Part 6.
The Chemistry of Carboxylic Acids and their Derivatives
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US
K12 grade 11 grade 12 organic chemistry
6.10 The manufacture,
molecular structure, properties and uses of polyesters -
condensation polymers
Sub-index for this page
6.10.1
Condensation
polymerisation compared to addition polymerisation and the structure of a
polyester
6.10.2
The manufacture,
molecular structure of
Terylene/ PET (Poly
Ethylene Terephthalate) and their uses
6.10.3
Examples of
homopolymeric polymers - structure and uses of polylactic acid and Biopol
6.10.4
Disposal and other
issues involving using polyesters
INDEX of all carboxylic acids
and derivatives notes
All Advanced A Level Organic
Chemistry Notes
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BASIC GCSE notes
Condensation polymers, Nylon, Terylene, comparing thermoplastics, fibres,
thermosets
6.10.1 Comparing condensation polymerisation
and addition
polymerisation and the structure of a polyester
Condensation
polymerisation involves linking lots of small
monomer molecules together by eliminating a small molecule.
This is
often water from two different monomers, a
H from one monomer, and an
OH
from the other monomer, the 'spare bonds' then link up to form the
polymer
chain plus H2O
molecules, or sometimes HCl
molecules if one of the monomers is an acid chloride derived from a
dicarboxylic acid.
By contrast, in addition polymerisation there is
only one monomer molecule with a double bond (e.g. alkene) and one product, the polymer,
and the linking occurs via the reactive double bond - illustrated
below.
Condensation polymerisation
Polyesters are made from dicarboxylic acids and
diols (though can use one monomer with the two different groups present
in its molecular structure See
homopolymeric polyesters).
Polyesters are condensation polymers because of
how they are formed - by this condensation reaction that eliminates a
small molecule to form the ester bond between the monomers.
This involves monomers
with two functional groups, one at each end of the molecule - they
can be the same or different, as long as one functional group and join
and condense with the functional group of another monomer.
When
these types of monomers react, they join together (polymerise) and small
molecules such as water are eliminated in the process, and so the
reactions are called condensation reactions, since a polymer is
formed, the process is
called condensation polymerisation.
e.g. in these three simplified cases, there are
two different monomers, but each has the same functional group (hydroxy,
amine, carboxylic acid or acid chloride) ....
n HO-[][]-OH
+ n HOOC-[][][][]-COOH
==> -(-[][]-OOC-[][][][]-COO-)n- +
2n H2O
n H2N-[][]-NH2
+ n HOOC-[][][][]-COOH
==> -(-[][]-NHOC-[][][][]-CONH-)n- +
2n H2O
n H2N-[][]-NH2
+ n ClOC-[][][][]-COCl
==> -(-[][]-NHOC-[][][][]-CONH-)n- +
2n HCl
[][]
and [][][][]
represent the rest of the molecules, and n is a very large number
!
A
polyester
can be made from a
diol (an alcohol with two hydroxy groups two -OH's) and a dioic acid
(a carboxylic acid with two -COOH groups, a dicarboxylic acid).
These are the two starting monomers prior to
polymerisation and both must have a reactive group at each end - and a
different functional group that can react with the other.
An example of polyester (which is a commercial
product) is made from the two monomers ...
The ethanediol monomer molecule
HO-CH2-CH2-OH
This type of molecule is called a diol,
because it has two alcohol groups -OH.
The hexanedioic acid molecule monomer molecule
HOOCCH2CH2CH2CH2COOH
This type of molecule is called a
dicarboxylic acid, because it has two carboxylic acid groups -COOH.
Alcohols react with carboxylic acid to form
esters with the elimination of water.
In this case the ester linkage is formed at
both ends of each molecule with the elimination of water molecules.
Diagram to explain how a diol (alcohol)
and a dicarboxylic acid condense together to give an ester linkage.
The
OH
group of one diol monomer and a COOH
group of the other monomer react to form an ester linkage COOC
and a water molecule
H2O
is eliminated.
Here just one of
each monomer have condensed together to make a bigger molecule -
the water is eliminated as the new linking covalent bond is
formed - an ester
bond.
BUT,
at each end of this molecule, the functional group (alcohol -OH,
on left) can link with the other functional group (carboxylic
acid, -COOH, on right) to create an even bigger molecule -
eventually, a long chain polyester polymer consisting of
hundreds and thousands of monomer units.
Where n is the very large number of monomer
molecules, the condensation polymerisation of ethane-1,2-diol and
hexanedioic acid can be represented, where n is a very
big number, as ...
n HO-CH2-CH2-OH
+ n HOOCCH2CH2CH2CH2COOH
===> -(-CH2-CH2-OOC-CH2CH2CH2CH2-COO-)n-
+ 2n H2O
6.10.2 The
formation, molecular structure and uses of the polyester PET/Terylene
PET is the acronym for
Poly Ethylene Terephthalate, and it has the same molecular structure
as Terylene!
Terylene (a polyester)
is
good for making 'artificial' or 'man-made' fibres
used in the clothing
industry.
benzene-1,4-dicarboxylic acid,
(terephthalic acid) a carboxylic acid functional group at each end of
the molecule
Ethane-1,2-diol
(ethylene glycol),
two primary
alcohol groups, one at each end of the molecule;
benzene-1,4-dicarboxylic acid forms a
condensation polymer with ethane-1,2-diol, the polyester Terylene and
the eliminated molecule is water
H2O, 1st diagram below, and the 4th diagram shows
several of the linked ester units.
You can carry out the reaction with a different
aromatic acid derivative to make PET/Terylene e.g. using ...
the diacid chloride of benzene-1,4-dicarboxylic
acid and the eliminated molecule is hydrogen chloride (HCl).
the dimethyl ester of benzene-1,4-dicarboxylic
acid and the eliminated molecule is methanol (CH3OH)l.
(see the two middle diagrams below)
More advanced displayed formula
representations of Terylene/PET and its formation
The plastic PET is the same as
Terylene!
The acronym
PET stands for
Polyethylene
terephthalate.
When used in clothing
fibres, it is called Terylene, but when used in bottles, it is called
PET !!
PET is a clear, strong and
lightweight plastic belonging to the polyester family.
It is typically called
"polyester" e.g. the brand Terylene when used for fibres or fabrics, and "PET" or "PET Resin"
when used for bottles, jars, containers and packaging applications - but
they have the same molecular structure - its just how its processed into a
particular product.
PET
is a thermoplastic, softens on heating, that can be extruded into different shapes,
particularly for water and fizzy drinks. The bottle keeps the dissolved carbon
dioxide from escaping and it is shatterproof if dropped on the floor - so its
quite a tough plastic.
PET is the world's packaging
choice for many foods and beverages because it is hygienic, strong,
lightweight, shatterproof, and retains freshness.
PET is most commonly used to
package carbonated soft drinks and water.
The most common use of polyester today
is called PET (for short!) and is used to make the plastic
bottles for storing liquids in like soft drinks. PET is very useful
because it is transparent, shatterproof and cheap!
Terylene
polyester is a typical synthetic fibres which have, in many cases,
replaced cotton, silk and wool fabrics in the clothing industry.
Fine polyester fibres can be
made into a variety of articles of clothing which are lighter and
cheaper than traditional materials like wool.
Shirts, sheets, socks and trousers are manufactured
from a mix of a polyester and a natural fibre such as cotton and wool.
The polyester gives fabric strength and creases
resistance and the natural fibre gives the material a softer more natural
feel and also allows the material to absorb some perspiration from the
wearer - remember plastic materials are usually hydrophobic - water
repellent.
drawing out fibres makes them stronger
In the manufacturing process the polymer chains are made to
line up by drawing out the polymer into a fine fibre.
Increasing the polymer chain alignment greatly increases the intermolecular forces between the
'aligned' polymer molecules and strong fibre strands of the
plastic can be made - the process increases the surface - surface
contact, so increasing the instantaneous dipole - induced dipole forces
and the permanent dipole - permanent dipole forces.
Terylene fibres are cheap to make on an industrial
scale compared to cotton from fields, silk from silkworms and wool from
sheep.
As well as being cheaper, the physical
properties of synthetic fibres have several advantages compared to their
natural predecessors like cotton, silk and wool.
Compared to natural fibres, synthetic
fibres tend to be ....
lighter - outdoor or indoor clothing,
more durable - harder tougher wearing
fibres,
water-resistant - better water-proofed
fabrics,
Polyester fibres are good thermal insulators - trap air
e.g. the stuffing in duvets.
However, there are some disadvantages
e.g.
they are not very breathable and sweat
builds up making you feel uncomfortable.
The use of 'breathable' fabrics like
GORE-TEX are described and discussed on the
smart materials page 6. Gore-Tex and
thinsulate etc.
6.10.3 Homopolymeric polyesters
A homopolymer is a polymer produced from a single type
of monomer e.g. the production of poly(ethene) from ethene.
To produce a condensation homopolymer, the single type
of monomer must have both functional groups (one at each end) in its
molecular structure to enable it to condense together with another monomer
and eliminate a small molecule when the linking bond is formed.
Hydroxy-carboxylic acids have stable functional groups
at (or near!) the ends of the molecule i.e. HO-R-COOH.
These can be obtained from renewable resources such as
corn starch and sugar cane.
The polymer products are sometimes referred to as 'bioplastics'.
Polylactic acid - structure and uses
2-hydroxypropanoic acid (lactic acid) is a
hydroxy acid that can be polymerised to make a useful biodegradable
polymer.
Lactic acid has two functional groups:
Hydroxy - alcohol and carboxylic acid,
these can condense to form an ester linkage.
2-hydroxypropanoic acid can be manufactured by
the fermentation of cornstarch.
Lactic acid is naturally formed e.g. in milk,
so microorganisms can 'recognise' the polymer's structure and break
it down.
Polymerisation equation and structure of
polylactic acid
Uses of
polylactic acid
Polylactic acid has many uses. It can be processed into
plastic films in food handling, bottles, and
biodegradable medical implant devices (e.g. screws, pins, rods, and
plates that are expected to biodegrade in the body within 6-12
months). Polylactic acid constricts under heat and is suitable for
use as a shrink wrap material. One disadvantage is its low
softening temperature makes polylactic acid products unsuitable to
hold hot liquids
e.g. plastic cups.
Biopol - structure and uses
Biopol is a brand
name for PHB (Polyhydroxybutyrate). It is an environmentally friendly,
quality biodegradable plastic
As an energy store, 3-hydroxybutanoate, CH3CH(OH)CH2COOCH3,
is produced by the microorganism Alcaligenes. From this you
obtain 3-hydroxybutanoic acid
.
Polymerisation equation and structure of Biopol
Uses of Biopol
Biopol
is widely used in products such as packaging, shampoo
bottles, disposable cups, surgical stitches, disposable
knives and forks, woven medical patches and nappy linings.
Many of these
products end up in waste bins and hopefully disposed of
safely, but they will biodegrade.
6.10.4 Disposal and other issues involving using polyesters
Plastic bottles made from polyester
can recycled and turned into fibres again and reused in
clothing, so!, are you wearing a plastic bottle!
The bottles must be collected, sorted, and washed and after purification
they can be recycled and softened - extruded once again into a useful
product.
However, polyesters are not biodegradable, despite the ester linkage,
which is present in compounds of all organisms, microorganisms have not yet
evolved to hydrolyse the ester linkage in synthetic fibres.
There is growing concern of the build-up of microplastic particles in
the environment, much of which seems to end up as 'fibres' in the sea.
Also, unfortunately, when burned, toxic fumes are easily produced unless
the material is incinerated at a very high temperature with excess air
(oxygen supply) under very carefully controlled conditions.
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INDEX of all carboxylic acids
and derivatives notes
All Advanced Organic
Chemistry Notes
BASIC GCSE notes
Condensation polymers, Nylon, Terylene, comparing thermoplastics, fibres,
thermosets
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