Unit 3 - Conventional Solvents for Reactions and Their Alternatives Flashcards
Why are solvents used?
To dissolve reactants so that they collide.
To ensure efficient mixing of reagents (reduce mass transfer).
To facilitate heat transfer.
To participate and drive chemistry such as through the stabilisation of transition states.
To facilitate product isolation.
What quantity of solvents are used per year?
18 million tons. This equated to $22 billion in 2020 but is expected to be $30 billion by 2025.
What percentage of atoms in a process come from solvents?
Around 60%.
What factors should be considered when choosing a solvent?
Literature - will the reaction work?
Solubility of substrate/product.
Boiling point
Vapour pressure
Cost
Availability
What is a problem with using data tables to choose a solvent?
They do not consider sustainability.
What factors affect the sustainability of a solvent?
The origin of constituent atoms.
The number of synthetic steps involved.
The energy invested in the solvent.
Can it be reused or is energy recovery more effective.
The toxicity of the solvent.
What are the problems with recycling solvent?
It sometimes costs more energy to distil and purify the solvent than to incinerate and collect the energy.
Why are solvents cheap?
They are co-produced when crude oil is refined and other materials are made.
What effects does minimising the importance of economy have?
The decisions made are more based upon technical performance, sustainability, health, wellbeing and the environment rather than money.
What does supercritical mean?
The defined state of a compound, mixture or element, above its critical pressure (pc) and critical temperature (Tc).
What are the properties of supercritical fluids?
Nearly as dense as liquids.
Diffuse like gases.
Liquid-like thermal properties.
They have tunable solvent properties; the solvation is dependent on density which is changed by varying temperature and pressure.
What are the properties of scCO2?
It is a relatively poor solvent (similar to hexane) and can only dissolve small-medium sized molecules.
Solvation power increases rapidly around the critical point.
The solubility is vapour pressure dependent and more volatile substances will dissolve easier.
Solubility can be increased by adding polar co-solvents or entrainers (such as methanol) or by designing the substrate to be soluble.
Despite scCO2 being a poor solvent, why is it still used?
It is an abundant natural resource that is free to obtain.
What is scCO2 used for in industry?
The decaffeination of coffee.
What are the benefits of using scCO2 for the decaffeination of coffee?
Previously chlorinated solvents were used (dichloromethane) which are toxic whereas scCO2 removes the addition of toxic solvents.
It is also much more specific to removing purely caffeine and is of such a high purity that it is sold.
What is high-pressure hydrogenation?
The reduction of an unsaturated functional group: alkyne, alkene, ester, ketone, aldehyde, nitrile etc.
It is often carried out stereospecifically with the aid of chiral catalysis.
What is the problem with high-pressure hydrogenation and what is the solution?
The reaction requires high pressures (>50 bar) and high temperatures (>50 °C). This poses a high level of risk due to the stored energy (pressure x volume).
The solution is to minimise the volume at high pressure by using a continuous flow reactor.
How much energy does distillation use in the USA?
Three times as much energy as the total energy consumption of Switzerland.
