Download Polymers and Amino Acids

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What are polymers?
A polymer is a long molecular chain built from monomer
units. The two main types of polymer are addition and
condensation.
Feature
Addition polymer
functional
group(s) of
monomer(s)
Condensation polymer
Polyester
Polyamide
C═C
–COOH and
–OH
–COOH and
–NH2
types(s) of
monomer
1
1 or 2
1 or 2
product(s)
poly(alkene)
polyester + water
polyamide + water
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What are addition polymers?
During addition polymerization, unsaturated molecules
add together forming a polymer.
Formation of poly(propene):
Formation of poly(chloroethene) (PVC):
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Naming addition polymers
Addition polymers are named after the monomer used to
make them:
is prepared from
ethene
poly(ethene)
is prepared from
poly(propenenitrile)
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propenenitrile
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Common addition polymers
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Naming addition polymers
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Disposal of addition polymers
The unreactivity of addition polymers,
which makes them useful for storing
food and chemicals, means they are
not biodegradable and persist in the
environment for many years.
Incineration of addition polymers is
controversial as it produces carbon
monoxide and carbon (soot), as well
as carbon dioxide, contributing to
global warming.
Polymers can also produce toxic chemicals when burnt.
For example, PVC produces hydrogen chloride, and nylon
produces hydrogen cyanide.
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What are condensation polymers?
Condensation polymers, such as polyesters and
polyamides, are formed by a condensation reaction, which
releases small molecules such as water.
Polyesters can be formed from monomers of:

a dicarboxylic acid (a carboxylic acid with two –COOH
groups) and a diol (an alcohol with two –OH groups)

substances containing both –COOH and –OH groups.
Polyamides can be formed from monomers of:

a dicarboxylic acid and a diamine (an amine with two
NH2 groups)

amino acids, which contain both –COOH and –NH2 groups.
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Formation of polyesters
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Formation of polyamides
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Disposal of condensation polymers
Unlike addition polymers, which don’t readily degrade,
condensation polymers can be designed that biodegrade,
photodegrade or which can be recycled.
Condensation polymers biodegrade by hydrolysis, which is
the reverse of the condensation reactions that form them.
Some condensation polymers, such
as nylon, degrade very slowly, which
is necessary for them to be useful.
Condensation polymers such
as poly(lactic acid) have been
designed to biodegrade more quickly and are used
in disposable plastic items such as packaging.
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Hydrolysis of polyesters
The hydrolysis of polyesters with pure water is too slow to be
useful, so an acid or base catalyst is required.
acid hydrolysis: reflux with dilute H+ (e.g. HCl, H2SO4)
base hydrolysis: reflux with dilute NaOH
Hydrolysis with a base is preferable as it is not a reversible
reaction like hydrolysis with an acid.
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Hydrolysis of polyamides
Polyamides can be split into their constituent carboxylic acids
and amines by hydrolysis with a hot aqueous acid or base.
acid hydrolysis: reflux with dilute H+ (e.g. HCl, H2SO4)
base hydrolysis: reflux with dilute NaOH
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Recycling polymers
Some polymers, such as PET, are
widely recycled. This reduces the
amount of plastic disposed of in
landfill sites and conserves crude
oil, which is the source of most of
the monomers used.
Some polymers are simply melted
and reused but condensation polymers
can be hydrolyzed back to form monomers.
The disadvantage of recycling is that the plastics must be
collected, sorted and processed, which is expensive.
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Polymers summary
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Type of polymerization
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Which polymer?
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Proteins and amino acids
Proteins are a diverse group of large and complex polymer
molecules, made up of long chains of amino acids.
Amino acids contain both amine
(NH2) and carboxyl (COOH)
functional groups.
In alpha amino acids, these groups
are attached to the same carbon atom.
glycine (gly)
The R group, also attached to the same carbon atom, can vary.
There are 22 amino acids that are used to make proteins
(proteinogenic). The simplest is glycine, where R = H.
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Proteinogenic amino acids
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Zwitterions
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Acid–base properties of amino acids
The presence of a carboxyl group and an amine group mean
that amino acids have both acidic and basic properties.
NH2 group
acts as a base
COOH acid group
acts as an acid
When acting as an acid, the COOH group loses a H+ ion:
H2NCHRCOOH + OH-  H2NCHRCOO- + H2O
When acting as a base, the NH2 group gains a H+ ion:
H2NCHRCOOH + H+  H3N+CHRCOOH
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Effect of pH on amino acids
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Peptide formation
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Hydrolysis of peptide bonds
A peptide bond can be split by refluxing with hydrochloric acid.
During hydrolysis, the water molecule adds across the peptide
bond, forming a mixture of the two amino acids.
Peptide links can also be broken using a solution of alkali,
such as aqueous sodium hydroxide at above 100°C.
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Protein structure
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Bonds in proteins
The 3D shape of a protein is maintained by several types of
bond, including:
hydrogen bonds:
involved in all levels of
structure.
hydrophobic
interactions:
between non-polar
sections of the protein.
disulfide bonds: one of
the strongest and most
important type of bond in
proteins. Occur between
two cysteine amino acids.
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Types of protein
There are two broad classes of protein: fibrous and globular.

Fibrous proteins are formed from parallel polypeptide
chains held together by cross-links. These form long,
rope-like fibres, with high tensile strength and are
generally insoluble in water.
Examples of fibrous proteins include collagen, keratin and
silk.

Globular proteins usually have a spherical shape caused
by tightly folded polypeptide chains with hydrophobic
groups on the inside, and hydrophilic groups on the outside.
Examples of globular proteins include enzymes and some
hormones.
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Structure of enzymes
All enzymes are globular proteins. They are soluble in water
due to the presence of many hydrophilic side groups on their
constituent amino acids.
Most enzymes are very large
molecules but only a small
part of them is involved in
catalysis. This is called the
active site and it may consist
of just a few amino acids.
active site
The remainder of the amino acids maintain the precise
shape of the enzyme and the active site.
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Amino acids and proteins: summary
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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