lecture 4: solvents
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Imperial College
LondonModule 4I10: Green Chemistry
Lecture 4: Solvents
4.I10 Green Chemistry Lecture 4 Slide 1
Imperial College
LondonLecture 4 - Learning Outcomes Imperial College
London
By the end of this lecture you should be able to
• describe the advantages and disadvantages of traditional organic
solvents
• list the characteristics of 4 different types of green solvent and for
each one describe an example of a suitable process
• suggest alternative solvent choices for reactions.
4.I10-4-2
Imperial College
LondonThe big problem with organic solvents…
VOCs - volatile organic compounds
• form street-level ozone and smog via free radical air oxidation processes.
According to GlaxoSmithKline, solvents make up ca. 85 % of all their non-
aqueous waste. Typical recovery efficiencies are 50 - 80 %.
The main alternatives to organic solvents are:
• solvent-free processes
• water-based chemistry
• supercritical fluids (particularly water and CO2)
• ionic liquids
• fluorous biphasic systems
All solvent waste must be contained and treated (e.g. incineration)
4.I10-4-3
Imperial College
LondonReplacing organic solvents is not always green!
Organic solvents
• good heat and mass transfer
• low viscosities (good for kinetics)
Replacing organics may incur an increased energy input
Also, not all organic solvents are harmful, e.g.
• isopropanol,
• ethyl acetate
• ethanol
• 2-butanone
• limonene (extracted from citrus fruit peel)
Therefore industry has concentrated on eliminating the most toxic
solvents first:
e.g. chlorocarbons, benzene, toluene, hexane, dioxane, pyridine,
methanol
4.I10-4-4
Imperial College
LondonAlternative 1. Solvent-free systems
Many high-volume chemicals are already produced without solvents
e.g. polymerisation of propene:
Catalyst is soluble in liquid propene
e.g. synthesis of MTBE:
(fuel additive in USA)
Liquid phase reaction (90 °C , 8 atm)
Fewer solvent-free examples exist for fine chemicals / pharmaceuticals.
Main disadvantages of solvent-free syntheses:
• solvents are often still required during work-up (e.g. extraction)
• poor heat transfer in the solid state (although this may be
overcome using microwaves)
4.I10-4-5
Imperial College
LondonA rare example - a terpyridine synthesis
1 2
3Step 1: Aldol - no solvent.
Step 2: Michael Addition - no solvent.
Both steps 1 and 2 are fast and quantitative,
(in EtOH, yields of both steps are ca. 50%)
Step 3: the only stage that requires solvent, but
no purification of the dione precursor is required4.I10-4-6
Imperial College
London2. Water-based chemistry - the ultimate green solvent?
Advantages Disadvantages
• Non-toxic
• Cheap
• Biorenewable
• Non-flammable
• High specific heat capacity
• Removal requires distillation
∴ energy intensive
• Waste streams may be difficult to treat
• Many reagents are water-sensitive
• Generally a poor solvent for organics
Despite the disadvantages, water-based organic
synthesis is a very popular area of research
4.I10-4-7
Imperial College
London2. Water-based chemistry
e.g. Diels-AlderSolvent Relative rate
octane 1
MeOH 12.5
water 740
LiCl(aq) 1800
Why do you think the rate is so much faster in
(i) water?
(ii) aqueous lithium chloride solution?
4.I10-4-8
Hydrophobic
effect
Salting in effect
Imperial College
LondonHigh temperature water
At high temperatures water becomes less dense and less polar
∴ becomes more like an organic solvent (due to reduction in H-bonding).
At high temperatures water also becomes more ionic
becomes more acidic and more basic (increased [H3O+] and [OH-]).
At 300 °C water behaves similarly to acetone
e.g. geraniol isomerisation - a source of fragrances without organic solvents.
220 °C
a-terpinol linalol
4.I10-4-9
Imperial College
LondonWater can also be used in biphasic systems
Traditional hydroformylation chemistry:
Separation of catalyst from medium chain (≥ C8) aldehydes is difficult
Biphasic approach (phase transfer catalysis):
water
organic solventalkene aldehyde
water-soluble
catalyst
Rh-catalyst bears water-solubilising P(Ar)3 ligands:
Catalysis occurs at the interface
4.I10-4-10
Imperial College
London3. Supercritical solvents ("sc-fluids")
Phase diagram:
supercritical
fluid
Gas
Liquid
Solid
triple point
critical point
Temperature, T
Pressure, P
Tc
Pc
Above Tc and Pc sc-fluids
have densities of liquids,
but viscosities of gases
H2O CO2 NH3 C2H4 C2H6 C3H8 CHF3
Tc / °C 374.2 31.1 132.4 9.2 32.2 96.7 25.9
Pc / bar 220.5 73.8 113.2 50.4 48.7 42.5 48.2
Tc often quite low, but Pc is usually high
4.I10-4-11
Imperial College
Londone.g. Supercritical CO2 - Tc = 31.1 °C, Pc = 73.8 bar, ρc = 0.477 g cm-3
Liquid CO2
CO2 vapour
Sub-critical
Approaching
critical
At, or above,
critical point
meniscus poorly
defined homogeneous
supercritical CO2
A major advantage of sc-solvents is their ease of removal
- decrease the pressure and vent off the gas
4.I10-4-12
Imperial College
LondonSupercritical CO2
Advantages Disadvantages
• Non-toxic
• Readily removed (and recyclable)
• Non-flammable
• Low viscosity (fast diffusion)
• CO2 is cheap
• Good solvent of gases (e.g. H2)
• High pressure equipment is expensive
and potentially dangerous
• CO2 is a relatively poor solvent
• Reacts with strong nucleophiles (e.g.
amines)
Uses of sc-CO2:
• extraction of caffeine from coffee (traditional method uses CH2Cl2)
• extraction of fatty acid triglycerides from crisps (low-fat crisps)
• dry-cleaning (traditional method uses C2Cl4)
• spray-painting
4.I10-4-13
Imperial College
Londonsc-CO2 as a reaction solvent
One area where scCO2 has found particular use is in hydrogenation
chemistry (mainly because it is very miscible with H2 gas)
time (hr)
convers
ion (
%)
sc-CO2
CH2Cl2
e.g. imine hydrogenation (20 x faster in sc-CO2 than CH2Cl2)
4.I10-4-14
Imperial College
London4. Ionic liquids
Liquids at room temperature (large non-coordinating ions pack poorly)
Advantages Disadvantages
• Readily prepared
• Very low vapour pressure
• Can act as catalysts
• Tuneable viscosity (via anion)
• Stable at high temperature
• Highly solvating
• Recyclable
• Non-biodegradable
• Concerns over toxicity
• Synthesis often requires haloalkanes
• Product isolation often requires
distillation or extraction into an organic
solvent
Common examples:
4.I10-4-15
Imperial College
LondonIonic liquids - e.g. Pd-catalysed Heck arylation
Work-up procedure:
• (i) add cyclohexane and water
• (ii) physically separate into
three components
• (iii) distill off cyclohexane to
obtain Heck product
• (iv) recycle catalyst without the
need to extract from ionic liquid
water
ionic liquid
cyclohexane
HNEt3I
Pd catalyst
product
yield > 95 %
4.I10-4-16
Imperial College
London5. Fluorous biphasics
Fully fluorinated solvents (e.g. C6F14) are non-polar and immiscible with
organic solvents. Ideal if reactants are non-polar, but products are polar:
e.g. hydroformylation:
fluorinated
solvent
organic
alkene +
catalyst
aldehyde
Catalyst: Rh(H)(CO){P(CH2CH2(CF2)5CF3)3}2
Disadvantages:
Fluorinated solvents are expensive and concerns exist for their
long-term environmental impact.
4.I10-4-17
Imperial College
LondonConclusions
There are several ways in which organic solvents may be replaced, and a
good argument can often be made for doing so on green chemistry
grounds.
However, it is important to remember that changing solvents may require
additional energy (e.g. stronger heating), and organics may still be needed
for work-up / purification steps.
The choice of green solvent depends upon the reaction, upon the catalyst(s)
and upon the method of product separation.
4.I10-4-18
Learning outcomes
(i) describe the advantages and disadvantages of traditional organic
solvents
(ii) list the characteristics of 4 different types of green solvent and for
each one describe an example of a suitable process
(iii) suggest alternative solvent choices for a previously unseen
reaction.
Try the practice exam question on the next slide!
Good heat transfer and good diffusion of reactants.
Potential source of VOCs & waste; not always easy to recover / recycle.
No solvent (terpyridine synthesis); water (Diels-Alder, Hydroformylation)
sc-fluids (Hydrogenation in sc-CO2); ionic liquids (Heck coupling)
Imperial College
LondonLearning outcomes
4.I10-4-19
Imperial College
LondonPractice exam question
The following is an experiment from an undergraduate laboratory course:
"In a 50 cm3 round-bottom flask, dissolve 3.0 g of 2-methylcyclohexanone in 12.5 mL
of methanol. Cool in an ice bath and carefully add 0.5 g of sodium borohydride. When
the vigorous reaction has subsided, remove the flask from the ice bath and allow it to
stand at room temperature for 10 minutes. Then add 12.5 mL of 3 M NaOH and to the
resulting cloudy solution, add 10 mL of water. The product will separate as a clear
layer. Remove as much of this as possible. Then extract the remaining product from
the reaction mixture with 2 x 5 mL portions of dichloromethane. Dry the combined
organic layers with sodium sulfate. Filter the drying agent off. Remove the solvent by
warming under a stream of nitrogen. Be careful when boiling off the dichloromethane
and methanol to avoid boiling off the product. Methanol and dichloromethane have
low boiling points and will boil out of solution rather easily."
Suggest ways in which the method could be modified in accord with the principles of
Green Chemistry.
4.I10-4-20
Imperial College
LondonAnd the answer to last week’s question
The traditional synthesis of ethylbenzene is a Friedel-Crafts alkylation,
such as that shown below:
The modern industrial synthesis involves mixing ethylene and benzene in
the presence of a zeolite (ZSM-5). In what ways would you consider this
method to be greener than the Friedel-Crafts reaction?
Possible answers:
• Reduced waste (NB: Friedel-Crafts acylations require excess AlCl3);
• Reduced energy (catalysis);
• Improved recovery and reuse of catalyst;
• No solvent required.
4.I10-4-21
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