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Green Organic Solvents
We are working to solve the biggest problem in Organic Chemistry. Organic Chemistry relies on the petroleum industry. The vast majority of chemicals organic chemists use as reagents and solvents are components of crude oil. If you are paying attention to the energy crisis, then you know that we will eventually run out of oil. In 2010, British Petroleum released a report indicating that the world has 46 years of oil left, assuming that our consumption continues to grow the way that it has been and assuming we don’t find any more. If BP is right, many of you who are reading this paragraph will live to see that day.
Many organic chemists, generally intelligent folks, ignore this problem because it is easy to do so. The use of petrochemicals is so ingrained in the workings of organic chemistry that they see no way out. Someone smarter than they are will come along and fix it before it becomes too bad. Therefore, there is no sense worrying about it. For others, it comes down to money. Right now, there is not a lot of money to be made in solving this problem. Petrochemicals are still the least expensive reagents and solvents and there is no reason that we should change until it makes economic sense.
If organic chemists are sitting around, waiting until someone else solves the problem, or until money becomes available to do so, we will not be ready when the oil runs out. Fortunately, not all organic chemists are sitting around waiting and ignoring. Great efforts are being undertaken, sponsored by the National Science Foundation, the Environmental Protection Agency, the American Chemical Society, and others to make organic chemistry a sustainable endeavor.
The EPA and their chief chemist Paul Anastas developed the principles of Green Chemistry in the ‘90s as a set of guidelines to reduce pollution in the chemical industry and to make chemical processes safer and more sustainable. The EPA’s Green Chemistry website now lists the following Sustainable Chemistry Hierarchy as a set of guidelines. Products and processes should be designed to include as many of the following and be cost-competitive.
*chemicals that are less hazardous to human health and the environment are:
Our approach to sustainable chemistry is to replace the solvents used in organic chemistry. Organic solvents represent the majority of all petrochemicals used in organic chemistry, especially when complex purifications are necessary. They are also the major component of generated waste. Additionally, most solvents are volatile, flammable, and hazardous to humans, the environment, or both.
There are two main approaches to solving the solvent problem. In the first approach, the solvent is removed altogether. Many reactions can be done under these “neat” conditions, but many reagents and intermediates are not stable outside of solution. The second approach replaces petrochemical solvents with greener solvents.
The most popular green solvents are water, supercritical carbon dioxide, and room temperature ionic liquids. All three have their benefits and drawbacks. Water is abundant, nontoxic, and inexpensive, but it is a poor solvent for most organic compounds and it is difficult to remove. Supercritical carbon dioxide is abundant, dissolves most organic solvents, and can be removed easily, but it requires a large energy input to generate the pressures needed. Room temperature ionic liquids are nonflammable, nonvolatile, and easy to recycle, but they are made from petroleum and are toxic to aquatic organisms.
We want to develop a new kind of green solvent that can be prepared from abundant, inexpensive, innocuous, biorenewable components and can be reused or recycled. Our approach uses low-melting mixtures of compounds that meet these requirements, like choline chloride, proline, tartaric acid, and urea. All of these compounds are soluble in water, so products of reactions can be isolated by addition of water followed by filtration or simple extraction. The solvent mixtures can then be recovered by evaporation of the water. You can read more about the science and some of our experiments in the Science section.
Our approach harnesses the following principles from the Sustainable Chemistry Hierarchy:
This project is an excellent way to get involved in experiential learning and undergraduate research. Students working on this project have presented their research at local and national conferences. Contact Dr. Norris if you are interested. You do not need to be a Chemistry major, but you do need to have taken at least one semester of Organic Chemistry.
You can sign up for credits of CHEM 493 Advanced Chemistry Research or CHEM 499 Special Problems in Chemistry. Each credit-hour corresponds to 2-3 hours of research work per week. If you sign up for credits, then your work will show up on your transcript.
This project has received funding from the President’s Experiential Learning Excellence Fund, the FSU Foundation, and the Chemistry Department.
Deep eutectic solvents (DES) are being investigated as potential green solvents for organic reactions. Choline chloride forms low melting polar mixtures with hydrogen-bond donors, such as carboxylic acids1 and urea,2 some of which have melting points below room temperature. The components of these mixtures are inexpensive, innocuous, and biorenewable, unlike those of conventional organic solvents.
Deep eutectic solvents
A eutectic mixture is a combination of compounds that exhibit a lower melting point than either compound does separately. Deep eutectic mixtures have melting points far below those of the component compounds. For example, choline chloride (decoposes at 302 oC) and urea (MP 133 oC) form liquids with melting points as low as 12 oC.2 Choline chloride also forms low melting mixtures with carboxylic acids and other hydrogen-bond donors.1
These deep eutectic mixtures should be usable as solvents for organic reactions. They are two-component room temperature ionic liquids, but unlike the more common imidazolium salt ionic liquids, these deep eutectic solvents are formed from innocuous and biorenewable compounds. Given the significant difference in composition between the two types of liquid, and the strong association of the words “ionic liquid” with a particular group of substances, we will continue to use the phrase “room temperature ionic liquid (RTIL)” to refer to single component liquids such as the imidazolium salts. We will refer to our mixtures as deep eutectic solvents (DES).
These deep eutectic solvents are polar and protic, and should lend themselves to reactions that require such conditions. If the hydrogen-bond donor is a carboxylic acid, then the solvents are also acidic. For this reason, we are targeting acid-catalyzed condensation reactions (e.g. aldol, Mannich, etc.) in our initial studies.
We are preparing DESs from choline chloride and chiral hydrogen-bond donors such as proline and tartaric acid, which are also innocuous and biorenewable. These chiral DESs should influence the stereoselectivity of the reactions done in them. Proline is an especially attractive component, given the great number of stereoselective condensations catalyzed by proline or proline derivatives.
Choline chloride and tartaric acid form mixtures that are molten at room temperature, but they are too viscous to allow stirring. Choline chloride and proline do not form mixtures that remain molten in any proportion. We are also investigating ternary mixtures of choline chloride, proline, and urea, some of which are molten and fluid at room temperature.
We initially investigated Biginelli condensations in DESs. The Biginelli reaction had previously been done in eutectic mixtures of tartaric acid and either urea or dimethyl urea,4 which are molten at 90 oC and 70 oC, respectively. We have been unable to match these results in DESs composed of choline chloride and urea, with or without catalytic tartaric acid, or choline chloride and tartaric acid.
We have also investigated a double Mannich cyclization that has been previously reported in both ethanol and in RTILs.5 We have been able to obtain similar results in the choline chloride-urea DES. In all cases, the yields are low.
Aldol and Knoevenagel Reactions:
We are beginning to investigate the simpler (fewer components) aldol and Knoevenagel condensations. We have successfully performed aldol reactions in the choline chloride-urea DES. The yields of these reactions dramatically improve in the presence of catalytic proline. We also have successfully performed Knoevenagel condensations in choline chloride-urea-proline ternary mixtures. We still need to optimize the conditions to obtain good yields.
We have briefly examined the reusability of DESs. We have found that we can remove added water without resorting to vacuum pumping or freeze-drying. Heating in a 100 oC oven for a few hours removes 99% of the water without decomposing the mixture. We have not yet investigated reusing the solvent mixtures after reactions.
Dr. Benjamin Norris (Advisor)
Sarah Russell (Senior ’12)
Emmett Kitchen (Sophomore)
Ryley McBride (Junior)