JiuguiA and Y. WangA*
Abstract: If you liked palladium-on-carbon, you’ll love the all-new palladium-on-lemon!
Introduction
Green chemistry is one of the most potent buzzwords in the modern chemical lexicon. Such has been the flap and clamour surrounding this recent fad that most people have forgotten about its predecessor: yellow chemistry. Despite being maligned and traduced by certain members of the internet community,1,2 yellow chemistry has been the foundation of organic synthesis for decades. Its tenets are that i) the reactions should actually fucking work, and ii) none of the reagents or solvents should be more deathy than necessary.3
The other day, my colleague Jiugui and I were reflecting on the nuances of green and yellow chemistry while sipping lab gin.4 Jiugui observed that the bubbles of CO2 nucleating on his slice of lemon resembled the effervescence of the Fischer-Whirlitzer reaction. As so often occurs when under the influence of short-chain alcohols, we were struck by a bolt of sublime inspiration: what would happen if we put lemons in chemical reactions?
Results and Discussion
The conventional workflow in synthetic chemistry is as follows: have a great idea >> get excited about said idea and start ordering chemicals >> search idea in SciFinder >> discover that it’s already been done by someone in 1969 >> weep.5 It should therefore come as no surprise that when we typed “applications of lemon juice in synthetic chemistry” into Google, we got more hits than cats and porn (Scheme 1).
The citric acid in lemon juice has been harnessed to cleave protecting groups such as silanes and acetals.6 The juice is administered as a 1:1 mixture with ethanol, to improve the solubility and the taste. As a catalyst, lemon juice has also been demonstrated to facilitate the oxidation of sulfides and boronic acids.7 Even as little as ten drops of lemon juice can catalyse an aldol condensation.
Despite the wealth of existing knowledge on the applications of lemons, we remained undeterred. After all, there is no scientific niche so small that it can’t accommodate a Tet. Lett. paper. It is notable that the existing procedures make use of the juice alone, and discard the remaining 70% of the lemon’s mass. This is very wasteful, and hardly consistent with the principles of green chemistry. As such, we utilised a holistic approach in a search for the applications of whole lemons in the laboratory.
1. Customisable Septa for Air Sensitive Reactions
The pith of a lemon is compressible, springy and easily cut. This makes it an ideal replacement for synthetic rubber in the manufacture of septa for air-sensitive reactions. Conventional septa become brittle and perforated with age and must be discarded, which contributes to laboratory waste and dead dolphins and shit. As lemons are biodegradable and infinitely replaceable, they are a far more sustainable alternative. Figure 1 shows the use of a lemon septum to facilitate the air-free transfer of samarium(II) iodide into a flask.
2. Heterogeneous Catalyst Support
Heterogeneous catalysts are ubiquitous tools for chemistry performed on the milligram and the megaton scale. Palladium-on-carbon is one of the most frequently employed such catalysts, but the carbon onto which the palladium is coated typically derives from burnt organic matter or coal. Again, this is inconsistent with a sustainable approach to synthesis. Like activated carbon, the surface of a lemon is pitted and porous, giving it a large surface area. To demonstrate this, we obtained NIST standardised lemons from SigmaEldrich,8 and coated them with 800 mesh Pd powder (Figure 2).
We found that ~300 mg of palladium adhered to a 300 g lemon, giving the resultant catalyst a loading of 0.1% w/wt. Aside from the environmental superiority of lemons, another benefit of our catalyst system is that it is much easier to weigh out. Specifically, the lemon can be sliced proportionally to the desired mass of active catalyst.
3. Sample Holders
The malleable nature of lemons makes them amenable to use as racks for a wide array of laboratory items, such as NMR tubes and eppendorfs (Figure 3). Furthermore, lemons are buoyant, and can be used as floats for sonicating samples. Traditional floats are made of polystyrene, which is in turn synthesised from petrochemical and the tears of pandas.
4. Base Neutralising Agent
Citric acid and other mild proton sources are often used in solvent extractions, or in quenching reactive bases such as NaH and BuLi. In the case of solvent extractions, a slow addition of acid is sometimes desirable, particularly if the aqueous layer contains a carbonate salt. Slices of lemon can be used for this purpose. We demonstrated that 100 mL of saturated NaHCO3 could be neutralised with 6 slices of lemon, without excessive foaming or overflow from the separating funnel. The lemon can then readily be removed by filtration, when the bubbling ceases.
A single slice of lemon is also capable of converting 20 mL of 1.6 M BuLi into lithium citrate in a controlled manner. For larger quantities of reactive bases, we recommend throwing lemons for a distance of 5-10 meters away.
Experimental
Preparation of lemons Seeds of Citrus limon were added to a ceramic pot (50 L) containing vermiculite (20% w/wt), perlite (5% w/wt), sphagnum peat (20% w/wt) and composted bark (55% w/wt), maintained at room temperature in air. The pot was then irradiated (250–1750 nm) for three years. Aliquots of water were added daily, and aliquots of ammonium nitrate and potassium phosphate were added monthly. The progress of the reaction was monitored by visual inspection, and when sufficient product had formed, it was collected by manual extraction. The product thus obtained was identical to an NIST Standard Lemon.
Preparation of lemon juice
Lemons were washed 3-4 times with deionized water, cut in half, and squeezed to get the crude juice. The crude lemon juice was filtered through medium-speed filter paper, left to stand in the fridge, centrifuged for 5 min at 8000 rpm, and the clear supernatant without any fibre was collected and stored in the 2-8 ◦C fridge for further use.
Conclusion
Despite being superficially yellow, lemons are one of the greenest reagents available to the modern chemist. We have demonstrated their applications in a range of practical roles throughout the laboratory, and are currently working on commercialising technical-grade lemons through SigmaAldrich.
Acknowledgements
Mr Wang acknowledges himself for helpful discussions.
Conflicts of Interest
The authors categorically deny that this paper was funded by “big lemon”.
Notes and references
1. For examples of the defamation of yellow chemistry, see https://www.youtube.com/@ExplosionsAndFire
2. https://www.youtube.com/watch?v=PDapGJ9jWZk
3. The IOUPAC Yellow Book
4. Note: “Lab gin” is a mixture of four-parts ethanol with six-parts water, garnished with whichever chemicals in your lab smell the nicest.
5. “What a great ide-awwww shit never mind” E. V. One, 1970, J. Org. Chem. 3, 69704–69797.
6 “A green deprotection strategy: Removing acid-labile protecting groups using lemon juice/ethanol as the solvent” W. Wang, Y. Chen, X. Li, Q. Zhang, Tet. Green. Chem., 2023, 2, 100026.
7. https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/slct.202103701
8. Note: NIST standard lemons can be purchased from SigmaAldrich for a million billion dollars. Probably.