Solvents Flashcards
List 5 chemical needs for solvents
1) To dissolve reactants/reagents
2) To ensure efficient mixing of reagents (reduce mass transfer)
3) Facilitate heat transfer
4) Participate + drive chemistry (transition state stabilisation)
5) Facilitates product isolation
Why would we need to dissolve reactants/reagents to allow chemistry to occur
Reaction components need to be able to move so particles can collide to have a chemical reaction
How can solvents facilitate heat transfer?
Solvents have defined boiling temperatures, meaning we can heat them up to their boiling point and it delivers and aliquot of energy to the chemical reaction
We can the use the variation in boiling points to titrate the appropriate amount of energy so that we can deliver the activation energy for a process
How can a solvent particupate and drive chemistry?
Some solvents like ethers, and THF, have LP on the oxygen and can participiate in chemistry, hence can stabilise intermediates and solids within solution
Polar solvents can be used to manipulate some reactions with active transition states
How can solvents facilitate product isolation
We can remove the solvent quite effectively by an evportator, or perhaps by manipulating the product solubility, so the product itself crystalises
Solvents have a staggering effect due to the huge demand for the chemistry created facilitated through them in everyday life
What is the approximate global demand for solvents ?
18 million tones per annum
What are 5 things we might consider when choosing a solvent?
- Literature …. will the reaction actually work?
- Solubility of substrate/product/H₂O
- Boiling point
- Vapor pressure
- Have we got it in the lab?
- Cost (£££)
Why might the solubility of substrate/product/H₂O, effect which solvent we use?
Ideally the starting material would be soluble and product would not be
As it would drive the reaction away from equilibria and to completition by removing the product
AND reduce energetic demands for isolation of product at the end
Why might the boiling point effect the solvent we use?
- So we can put in the appropriate amount of energy
- This would also avoid starting side reactions + degradation occuring
Why might the vapor pressure effect the solvent that we use?
As this effects how much energy is needed to remove or clean product from solvent
BUT also on a health&saftey and risk of explosive environments if the solvent evaporated
Why might the availability of the solvent effect of what solvent we use?
The best solvent might not be available now
Why might the cost of a solvent, effect which solvent we use?
If the ideal solvent is really expensive, in particular at industrial scale it probably not going to be used
What is the type of physical chemical data which would be in an informational book about solvent?
- Solubility driven
- Activation energy
- Ease of removal/recovery
- Classes of solvents
However, none of these data tables really consider sustainability and the impact of these molecule on the environment
What are the 4 classes of solvents?
- Polar
- Non-polar
- Protic
- Aprotic
What sorts of questions may we ask to determine if a solvent is sustainable or not
- Where do the atoms in the solvent come from?
- Is the source sustainable?
- Where will the solvent end up after use?
- Toxicological impact….
- Embodies/investment energy…..
Philiph Jessop is a pioneer in the field of green chemisty
What was a big point which came out of his paper ‘searching for green solvents’
His paper covers the topic of sustainability and solvents
He has concerns over the amount of energy used within the creation of a solvent
And makes comparision of the greenness between solvents
This graph shows the measurement of energy used to create a kilo of solvent
There is real variation in these values, which is down to the synthetic processes involved
How can we reduce this energy?
We can reduce this energy by reducing the number of steps involved
This graph measures the amount CO₂ which is emitted to the environment when we produce a kilo of solvent (these numbers are quite reasonable due to process being done on industrial scales)
Why should we only really consider the value of the white and black bar, not just the black?
The white bar is the additional CO₂ emission if we make the assumption the solvent is then flared (the more carbon atoms the higher this is)
It is assumed the solvent is only used once because the energy required to recycle them is very large
Once the solvents are flared, the remains are collected and used within other chemical reactions
This graph show how many synthetic steps are required to make a simple molecule available in large scale
How can the number of steps relate to the greenness of the solvent?
The more steps, the higher the energy cost, and the cost to the environment
Hence the more steps, the less sustainable a solvent may potentially be
(All the solvents listed here also come from petrochemical processes)
Why are solvents so cheap
Solvents are co-produced along with other organic material when crude oil and natural gas is refined through cracking and reforming
How have solvents impacted the sustainable development goals?
Solvents impact many chemical processes
e.g. production of pharmaceuticals which would improve (3) good health and well-being
As information and awareness becomes better over time, the roles of molecules may change as our awareness changes
Analytical chemistry has made huge steps towards measuring molecule at much lower concentrations
Why is this important in term of chemical greenness?
We can see the impact that molecule have as a whole on organisms and the environment (e.g. pharmaceuticals in the water ways)
And are more aware of negative impact of the molecules that we have used in the past
e.g. recalcitrance = when a molecule will not degrade and build up in concentrations
Given an example of a pharmaceutical where there was increased awareness over time which effected use
DDT in the 1940s saved million of lives by killing the vectors of malaria, typhus & others
However by the 1970s was banned due to causing population collapse in birds
Name a solvent which is widely used by has real health and safety issues
Dichloromethane is a solvent with a variety of applications
But had risks associated with inhalation, neurotoxity, carcinogens and skin irritation
What particular principles of Green Chemistry may we apply to evaluating the sustainability of solvents?
1) Prevention of Waste
3) Less Hazardous Chemical Synthesis
5) Safer Solvents and Auxiliaries
7) Use of Renewable Feedstock
12) Inherently Safer Chemistry for Accident Prevention
How could we use the ‘classic model’ of sustainability to evaluate the greenness of solvents?
Use the Venn Diagram showing the cross over between benefits
In history they have not been treated as equals when considering the benefit to society, which is why we have the challenges we do - but they are often interlinked
(Do these spheres have an equal importance/quotient value?)
How could we reimagine the ‘classic model’ of sustainability to evaluate the greenness of solvents?
Instead we can have the 3 spheres appear as a combination of concentric circles, confined by the contribution of the environment
The resources and the energy used to do a chemical reaction/process are defined by the planetary boundaries in our system
Use this to create a solvent framework
What is a really ‘radical’ way to reimagine the ‘classic model of sustainability’ to evaluate the greenness of solvents?
There is a suggestion to eliminate the impact of the economy
Take away the polarisation of the ‘how much does it cost’ question
If a material is too expensive, we might be persuaded to use it and look for a cheaper alternative
BUT there will always be an economic benefit
This is what derives a safety-process approach for solvent production and allows decisions to be based upon technical performance, and measurable impacts upon health, wellbeing and the environment
Companies have considered their use of solvents and published their results within papers which essentially will help inform and make decisions
What does this solvent selection guide look like?
The power of the solvent guide is it translates the information about different solvents into a recommendations
Classically done by giving the solvent a series of Red, Amber and Green ratings against society, health and the environment
Simple to use and can help to change and adapt chemical processes
Name a pharmaceutical which has benefited from the solvent guide
The evolution of the synthetic processes of Viagra, where the environmental impacts have been driven down
Paul Anastas came up with this Chemistree which related to the research activities which contribute towards a cleaner, lower impact chemical industry
What does the branch related to solvent use focus on?
The focus is on reducing the impact of chemical processes and the impact of creating and using solvents
The leaves are different areas of research and supercritical fluids are just one of the leaves on this tree
Which Solvent does Philip Jessop in his paper did he suggest will be responsible for the greatest reduction in environmental damages
At the time it was though to be CO₂ related solvents which would lead to significant improvements in our environment
However at the time (2011) there was very little work being done on it (most of it was ionic liquids)
What is a Supercritical fluid?
Supercritical fluids have properties which are similar to both liquids and gases
It is the defined state of a compound, mixture or elements, above its critical pressure (Pc) and critical temperature (Tc)
What is the critical point (which is defined by the composition) for pure CO₂
the critical temperature would be 31°C and 73bars of pressure
Anything above that temperature and pressure is defined as supercritical
What are some key properties of supercritical fluids
They are highly compressed gases - CO₂, H₂O, C₂H₄
Nearly as dense as liquids
Diffuse like gases
Liquid like thermal properties
Solvent properties tunable (changes in temp/pressure)
Solvation is dependent upon density and vapor pressure
What is the real benefit of CO₂
It doesn’t cost use anything in terms of invested energy and chemical processing
Heavily abundant and can be captured from source
What is the solubility of scCO₂ like
Solubility similar to hexane (non-polar, small/medium sized molecule) - quite a poor solvent
How does solubility change around the critical point
Solubility increases rapidly around the critical point
How can the solubility of a supercritical solvent be increased?
Solubility can be increase by designing them to be volatile/other properties which are liked by supercritical fluids
The addition of polar co-solvents or entrainers (MeOH)
Designing substrates to be soluble
Historically, how was coffee made to be decaffeinated and what was the issue with the process?
Historically dichloromethane was used (known to be toxic) and had trace residues
What is now used instead to form decaffeinated coffee
Supercritical CO₂ is now ubiquitously used (or water)
It removes just the caffeine (unlike dichloromethane which would remove other things like flavouring) and then is washed in water as caffeine is more soluble
The caffeine can then be retrieved and then be sold (no waste)
The CO₂ is the recycled and used continuously
What is a common process in the fine and pharmaceutical industry which uses supercritical fluids
High pressure hydrogenation (reduction of functionality, alkyne, alkene, nitrile, aldehyde, ketone, ester etc)
Describe the process of continuous flow hydrogenation
- Substrate and H₂ mixes with scCO₂ solvent
- Solution passes over heated fixed catalyst bed
- Products collected by expansion of solvent (quality of products measured online by IR)
- Solvent then recycled
Explain why in this process there is a benefit to using supercritical fluids to form trimethylcyclohexanone from Isophorone
- The issue here is that there is multiple sites where the hydrogen could react on isophorone
- However, through manipulation of supercritical fluids by changing the temperature, as well as the amount of hydrogen which we put in, we can obtain just the trimethylcyclohexanone
- This has eliminated the requirements of downstream separation (decrease energy requirement)
(however in the end this process was not economic due to market price for product decreasing)
Define an ionic liquid
It is a salt in the liquid state
The fluid is composed of entirely of ionic species which may be simple ions or short-lived ionic aggregates or clusters
(room temp ionic liquids have a melting temp typically less than the BP of water (373K)
Why is there a benefit of reducing the number of distillative steps which would come from using a supercritical fluid
Product separation and purification is one of the single largest consumers of energy and materials in manufacturing processes
(total energy consumption of distillation column in operation in the USA is more than 3x as high as the total energy consumption of Switzerland
In Philip Jessops paper, by how much did he suggest that ionic liquids could contribute to doing cleaner chemistry
15%
Why however is just short of 50% of experiments done in literature use ionic solvents
Due to the fact they are relatively simple materials - hence no high pressure is needed
What are some general properties of Ionic liquids?
(note: not all ionic liquids share all of the listed characteristics)
- Low Melting point
- Very low vapor pressure
- Moderately conductive
- Thermally stable
- Wide electrochemical window
How can an ionic liquid have a low vapor pressure
It is composed of quite unorthodox ions
Achieved by making ions asymmetric and delocalising charge, you can decrease the lattice enthalpy and hence the melting temperature
What are some of the main uses of ionic liquids
- Green solvents (obviously) + Seperation media
- Electrolytes
- Working fluids (heat storage, lubricants)
Why are ionic liquids known as designer solvents
Because you can exchange the anion to meet the purpose
Resulting is a diversity of ionic liquids
Each partnership between anion and cation will have its own unique set or properties, which can related to a specific reaction/function
What are some issues with ionic liquids however
- Cannot measure vapor pressure and boiling point of an ionic liquid because so low (solvent choice survey)
- Toxicology surveys have not been done yet (unknown risks)
- More often not found in labs but can be easily obtained from chemical companies
What things in particular may we consider when looking at the sustainability of ionic solvents?
- Can ionic liquids be sensible alternative solvents?
- Green or Sustainable?
- Comparison to other solvents?
What is the energetic demands of synthesis like for ionic liquids compared to the petrochemical alternatives
A typical ionic liquid requires many more synthetic steps than typical petrochemical solvent = more energy
Furthermore some of them are only created on small scales by one company
the in the retrosynthesis of simple ionic liquids (most common) the precursors for this material all come back to petrochemicals
What are some other drawbacks of ionic liquids
- not easily recycled
- purification is challenging (usually not done)
- impurities can impact stability
What is BASIL
Biphasic Acid Scavenging utilising Ionic Liquids
It solves the problem of the hydrochloride salt (separation costs) which is formed from scrubbing acidic gas with an organic base
The smart acid scavengers uses an ionic liquid which can easily be seperated and recycled
What is BASIL used in
Dyes used in photocopier, Lazers and printer (cure coatings and printing inks by exposure to UV)
What organic reaction has been revonlutionised due to the use of BASIL
- Reacting dichlorophosphine with ethanol (to form dimethylphenylphosphonite) in the presence of organic base which resulted in the formation of a slurry which is the suspension of the products
- The challenge is then to break down the slurry and extract the product which is done with sequential filtration and washing with soap (losses or products)
- By exchanging the base for an ionic liquid, it resuts in a liquid emulsion forming rather than a slurry, which seperated naturally under gravity (salt at bottom is run off)
What is the benefit of using the BASIL when reacting dichlorophosphine with ethanol, over an organic base
- Recyclable ionic liquid product (used in the chlorination of diols after this reaction)
- Decantation of immiscible phase
- 98% yield
- nucleophilic catalystic effect (quick and effecient chemistry)
- 690000 kg m³ h⁻¹
If not solvents, what else could ionic liquids be used for?
- Ionic liquids as performance molecules and modifiers
(chemical and process advantages where low volumes can be use = reduced environmental impact)
e.g. reagents, catalysts, electrolyytes , lubtricants, heat transfer fluids
What are 5 key necessities for a solvent?
- The primary function of a solvent is to facilitate or enable a chemical reaction
- Solvents should be relatively invert
- Solvents must be available on scale
- Solvents should be low cost
- Renewable routes should not impact food vs chemical
Where do bio-derived renewable based solvents come from?
Solvents which are produced from biomass in a biorefinery which is defined as:
“a facility that intergrates biomass conversion processes and equipment to produce fuels, power and chemicals from otherwise waste biomass”
Hence adding value to originally where there wasnt any
What are terpenes and terpenoids used for in plants?
In plants, terpenes and terpenoids are important mediators of ecological interactions, i.e. plant defence, disease resistance, semiochemicals - pheromones
Terpenes have similar chemical properties to which other chemical solvent?
Similar Mr to hexane
Terpenes can be used as solvents for small molecule like hexane and has been used for extractions in chemical processes on small scale as well as in industrial solvents like paints and coatings
Why is there a limited commerical application to biorefined solvents like Terpenes
Because of the scale of supply
They are dependent on biorefining from waste citrus materials, and the suppling of these materials is somewhat finite
What types of products can be made from the biorefinery of citrus
- Biofuels and Essential Oils
- Pectin (jelling agent & stabilizer)
- Kraft Paper (biodegradable packaging)
- Activated Carbon (electrocatalyst, high energy density fuel)
There other products (other than solvent) formed from citus can be used to make other value-added materials
What is a big issue with biorefinery?
Only a small amount of the solvent product can be extracted from biocrtius and hence there is a need for a really large supply
How can you form Cyrene (an alternative dipolar aprotic solvent) from bioderived processes
- Cellulose is the most abundant organic polymer on Earth, hence can be obtain from heating biomass in the presence of a catalyst
- Volatiles are recovered by distillation of levoglucosenone (LGO)
- Hydrogenation yields cyrene - industrial scale
What is a key benefit of the use of Cyrene as an alternative solvent
It has a low toxicity and a high flash point
What other types of reaction can Cyrene be involved in
It is susceptible to nucleophilic attack (can be used in key reactions like amide formation and processing of graphene)
What are Deep Eutectic Solvents (DES)?
DES incorporate one or more compounds in a mixture, yielding a eutectic with a melting temperature/point much lower than eiher of the individual components
Classically they are based upon organic salts (Chlorine chloride) and a wide range of hydrogen bond donors (HBD) i.e. urea, glycerol, diols
(alternative to ionic liquids)
So what actually goes on to form deep eutectic?
Hydrogen bond donors (which could be taken from nature) and hydrogen bond acceptors
How are the properties of deep eutectics similar to ionic liquids
Similar to ionic liquids as the diversity of these compounds is really broad
This allows unique application of these compounds (designer solvents) with a lower embedded energy and a lower impact on our environment
Why is the connective of deep eutectic solvents not quite as high as that of an ionic liquid
Because DES have fewer charge carriers (diluting H-bond donors)
(still can be used as an electrochemical media and often used in techniques of painting of metals on to surfaces and polishing of metals
What is the benefit of DES of ionic liquids
The benefit of DES is due to the reliance of peterochemical resources are reduced, due to drawing on other materials drawn from nature
(however in terms of sustainability it is important to consider where these atoms are coming from
Even though Choline chloride is considered a natural material (provitamin) does it predominately come from industrial processes
The industrial feedstock for it far outstrips the biological feedstock
Choline chloride is produced industrially on the megaton scale in a single-stage gas phase reaction (ethylene oxide + HCl + trimethylamine)
Hence the claim that the proton donors/acceptors for DES are not always true
