3 Ionic liquids Flashcards
Which structural and electronic factors contribute to lowering the melting point of a salt?
Charge dispersal and low symmetry are key to reducing packing in the solid state and lowering the melting point of salt.
List the two main advantages of using ionic liquids.
● ILs offer an answer to the release of toxic and/or smog-generating VOCs
● ILs are a virtually infinite variety of new reaction media (nearly 10^18 candidates available) with tunable properties, which offers a great opportunity for synthetic chemists to enrich their toolbox.
Describe the Menshutkin reaction and what it is used for.
(Reaction: slide 15, 3rd ppt)
The Menshutkin reaction converts a tertiary amine to a quaternary ammonium salt by reaction with an alkyl halide. The ammonium cations of the tertiary amine attack the R4-X bond in the alkyl halide, breaking it off and generating a quaternary ammonium salt (organic salt, also called a QUATs) and a free X- atom. It is the method of choice for preparing QUATs.
How can we tune the hydrophilicity/hydrophobicity profile of an IL?
The hydrophilicity/hydrophobicity profile is easily tunable through a proper choice of the ion pair. For example, for the anion, [PF6]- will produce a more hydrophobic IL, and [NO3]-, Cl-, Br-, and I- will produce a more hydrophilic one. The same result can also be obtained by mixing one IL with molecular liquids or other ILs.
Write down the structure of a lipophilic IL and one example of a hydrophilic IL.
Hydrophobic/lipophilic:
Hydrophilic:
VER ESQUEMAS
How can we exchange the anion in an imidazolium-based IL?
(Mechanism: slide 17, 3rd ppt)
The hydrogen in C-2 is quite acidic and can be easily removed to form a carbene. Imidazole carbenes, or imidazol-2-ylidene, rapidly dimerize unless bulky substituents are located on nitrogen atoms.
How can we explain the ability of [bmim][Cl] to dissolve cellulose?
(Mechanism: slide 19, 3rd ppt)
The ability of [bmim][Cl] to dissolve cellulose depends on its power to break the intermolecular H-bond network connecting the cellulose chains. The [bmim][Cl] is able to break the intermolecular H-bond network, linking [bmim]’s C2’s H to the cellulose’s O, while the Cl- creates a bond with the cellulose’s H, therefore separating the connected cellulose chains and effectively dissolving it.
Provide one example of a protic (acidic) IL and one example of basic IL.
VER DIAGRAMAS
Describe the chlorination of alcohols with HCl in ILs.
(Slide 32, ppt 3)
BASIL™ process (BASF): for the synthesis of alkoxyphenyl-phosphines. HCl is formed as a by-product during the synthesis of diethoxyphenyl-phosphine. Using the BASF product 1-methylimidazole as an acid scavenger, an ionic liquid is formed: 1-methyl-imidazolium chloride (Hmim), with melting point of 75°C. Above this temperature, two distinct phases occur: the upper one is pure product, no extra solvent is needed, while the bottom one is the pure ionic liquid.
Describe an industrial process based on the use of [Hmim][Cl].
When trying industrially to chlorinate alcohols, phosgene provides excellent yields. Nevertheless, the safety risks are huge. In principal, the chlorination can also be achieved with HCl gas but with very bad results. Nevertheless, when the reaction is performed in an ionic liquid the ether by-products are cleaved, allowing for a further reaction to the desired bischlorinated product, particularly the 1,4-dichlorobutane (DCB). Furthermore, after complete conversion, the organic phase separates off, water co-product can be distilled off from the remaining [Hmim][Cl-] and it can be used for the next run without any extra work-up.
In terms of EHS issues, quote the advantages of ILs over traditional organic solvents, and the possible negative.
No vapor pressure, therefore they do not evaporate. They have favorable thermal stability and operate over large ranges. They dissolve many metal catalysts, polar organic compounds, and gases. They are immiscible with many organic solvents and water. They may increase catalyst lifetimes and stabilize sensitive catalysts. They can be burned to give specific (and predictable) properties for specific applications.
Nevertheless, stability depends upon cation and anion choice, and impurities such as chloride and halide decrease the chemical stability. The biggest negative is their very high viscosity, which lowers the mass transport.
Define what a Task Specific Ionic Liquids corresponds to and provide an example of application.
A TSIL is an ionic liquid which incorporates a covalently linked reactive functionality. They are more sophisticated solvents (by design) composed by an ion pair, a spacer, and the functional group. One example of application is in extractive chemistry, such as the extraction of Ni2+ ions from aqueous solutions.
Define what a deep eutectic solvent is with the aid of a proper graph.
DES are eutectic mixtures of QUATS (quaternary ammonium salts) and hydrogen bond donors (HBD, can be molecules with many groups that have loose electron pairs, usually O and N).
Why IL and TSILs stabilize metal nanoparticles towards aggregation.
The agglomeration of MNPs is the result of the high surface area to volume ratio. This ratio turns the particles unstable if they are kept in the nanoscale, so they tend to form bigger clusters to reduce this energy and thus be more stable. Since the IL are formed by one negatively and other electropositively charged parts, these can stabilize the MNP surface charge by surrounding it and creating a electro steric coat. On the other hand, the TSILs have the same effect as if it was about a surfactant, with the difference that instead of relying on hydrophobicity and philicity, the TSILs take advantage of the charged sections. While the regular ILs create the electrostatic spheres for the MNPs stabilization, the TSILs also take advantage of their tunable groups and their steric hindrance. Therefore, by formation of an external ligand shell. QUATs may have electro-steric effects, while TSILs have surface ligation and, therefore, stabilize metal nanoparticles and make them precipitate.
Task specific ionic liquids in metal ions extractive chemistry.
One attractive use is an environmentally friendlier process for the separation of transition metals (Cu, Co, Fe, Mn, Zc) from rare earths by solvent extraction with IL trihexyl(tetradecyl) phosphonium chloride. No extra organic diluents or extra extraction agents is needed.
