you might want to consider using Radleys’ Carousel™ Stirring Hotplates,along with products optimised for heat transfer such as Heat-On™ blocks.
- If you’re dealing with a gaseous reaction, does the pressure need adjusting?
- Has your reaction gone as near to completion as it can? Perhaps it needs more time, or more catalyst?
- If you’re dealing with a reversible reaction, does the reaction's equilibrium position lie on the side of the products? Consider adjusting the amounts of the reagents you’re using, to shift the reaction in your favour.
- Could side reactions be generating by-products which are reducing your yield? Separating out by-products is often costly, so it’s best to minimise them in the first place by reducing the temperature for example.
- Could you reduce the number of transfers you are carrying out?
- Do you need to slow things down by adding reagents dropwise for instance?
- Are you stirring thoroughly enough? This is particularly important for viscous samples which may need mechanical (overhead) stirring instead of magnetic stirring. You could even increase your overhead stirring productivity by up to 600% with Radleys’ Tornado™ Overhead Stirring System which uses a single stirrer for six reactions.
Work-up, quenching and purification
- Do you need to try an alternative solvent? You may be losing product at the extraction stage. Try adjusting the amount of solvent too.
- If you are filtering, do you need to use a different membrane that the product is less likely to stick to?
- If you are dealing with a volatile product, could you be losing too much to evaporation? Try adjusting the temperature and/or pressure to minimise such losses.
- Has the product dried sufficiently?
- Are you leaving some product in the vessel or reaction mixture?
- Are you weighing your product on a properly calibrated machine?
Do you have any tips you’d like to share? Why not let us know by tweeting us @Radleys?