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Brown's Chemistry Clinic
My unofficial
support for Salters
A2
Advanced Chemistry
Salters A2 Chemistry
-'exam
bashing' thoughts for unit DP "Designer Polymers"
- part of module 2849
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other
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PLEASE REMEMBER, THESE ARE NOT 'STAND ALONE' NOTES, and were designed for my
classes for use alongside the Salters resources - Chemical Ideas, Chemical
Storylines, Practical Activities-Investigations and the AS-A2 Revision guides
all published by Heinemann Secondary Series, to reduce the reading workload and
offer a study strategy. From your
teacher (not me!), its handy to have the answers to the Chemical Ideas,
Storylines Assignments and Activities Questions side by side with the texts and
these strategy pages. You haven't time to redo the Q's but a quick read of the
Q's and connecting with the official answers is valuable revision - there is too
much hit and miss revision from doing past papers in my opinion.
Chemical
Storylines DP1 Designer Polymers
Polyvinyl Carbazole is a photoconductive
polymer ie it conducts electricity better when light is shone on it.
It is an addition polymer formed
from an alkene (>C=C<) monomer but the delocalised system of the rings is the
feature which allows this conduction. It is used on the picture reproduction drum of a
photocopier.
Most polymers in this unit are A-B
condensation polymers. The two different monomers
(e.g. diamine and diol plus a dicarboxylic acid) must have a functional group at each end and when they
condense together a small molecule is eliminated for each bond formed.
Chemical
Storylines DP2 The Invention of Nylon
Nylon came out of a desire
to make artificial fibres similar to wool and silk. It is not a case of 'serendipity', but
the chemists used existing molecular knowledge to systematically 'mimic' nature and if
possible improve on it!
Wool and silk molecules contain peptide or
secondary amide linkages -NH-CO- so scientists used monomers with -NH2 and
-COOH groups (e.g.* H2N-R-NH2, HOOC-R-COOH or H2N-R-COOH).
The condensation reaction to form the polymer
linkage in principle is ...
-
-NH2 + HOOC- ==> -NH-CO- + H2O
and so water is the small molecule eliminated and the resulting polymer is called a
polyamide of which
nylon is the most common example, and applying this to make the polyamide
polymers gives ....
This reaction is slow, but still used in
industry, so for laboratory demonstrations the diacyl chloride is used - more reactive but
more costly and produces hazardous fumes of HCl.
p123-124 equations AND p125 green box +
Assignment 1 for nylon-x,y or nylon-z naming and structure.
Nylon-6,6 became the pre-eminent nylon
because its physical properties allowed a wider range of applications and the monomers
could be made from readily available materials from the petrochemicals processed from
crude oil eg benzene.
Nylon can be made as a fibre or engineering
plastic eg replacing metal parts in machines. It is strong, tough, rigid and abrasion
resistant and chemically unreactive.
Nylons properties are superior to poly(ethene)
and poly(propene). The intermolecular forces between PP and PE are the weakest
transient dipole-induced dipole interactions BUT in nylon added to these attractive forces are
the hydrogen bonds from the >C=Od-||||d+H-N<, which are the strongest of the
inter-molecular forces.
BUT there was one particular problem to solve
for clothing applications. Nylon is hydrophobic ie it repels water, so in
nylon clothing couldn't 'breathe' leading to a 'sweaty' situation. By using thinner nylon
fibres, a 'delustrant' to make it look more natural, and 'puffing out' the nylon molecules
with high pressure air
into large numbers of loops, a softer material was formed AND it allowed the passage of
water vapour but NOT liquid water. These technological developments produce a more
acceptable artificial fibre for clothing and where required, a comfortable waterproof
material.
CI 13.3 Carboxylic acids and their derivatives
(revision)
Revise the naming and structure
representations of carboxylic acids, esters, amides and acyl chlorides in particular to
cover DP2-4, these are all examples of a homologous series and be able to
spot/identify the relevant functional group. Homologous
Series/Functional Groups
CI
13.2 Alcohols and Ethers plus 13.4 The -OH group in alcohols, phenols and acids (both revision)
Chemical
Ideas 13.8 Amines and amides
Compare the relative structures of
(0) ammonia NH3, and (1) primary amine RNH2, (2)
secondary amine R2NH
and (3) tertiary amines R3N (from 0-3 alkyl groups,
R, attached to the nitrogen
atom). The bonding of amines is similar to that for ammonia (Figs 13/14) and the H is
replaced by alkyl (or aryl) groups. If you can do a ox diagram for ammonia or methane, it should
be no problem to do methylamine.
Naming of amines: lower
members of the series are named as alkylamine's eg ethylamine, and higher members best named as
aminoalkanes eg 1-aminopentane.
Physical properties: The
lower members are gases or liquids, and like ammonia, have strong smell, in the case of
amines it is of decaying fish! The lower members are soluble in water due to solvation via
hydrogen bonding (Figs 14). Aromatic amines are insoluble, and the higher
aliphatic amines become progressively less soluble as the hydrophobic alkyl
group gets longer.
Chemical properties of amines:
As a base: In water the lone
pair on the nitrogen can accept a proton from water to form the protonated cation. eg
R-NH2(aq)
+ H2O(l) 
RNH3+(aq) + OH-(aq)
the equilibrium is mainly on the left as ammonia and amines are weak bases, but the
solution is quite alkaline. The amines are neutralised by acids to form alkyl ammonium
salts containing the cation above e.g.
As a ligand: The lone pair on the
nitrogen can be donated to form a dative covalent bond with eg a transition metal ion like
copper(II) eg with butylamine it forms
-
[Cu(CH3CH2CH2CH2NH2)4(H2O)2]2+
which is a dark blue complex.
-
In unit SS you will come across the
ammonia complex [Cu(NH3)4(H2O)2]2+
As a nucleophile: Amines can acts as a
nucleophile because of the ability to donate the lone pair of electrons on the nitrogen to
form a bond with a Cd+ carbon atom in
eg the
polarised Cd+-Cld-
bond in haloalkanes or acyl chlorides.
reacts with haloalkanes just like
ammonia (gives primary amine) to form a secondary amine (see CI p303/332).
reacts with acyl chloride to form a secondary
amide (p332). Note the structure of a primary amide like ethanamide compared to the
secondary amide shown.
Hydrolysis of amides: slow with water
but much faster when refluxed with a strong acid (eg HCl(aq))
or strong alkali (eg NaOH(aq)) catalyst. The permutations for secondary amides
are shown using abbreviated structural formulae - more detailed on p333.
with water: R-NH-CO-R' + H2O
==> R-NH2 + HOOC-R' (the free amine + the free acid)
with acid: R-NH-CO-R' +
H2O + H+
==> RNH3+ + HOOC-R' (the cation of the amine salt +
the free organic acid)
with alkali: R-NH-CO-R' + OH-
==> R-NH2 + -OOC-R' (the free amine + the anion of salt of
the carboxylic acid)
Formation of condensation polymers:
A carboxylic acid and an amine group can
condense together to form a secondary amide linkage (same as a peptide linkage in
proteins)
-NH2 + HOOC- ==> -NH-CO- + H2O
(so water is the small molecule eliminated in the condensation reaction)
see above in CS DP2 for more equation
details
If a diamine and a dicarboxylic acid are used
the resulting polymer is called a polyamide of which nylon is the
most common example (see p334 Figs 18/19). You can also use as an amino carboxylic acid as
a single monomer.
This reaction is slow, but still used in
industry, so for laboratory demonstrations a diacyl chloride is used - more reactive but
more costly and produces hazardous fumes of HCl. Be able to work out the equations using
either the acid or the acid chloride.
CI 5.4 Forces between molecules: hydrogen bonding
(revision)
Activity
DP2.1 Making nylon
Writing the equations and naming the
nylon. equation
examples above in CS DP2
HCl is eliminated in this condensation
polymerisation NOT water.
Appreciate that using the more reactive
di-acid dichloride is fast and good for laboratory demonstrations but commercially the
acid itself reacts slower BUT is less costly and no hazardous HCl gas to deal with!
Activity
DP2.2 Taking nylon apart
This reaction can be overall described in
two ways ...
Revision of important procedures for
purifying an organic compound and you need to be able to briefly describe and explain ...
Heating under reflux - vertical
condenser over flask, allows higher reaction temperature without losing any volatile
solvent, reactant or product.
Vacuum filtration - fast filtration
due to pressure differential and a little washing of the solid with eg the solvent.
Recrystallisation - the method
relies on solubility increase with increase in temperature and hopefully it is not too
soluble in the cold solvent and the traces of impurity stay in the solvent. The solid is
dissolved in minimum volume hot solvent (water here), cool to crystallise, re-filtered and
washed with a little solvent, collected and dried (usually left out in fume cupboard ok).
Melting point - small sample in
capillary tube sealed at one end, carefully heated in oil bath, slow to 1o/minute
rise near melting point. If it melts sharply over a narrow range close to the data book
value its pure BUT if it melts at a lower temperature and over a wider range its impure!
Be able to write out the hydrolysis
equations ...
Irrespective of hydrolysing agent the basic
reaction at the 'end' of each 'monomer' in the chain is ... but
immeasurably slow if at all, with pure water
secondary amide + water ==> amine + acid
(but three variations on the equation!)
(-NH-R-NH-CO-R'-CO-)n + 2nH2O
==> n H2N-R-NH2 + n HOOC-R'-COOH
(free amine + free acid)
Note (i) that this is the reverse of the
condensation polymerisation, and (ii) in fact by using acid or alkali
catalyst under reflux the
equations are actually ....
-
(-NH-R-NH-CO-R'-CO-)n +
2H2O + 2nHCl
==>
n Cl-+H3N-R-NH3+Cl- + n HOOC-R'-COOH
(amine salt + free acid)
-
(-NH-R-NH-CO-R'-CO-)n +
2nNa+OH-
==>
n H2N-R-NH2 + n
Na+-OOC-R'-COO-Na+
(free amine + salt of acid)
To detect the amine, the acid is
neutralised by the sodium hydrogencarbonate and then sodium hydroxide is added to free the
amine, which has a fishy odour.
Chemical
Storylines DP3 Polyesters: From clothes to bottles
Polyesters were developed along a similar
storyline to nylon. The most well-used is PET, old name polyethylene terephthalate.
This made from the monomers ethane-1,2-diol and 1,4-benzenedicarboxylic acid (write out
equation of formation CI page 114 and its + 2H2O).
The polyester is made of small granules which
are melted and squeezes through fine holes to form fibres known as Terylene and Dacron for
clothing. The fibres are good heat insulators.
PET is also used in packaging after being
stretched and heat treated (curing) to increase strength. This processing allows the
alignment of the polymer molecules to happen in 1-3 dimensions increasing the
permanent-permanent dipole (>Cd+=Od- ... >Cd+=Od-)
intermolecular forces. (Fig 11: 1D fibre, 2D film, 3D bottles - can you make the
molecular connection to the application?) This
PET has great strength and is impermeable to gases and is used in food packaging and
bottles for fizzy drinks etc.
A polyester made from a hydroxycarboxylic
acid* is used to make dissolving
stitches. It can also be used, for the same reason, as a
coating on a tablet implanted in the body. The medicine is slowly released at a rate
determined by the hydrolysis speed of the polyester - so scope for changing the polymer
structure to a desired rate of hydrolysis.
* n HO-CHR-COOH ==> (-O-CHR-CO-)n
+ n H2O and the polymers form strong threads but the water in the body
slowly hydrolyses the ester linkage, reverse of the above reaction, and the products are
non-toxic.
Chemical
Ideas 13.5 Esters (part new, part revision)
Basic equation of formation:
R-OH +
HOOC-R'
R-OOC-R' + H2O
R is alkyl for alcohols, or aryl for
phenols e.g. C6H5- for the ring of phenol, R' is H, alkyl or aryl for the carboxylic acid. Aryl means aromatic eg simplest
is C6H5- for the a benzene ring of benzoic acid.
Esterification is another example of
a condensation reaction. The usual catalyst is a small amount of concentrated
sulphuric acid and the mixture heated under reflux.
Be able to recognise the ester linkage
-CO-O- as a
functional group/homologous series and to write the equation in shorthand or full structural
formula style.
AND recognise it and name it which ever way
its written down!
Naming: The alcohol
bit forms the prefix of the name and the carboxylic acid the
suffix so the name becomes eg alkyl ...oate. so ethanol becomes ethyl and ethanoi giving ethyl
ethanoate.
Polyesters are made by esterifying
or condensing together a diol and a dicarboxylic acid, eg PET from ethane-1,2-diol and
benzene-1,4-dicarboxylic acid.
Esters from phenols: Phenols are not
as reactive as alcohols in esterification reactions and a more vigorous reagent is needed.
When an ethanoate is made the process is called ethanoylation and there are two ethanoylating
reagents (or acylating reagents, meaning they replace the H of the OH with an R-C=O
group). Water must not be present in the reaction mixture.
Ethanoic anhydride: This is an example
of an acid or acyl anhydride made by eliminating a water molecule between two of the
acid molecules. It readily reacts with phenols (and alcohols!) to give the ester eg
aspirin preparation top of p319. The mixture is heated under reflux and the ester and one
molecule of ethanoic acid are formed.
Ethanoyl chloride: This is an example
of an acid or acyl chloride where the OH of the carboxylic acid is replaced with a
chlorine atom. They are reactive reagents with phenols (and even more so with
alcohols) even at room temperature. Nasty acrid fumes of HCl are formed.
Ester Hydrolysis: The reverse of
esterification is usually slow with water but much faster when catalysed by strong acids
eg HCl or H2SO4 (provide H+) or strong alkalis like
sodium hydroxide (provide OH-) in aqueous solution. The hydrolysis is much
faster when the mixture is heated under reflux. The general equations are ...
with water or acid:
R-OOC-R' + H2O R-OH + HOOC-R' (equilibrium in forming the free carboxylic acid and alcohol,
but using a large excess of water virtually ensures completion)
with alkali:
R-OOC-R' + OH-
R-OH + -OOC-R'
(this goes to completion to form eg the sodium Na+ salt of the carboxylic acid)
Chemical
Storylines DP4 Kevlar
Kevlar
is related to nylon because it is a polyamide, but an aromatic amine and carboxylic acid
is used are used. Kevlar type polymers are very strong and heat resistant but still
flexible.
The
higher C:H ratio than 'aliphatic' nylon makes it more combustion resistant and its density
is low because it is made from light C H O N atoms.
The
planar aromatic rings result in rigid polymer chains which are held together by hydrogen
bonding forming strong sheets at the molecular level (Fig 16). Using -1,4- position
monomers produces straighter chains with closer alignment. The sheets can arranged in an
axial manner to make extremely strong fibres.
Unfortunately
in the early manufacturing there was a problem of too early precipitation from the monomer
solution due to its insolubility due to strong H bonding, but using sulphuric acid
as a solvent solved the problem because the H2SO4 disrupts H bonds by protonating the -NH- group to
-NH2+-, so the polymer was kept in
solution longer to grow much longer chains. When the sulphuric acid solution is poured
into water the proton from the -NH2+- is transferred to water to
form H3O+ (-NH2+- + H2O
=> -NH- + H3O+) and the polymer is
precipitated out of solution.
Its
light and strong nature allow it to be used for cords in car tyres, an aircraft
construction material and bullet proof vests!
Assignment
4 essential practice.
Activity
DP4 Comparing models of nylon-6,6 and Kevlar
Storylines
DP5
Taking Temperature into Account
Poly(ether-ether-ketone) or PEEK plastics
(Fig 18) are extremely strong and heat resistant. They can be used in machines like car
engines operating at high temperatures and as a combustion resistant construction material
for aircraft.
Other polymers like a poly(hydroxyamide)
,PHA, are designed to change to a more stable and heat resistant form if subjected to a
fire situation. You get a sort of 'intra-molecular' condensation in which the small
non-flammable molecule of water is eliminated to produce the more stable form (above Fig
20).
Because each polymer has its own
characteristic useful properties it is possible to mix them to form polymer alloys
(analogous to metals). These combine different properties to produce a useful range of
materials. You can also add plasticisers (small but non-volatile molecules) to make it
more flexible or make copolymers like lldpe (see CI 5.5 part 2).
Ch.
Ideas 5.5 The Structure and Properties of polymers
(revise
part 1, part 2 new)
Some revision points from Part 1:
explaining addition polymerisation, copolymerisation, elastomer, plastic,
fibre, factors affecting polymer properties, thermoplastic, thermoset, effect of chain
length, intermolecular forces, crystalline and amorphous regions, cold-drawing ... in
other words give it one good read before tackling Part 2!
Condensation polymerisation:
Terylene is a typical polyester and
nylon is
a typical polyamide (you should know the equations given on p114-115, they need 2H2O
to balance in my CI edition!).
They are both formed in a condensation
reaction in which monomer molecules link together via the elimination of a small molecule
(water).
They are linear structures ie no branching,
and are ideal for fibres. The molecules can line up and are held together by ...
Terylene: (transient dipole
- induced dipole) + (permanent dipole - permanent dipole) intermolecular forces.
Nylon: (transient dipole -
induced dipole) + (permanent dipole - permanent dipole) + ( hydrogen bonding) intermolecular
forces.
The effect of temperature on polymer
properties:
Polymer materials have amorphous and
crystalline regions (Fig 32 p113) and the relative proportions of them affect the polymers
physical properties as does change in the temperature of the plastic.
The variation of physical strength versus
temperature of a typical part-crystalline polymer is shown on p115 Fig 34.
Below the glass transition
temperature Tg the tangled polymer molecules of the amorphous region
are frozen and the plastic is quite hard and brittle, ie glass like.
Between the Tg
and the melting temperature Tm the polymer chains of the
amorphous region becoming increasingly less frozen and the plastic becomes quite flexible.
Above the melting
temperature Tm the
polymer chains of the crystalline regions break down and the plastic becomes a viscous
fluid.
Activity
DP5 Bubble gum - or bubble glass?
Chemical
storylines DP6 Poly(ethene) by Design
Reminders on poly(ethene)
ldpe low density
poly(ethene), made under high pressure and oxygen catalyst, is all very tangled and
branched
hdpe high density
poly(ethene) is made with Ziegler-Natta catalysts and produces chains with little
branching that can pack tightly together maximising and intermolecular forces. The
resulting hdpe is stronger and more dense than ldpe.
The two forms have different applications
because of their different properties.
However there is an increasing demand for
ldpe but the high pressure engineering costs have been economically prohibitive. So by
combining the chemically efficiency of Ziegler-Natta catalysts with using a small proportion
of a longer alkene with ethene (copolymerisation) results in the production of lldpe
linear low density poly(ethene).
The Ziegler-Natta catalyst produces uniform
long chains but the use of eg hex-1-ene in controlled amounts allows a controlled amount
of branching with -C4H9 groups sticking out from the main
chain. The
chains can't pack together as efficiently as in hdpe but they are more aligned than in
ldpe making it stronger but the branching makes it lower melting and more flexible than
hdpe. Check out assignment 3.
Chemical
Storylines DP7 Throwing it away ... or not
Plastics are a major waste
problem, partly
because most of them are not readily biodegradable in a landfill or any other increasingly
costly site! Apart from risky incineration producing toxic smoke there are two basic
solutions.
Recycling is suitable for
some thermoplastics as they can be reworked without decomposition, but there are problems
in sorting out the different plastics and this is expensive. It would be more efficient if
the used plastic could be recycled by the original manufacturer. It is also possible to
degrade the waste plastic back to its original monomer and repolymerising. A 3rd recycling
route is to thermally degrade (cracking) the plastic into basic organic feedstock. See Fig
27/28 for a summary.
Degradable plastics can be
divided into three types
Biopolymers: like
poly(hydroxybutanoate) PHB are made by certain bacteria (example of a
polyhydroxy alkanoates, PHA). They are readily degraded by other bacteria but are costly
to produce. However, research is going on to genetic engineer plants like cotton and oil
seed to produce different PHA's more cheaply. Mother nature will then happily recycle them
via bacteria into water and carbon dioxide!
Synthetic biodegradable plastics:
If a biodegradable material like starch is dispersed in a non-biodegradable plastic like
PE then as the starch degrades the PE is broken up into very small sections with a large
surface area that considerably increase the rate of natural degradation.
Photodegradable plastics: If
near uv light absorbing chemical groups like C=O are incorporated into the polymer
structure, the energy absorption causes bond rupture which triggers the degradation of the
smaller molecular fragments Assignment 6 poses a few Q's about their use.
Chemical storylines DP8 SUMMARY,
Activity DP8 Check your notes on DP, and the
learning
objective list should prepare you for the DP UNIT TEST and ultimately the module
2853 "Polymers, Proteins and Steel" exam.
GENERAL
REVISION
NOTES
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