5.5.3 · D5Green Chemistry & Sustainability
Question bank — Solvent selection — water, supercritical CO₂, ionic liquids
Before you start, hold three anchor pictures in mind:
- Water = the polar, hydrogen-bonding solvent that struggles with greasy things.
- scCO₂ = a fluid past the point where "liquid vs gas" stops meaning anything, whose dissolving power is a dial you turn with pressure.
- Ionic liquid (IL) = a salt that happens to be liquid, that essentially never becomes a vapour.
True or false — justify
Water is always the greenest choice because it is cheap and non-toxic.
False. Cheapness and low toxicity are only two green axes; water hydrolyses sensitive reagents, dissolves few non-polar substrates, and has a huge latent heat of vaporization making it energy-expensive to remove. See Life Cycle Assessment (LCA) — greenness is judged over the whole process, not one property.
A supercritical fluid is just a very hot gas.
False. "Supercritical" means above the critical point in both temperature and pressure. For CO₂ that is a mild ; the defining feature is that the liquid–gas boundary has vanished, not that it is hot.
Ionic liquids are automatically green because they have negligible vapour pressure.
False. Zero volatility only removes air pollution (VOCs). Many ILs are aquatic-toxic, poorly biodegradable, and energy-intensive to synthesise — the water, soil, and manufacturing impacts remain.
Using scCO₂ adds carbon dioxide to the atmosphere and worsens global warming.
False. The CO₂ is captured and recycled in a closed loop (often a by-product of another process). It is contained and reused, not newly emitted, so releasing it at the end returns it to the same pool it came from.
The solvent is usually a minor part of a process's mass, so its choice is low-leverage.
False. The solvent is typically the largest mass in a synthesis (often >80% of total material), which is exactly why solvent selection is the highest-leverage green decision — bigger than the catalyst choice.
Near its critical point, a small pressure change barely affects scCO₂'s solvent power.
False. Near the isothermal compressibility becomes enormous (because ), so a modest produces a large , and solvent power tracks density — a small squeeze is a big dial-turn. See Phase Diagrams & Critical Point.
Water's polarity is a fixed property, so it can never dissolve non-polar organics.
False. At high temperature water's hydrogen-bond network weakens and its polarity drops, letting superheated water behave like a mild organic solvent. Even at room temperature the Hydrophobic Effect can accelerate "on-water" reactions.
Recovering scCO₂ from a product is harder than recovering water.
False. scCO₂ is recovered by simply depressurizing — it flashes off as a gas leaving zero residue. Water must be boiled off against its large latent heat, which is why water recovery is the energy-expensive one.
Spot the error
"scCO₂ works because at 200 °C the CO₂ molecules move so fast they dissolve everything."
The temperature is wrong (scCO₂ operates near , not 200 °C) and the mechanism is wrong — dissolving power comes from density/pressure, not from thermal speed. High temperature would actually lower density and weaken solvent power at fixed pressure.
"To make scCO₂ dissolve polar caffeine better, lower the pressure."
Backwards. Lowering pressure lowers density and reduces solvent power. To handle a somewhat-polar solute you either raise density (more pressure) or add a small polar co-solvent like water to nudge the mixture's polarity up.
"An ionic liquid is any salt dissolved in a liquid."
Wrong — an IL is the liquid; it is a salt (cation + anion) that is itself molten below ~100 °C, with no molecular solvent required. Example: an imidazolium cation with a -type anion.
"Because water is polar protic, it stabilizes non-polar reactants and speeds every reaction."
Two errors. Water does not stabilize non-polar reactants; in "on-water" Diels–Alder it stabilizes the more polar, compact transition state, lowering . And it does not speed "every" reaction — it can hydrolyse or simply fail to dissolve many substrates.
"scCO₂ and ionic liquids are both good because they solve the same problem."
They solve different problems. scCO₂'s virtue is trivially easy recovery (depressurize); an IL's virtue is negligible vapour pressure keeping catalysts contained. Pairing them (IL layer + scCO₂ layer) exploits both distinct strengths at once.
"A biphasic IL/product system fails because the catalyst leaks into the product."
The whole point is that it does not leak: the catalyst stays dissolved in the IL phase while product collects in the other phase. Because the IL has negligible vapour pressure, it also never contaminates the product stream by evaporation. See Catalysis & Catalyst Recovery.
Why questions
Why is CO₂'s mild critical point (, 74 bar) a decisive industrial advantage over supercritical water ()?
Reaching costs almost no heating energy and protects heat-sensitive compounds (like coffee flavour molecules), whereas supercritical water demands enormous energy and destroys delicate solutes — so scCO₂ is cheap and gentle enough for food-grade use.
Why does the liquid–gas distinction disappear past the critical point?
On a P–T diagram the liquid–gas boundary line simply ends at the critical point. Beyond it, raising pressure smoothly increases density with no boiling and no phase boundary to cross, so there is no moment where "gas" becomes "liquid." See Phase Diagrams & Critical Point.
Why does the hydrophobic effect speed up certain reactions rather than just failing to dissolve reactants?
Water molecules around a non-polar solute lose hydrogen-bonding freedom (entropy penalty), so the system clusters the organics together — this raises their effective local concentration and, for reactions with a polar transition state, water further lowers by stabilizing that TS.
Why is "negligible vapour pressure" the headline green virtue of ILs but not sufficient to call them green?
It eliminates VOC air pollution, which is a genuine and rare benefit — but greenness is measured across toxicity, biodegradability, and synthesis energy too, and many ILs fail those. Low volatility fixes one axis, not the whole Life Cycle Assessment (LCA).
Why do we sometimes add a small amount of water to scCO₂ in extraction?
Pure CO₂ is non-polar, so it dissolves polar targets (like caffeine) poorly. A little water acts as a polar co-solvent, raising the mixture's polarity just enough to pull the target out, without abandoning scCO₂'s clean-separation advantage.
Why does selecting a greener solvent connect to atom economy and E-factor?
The solvent is usually the dominant discarded mass, so it dominates the E-factor (mass of waste per mass of product) even though it does not appear in the balanced equation that sets Atom Economy & E-factor. Cutting or recycling solvent slashes waste more than tweaking stoichiometry.
Why is a low boiling point a disadvantage for a conventional solvent but density-tunability an advantage for scCO₂?
A low-boiling organic escapes as a VOC (air pollution, VOCs and Air Pollution); scCO₂'s pressure-tunable density lets it dissolve on demand and then vanish cleanly on depressurization — the "escape" is now a controlled, closed-loop recovery, not an emission.
Edge cases
What happens to scCO₂'s solvent power exactly at the critical point?
The isothermal compressibility diverges ( because ), so density and thus solvent power become hypersensitive — tiny pressure fluctuations cause large swings. Operators therefore run just above , not exactly on the point.
If temperature is above but pressure is below , is CO₂ supercritical?
No. Both conditions must hold simultaneously ( and ). Above but low pressure you have an ordinary gas that cannot be liquefied by compression alone — but it is not the dense, liquid-like supercritical fluid.
Can water be a green solvent for a Grignard reaction?
No — the Grignard reagent is instantly destroyed by hydrolysis. This is the classic "right tool, wrong job" case: water's very reactivity (its protic O–H) disqualifies it here despite being cheap and non-toxic.
What is the limiting behaviour of scCO₂ density as pressure rises far above at fixed ?
Density climbs toward liquid-like values and the fluid becomes a strong solvent, but the sensitivity falls off — far from the critical point the "dial" becomes coarse, so fine solvent-power control is best kept near .
Is an IL that is toxic and non-biodegradable still worth using anywhere?
Sometimes — if it is confined in a closed, recyclable biphasic loop (catalyst reuse) where it never enters the environment, its in-process reuse can still lower overall waste. But it must be assessed by full life cycle, not assumed green from zero volatility alone.
For a reaction with non-polar, water-insoluble reactants, does insolubility rule water out?
Not necessarily. "On-water" chemistry exploits the Hydrophobic Effect precisely because the reactants are insoluble — they cluster at the interface, raising effective concentration, so a suspension in water can outperform a homogeneous organic solvent.
Recall One-line self-test
Name the single distinct problem each solvent fixes. Answer ::: Water fixes toxicity/cost, scCO₂ fixes recovery (depressurize to zero residue), IL fixes volatility (negligible vapour pressure). Different problems — hence selection, not a universal winner.