Ionic liquids Solvents reveal their ionic powers Katharine Sanderson unveils the proof that finally showed quite how special the green solvents ionic liquids really are

Tom Welton knows his solvents. In proof that ionic liquids didn’t alter taking into account concentrations of particular Welton knows more than In short reactions more than a conventional, the reacting molecules. The reaction your average chemist about room  Ionic liquids are touted molecular solvent would. That quest Welton chose was a nucleophilic temperature ionic liquids, a class of as green alternatives to for proof would force Welton to eat substitution reaction, a well chemicals being touted as the solvents conventional solvents his words, and show that ionic liquids known organic reaction where one of choice for environmentally friendly  In certain situations, are more special than even he had chemical group rich in electrons (the chemical reactions. ionic liquids can alter imagined. nucleophile), replaces another group Welton, currently the head of the rates of reaction in a way In a plenary lecture at the 2nd by attacking a molecule’s weakest, chemistry department at Imperial never seen for standard International Congress on Ionic electron poor part (the electrophile). College London, UK, has worked with solvents Liquids, in Yokohama, Japan, in Welton wanted to test different these strange liquids made from two  The reasons why, and August 2007, Welton told the audience chemical combinations, in different salts, which melt together at room uses for this phenomenon, of his hunch – that there was no such solvents both molecular and ionic temperature when they come into are still being studied thing as an ionic liquid effect. liquids. If the graphs following the contact, for most of his academic Meanwhile, one of Welton’s reaction rates and concentration of career. students was laboriously working reactants that were plotted all turned The hope for ionic liquids as green to prove this by following the speed out to have the same shaped line, this solvents comes from their lack of at which reactions progress when would show that the reactions were all vapour, and their ease of recycling. following the same trend, and Welton Researchers around the world would finally have proof that the ionic have spent years investigating the liquid effect does not exist. properties of these liquids, and doing Each reaction was performed in HIROYUKI OHNO HIROYUKI chemical reactions in them. seven different molecular solvents Yet for many of those years and seven different ionic liquids. First something didn’t seem right to came the reaction of a negatively Welton. He was uncomfortable with charged nucleophile with a neutral the notion that reactions done in these electrophile. As he expected, Welton special solvents performed differently saw no special ionic liquid effect – as a direct, general, consequence of the shape of the graphs tracing the the ionic liquid. As far as Welton could reactions were the same for all 14 see, after an exhaustive trawl of the solvents. literature, there was no such thing as Then came the turn of a neutral a general ‘ionic liquid effect’ and no nucleophile to react with a neutral quantitative evidence that such an electrophile. No ionic liquid effect was effect existed. seen. The third of four combinations to be tested was a neutral nucleophile Beyond reasonable doubt reacting with a positively charged ‘I spent years saying there was nothing ‘There is no such thing as an ionic liquid effect’ – Tom Welton has electrophile. Yet again, no special special,’ says Welton. But he needed had to eat his words since Yokohama in 2007 effect was seen. 54 | Chemistry World | May 2010 www.chemistryworld.org GREEN CHEMISTRY GREEN STEFAN WILDHIRT, DEGUSSA AG / / AG DEGUSSA WILDHIRT, STEFAN www.chemistryworld.org Chemistry World | May 2010 | 55 Ionic liquids BASF

Welton was pretty pleased with performing chemical reactions in Ionic liquids are significantly. ‘In the ionic liquid the himself, sure that he had proven ionic liquids, was impressed by this considered to be green same reaction happens so you get the himself right. All that was left was to proof of an ionic liquid effect, if solvents due to a lack same product out, but now instead check the final reaction: a negatively not too surprised that it exists. ‘It’s of vapour and ease of of the reactive anion and the reactive charged nucleophile, a chloride ion, a very important demonstration,’ recycling cation being stuck together in this ion with a positively charged electrophile, says Seddon. ‘Tom has done what pair that then goes on to react, they a sulfonium salt. Welton was somebody should have done years ago, only react when they happen to bump convinced that this last test was a mere which is a proper serious study, and into each other in the normal manner,’ formality. ‘We’d done three quarters he’s got very good results.’ says Welton. of the way through,’ recounts Welton. Theoretical chemist Ruth ‘I said to the student, “just do a couple Probing deeper Lynden-Bell from the University of of these because we need to do it for The next task was to explain this Cambridge, UK, has worked with completeness, then once you’ve done effect. Welton had some ideas. He computer models of ionic liquids for that we’ll move onto something more suggested that in molecular solvents, some time, and she was intrigued by interesting”.’ pairs of the reacting chloride and Welton’s observations. But in this final reaction something sulfonium ions were dominating Lynden-Bell stepped in to help different happened. The graphs for the reaction. ‘You dissolve in [to the explain what Welton was seeing. She the reaction in the ionic liquids and solvent] your reactive anion with an performed molecular simulations of in conventional solvents no longer unreactive cation, and your reactive Welton proposes that two charged spheres, one negative looked the same. cation with an unreactive anion,’ says charged reactants are and one positive, dissolved in an ionic ‘Famous last words,’ says Welton Welton. ‘They swap over so you then shielded from each other liquid, and then measured the forces of his previous instructions to the have your reactive anion and your by ionic liquid molecules between those charged spheres. student. ‘We were seeing a unique reactive cation together Her computer models showed that behaviour.’ For this reaction of but not reacted yet.’ the ionic liquid molecules charged reactants, in all the molecular It’s this so-called form a shell around solvents such as acetone, butanol, ion pair that goes the charged spheres. dichloromethane, the rate during the on to react, and Positive spheres course of the reactions followed the dominate the were surrounded by same kind of path. But for all the ionic reaction kinetics. negative ionic liquid liquids, those graphs were a startlingly But in ionic molecules and vice different shape, the reaction rates all liquids these ion versa. The shells followed another kind of path, and pairs weren’t shield the effect of were much slower. forming. The the ion within. ‘The After years of proclaiming there ionic liquid counter charge in was no such thing as an ionic liquid was somehow the sphere dilutes effect, Welton’s attempt to prove it had shielding the ions out the charge on outdone him. from one another. the central ion,’ says Ken Seddon, from Queen’s This changes Lynden-Bell. University Belfast, UK, and one the behaviour And these shells,

JOURNAL OF ORGANIC CHEMISTRY / AMERICAN CHEMICAL SOCIETY CHEMICAL AMERICAN / CHEMISTRY ORGANIC OF JOURNAL of the first researchers to suggest of the reaction formed by the ionic 56 | Chemistry World | May 2010 www.chemistryworld.org liquid, keep those charged spheres apart – they don’t form the ion pairs that are seen in molecular solvents. ‘What really surprised me was not that there was some of this screening that destabilised the ion pair, but how strong it was,’ says Lynden-Bell. With the shielding effect worked out by Lynden-Bell on a basic system, other researchers, including Patricia Hunt also at Imperial College London, are now performing calculations on more realistic systems, including the species that Welton used in his reactions. Seddon thinks caution should be taken as other explanations might also fit. ‘It makes sense. It doesn’t mean it’s correct,’ he says. ‘In other words it’s a simple way of explaining what [Welton] found, but it isn’t necessarily the only way of explaining what he found.’ But molecular simulations take enormous amounts of computing power, so for a first stab at the problem, Lynden-Bell’s simple approach is a sensible one, Seddon adds. ‘It’s very difficult to get a very precise study,’ he says. ‘You’d have to be sure why you wanted to spend that amount of computational time. To model on a discreet ion pair is a much simpler job.’ Even without proof that ionic liquids can have a general effect on reactions, they were still being widely

used. For example, organic chemist PHYSICS CHEMICAL CHEMISTRY PHYSICAL Steven Ley, from the University of Cambridge, has used ionic liquids at which the dye ions are associated Lynden-Bell’s models basics of how ions interact with as solvents in his total synthesis – paired together – or dissociated show that ionic liquid each other. He thinks that the chemistry. And Seddon showed in – nowhere near each other. This molecules form shells understanding of ion pairs will need 2004 that the reaction of toluene and dissociation/association behaviour is around charged reactants to be refined in light of his discoveries, nitric acid in three different ionic in equilibrium in the solvent. with practical consequences. ‘There liquids gives three different products. Welton investigated how the is a huge number of processes where Seddon also says that certain organic concentration of the dye changes the ion pairing is important,’ Welton says. reactions, like the Friedel–Crafts intensity of the absorption in different Ionic liquids could alter how those reaction, zip along much faster when solvents, which is related to the processes are controlled. ‘It’s forced performed in an ionic liquid, because equilibrium. A graph of concentration us to be a bit more precise about what of the specific ionic intermediate that and intensity gives a straight line we mean when we say ion pair.’ is made in the process of the reaction. only if the ions in the salt are always So Welton’s stance about the ionic But those are all one-off examples, completely dissociated, and no energy liquid effect has been completely not proof of a general phenomenon. is needed to rip them apart. And this reversed. He remains philosophical ‘What [Welton] has shown is much only happens in ionic liquids, where about this, and reflects on how close more general, and therefore more the ionic liquid’s ions surround the he was to uncovering the truth when important,’ says Seddon. dye ions. he stood up in Yokohama in 2007 and It could be that mixing dyes in ionic renounced any ionic liquid effect: ‘It Moving forward liquids, measuring their absorption can only have been a matter of months Welton is still looking at the special and looking for this simple straight before the first results started to come properties of ionic liquids and has line could be used as a diagnostic for in that ended up showing that I was come up with a system that could be ionic liquid behaviour. It could even wrong,’ he says. ‘Typical!’ used as a simple diagnostic for ionic be used to calculate how much ionic liquid-like behaviour. liquid might be needed in a mixture ‘Tom has done Katharine Sanderson is a science writer That system is the salt of a dye with conventional solvents in order to based in London, UK that contains an iodide ion and a get the benefits of an ionic liquid effect what someone pyridinium ion. When this dye salt is in a reaction, says Welton. should have Further reading dissolved in a solvent, its absorption Welton is now looking at the J P Hallett et al, J. Org. Chem., 2009, 74, 1864 can be measured by UV spectroscopy. behaviour of ions of different sizes, done years ago, R M Lynden-Bell, Phys. Chem. Chem. Phys., Different solvents affect the extent and continuing to investigate the a proper study’ 2010, 12, 1733 www.chemistryworld.org Chemistry World | May 2010 | 57