lecture 4: solvents

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Imperial College London Module 4I10: Green Chemistry Lecture 4: Solvents 4.I10 Green Chemistry Lecture 4 Slide 1

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Page 1: Lecture 4: Solvents

Imperial College

LondonModule 4I10: Green Chemistry

Lecture 4: Solvents

4.I10 Green Chemistry Lecture 4 Slide 1

Page 2: Lecture 4: Solvents

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

Page 3: Lecture 4: Solvents

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

Page 4: Lecture 4: Solvents

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

Page 5: Lecture 4: Solvents

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

Page 6: Lecture 4: Solvents

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

Page 7: Lecture 4: Solvents

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

Page 8: Lecture 4: Solvents

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

Page 9: Lecture 4: Solvents

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

Page 10: Lecture 4: Solvents

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

Page 11: Lecture 4: Solvents

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

Page 12: Lecture 4: Solvents

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

Page 13: Lecture 4: Solvents

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

Page 14: Lecture 4: Solvents

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

Page 15: Lecture 4: Solvents

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

Page 16: Lecture 4: Solvents

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

Page 17: Lecture 4: Solvents

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

Page 18: Lecture 4: Solvents

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

Page 19: Lecture 4: Solvents

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

Page 20: Lecture 4: Solvents

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

Page 21: Lecture 4: Solvents

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