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Aromatic hydrocarbons
Unit 2: world of carbon
HIGHER CHEMISTRY
Aliphatic hydrocarbons
• Where carbon atoms are linked together to
form a chain.
• Alkanes, alkenes & alkynes
Alicylcic hydrocarbons
• Carbon atoms are linked together to form
rings.
• Cycloalkanes, cycloakenes
Aromatic hydrocarbons
• Contain a ring of 6 carbon atoms.
• The simplest compound is benzene C6H6
Structure of benzene
Benzene
• Carbon has 4 outer electrons.
• 3 of these electrons are involved in covalent
bonding with the 2 adjacent carbon atoms
and the 1 hydrogen.
• The last electron enters a pool of
delocalised electrons.
Uses
• Benzene is an important feedstock to
industry.
• Many consumer products are
manufactured using benzene as a
feedstock.
Halogenalkanes
Uses
• Trichloromethane, CHCl3, more commonly
known as chloroform was used as an
anaesthetic.
• However it is known to damage the liver.
CFC’s
• Chlorofluorocarbons were developed for use where a
non-flammable volatile liquid was required.
• During the 80’s there was growing concern over the
depletion of the ozone layer. CFC’s were resposible.
• When UV rays met the CFC’s in the upper atmosphere
the molecules would break up leaving a Cl atom.
• This Cl atom then converts O3 (ozone) into O2.
World of Carbon
Fuels
Unit 2
Fossil fuels
- What are they?
- How were they
formed?
- Fractional distillation
Pollution
- Greenhouse effect
- Renewable / non
renewable energy
Hydrocarbons
- Alkanes
- Alkenes
- Cycloalkanes
Hydrocarbons
- Isomerism
- Saturation /
Unsaturation
- Cracking
Petrol
•This petrol
fraction is not
Ready to be used as
a fuel.
• Straight chain
hydrocarbons
do not perform well
as fuels
In petrol burning
engines.
•You can improve
performance by
adding aromatic &
branched chain
hydrocarbons
Aromatic & branched
Hydrocarbons
• Aromatic hydrocarbons contain rings of 6 carbon
atoms.
• Branched-chain hydrocarbons are not straight
chains. E.g.
Reforming
• Straight chain hydrocarbons undergo
rearrangement to form branched chain
hydrocarbons, without necessarily
changing the number of C.
CH3 CH3
CH3 CH2CH2CH2CH2CH2CH2CH3
CH3CCH2CHCH3
CH3
Refoming
• Passing naphtha over a catalyst ( platinum
or molybdenum (VI) oxide at about 500°C.
• Straight chain alkanes may also be
converted to aromatic hydrocarbons;
Aromatic hydrocarbons
• The Naptha fraction is an important
feedstock for the production of aromatic
hydrocarbons.
• Cyclohexane (present in naphtha) can
undergo dehydrogenation to produce
benzene.
Benzene
• Benzene is the simplest aromatic
hydrocarbon.
• This process is known as reforming
C6H12
C6H6 + 3H2
Benzene
• Ring structure. Each corner of the
hexagon represents a carbon atom with a
hydrogen atom attached.
• Toluene (methylbenzene) has one of
these hydrogen atoms replaced by a
CH
methyl group.
3
C6H5CH3
Methyl benzene
Making petrol
• Products made from reforming naptha are
combined with products of cracking longer
chain hydrocarbons to produce more
efficient fuel.
• Butane is dissolved in petrol in the winter,
this is to make it more volatile.
Petrol engines
• In a petrol engine,
air and petrol
vapour are drawn
into the cylinder,
compressed and
ignited by a high
voltage electric
spark.
Controlled
explosion of petrol
air mix forces piston down
Knocking
• Air-fuel mixture auto ignites or pre-ignites when
the engine is hot.
• It can be heard as ‘knocking’ or ‘pinking’ and is
potentially damaging to the engine and
inefficient.
• Leaded petrol contains anti-knocking agent, lead
tetra-ethyl Pb(C2H5)4 – toxic gases.
• Unleaded petrol requires a much higher
proportion of branched and aromatic
hydrocarbons to increase the efficiency of
burning.
Polymers
From previous work you should
know the terms:
1.
2.
3.
4.
5.
6.
7.
Alkenes
Addition reaction
Polymerisation
Monomer
Polymer
Thermoplastic
Thermosetting plastic
Early plastics & fibres
• Ethene & propene are very important
starting materials in the petrochemical
industry. Especially production of
polymers.
– Where do these starting materials come
come from?
– How can we produce more of them?
Approximately 80% of ethene comes from cracking ethane
Addition polymerisation
Condensation polymers
• Made from a monomer which contains 2
functional groups per molecule.
• The monomers join in chains by eliminating a
molecule of water.
• Examples we will learn about include
polyesters, polyamides and aramids.
Polyester…
• Is synthesised by
condensation of 2
monomer units.
• The first has 2 hydroxyl
groups, know as a
dialchohol.
• The second has 2
carboxylic acid groups
called a dicarboxylic acid.
Hexanedioic acid
Polyester
2H2O
New ester links
Uses of polyesters
• If the monomers link only linearly, the polyester
produced can be used for textile fibres, ropes,
cords and sailcloth.
• However if the monomers have additional
hydroxyl (-OH) and carboxyl (-COOH)
crosslinking occurs.
• This creates a 3D thermoplastic used in the
manufacture of bottles for soft drinks & fleece
jackets.
Hydrogen bonds in kevlar
Proteins
Proteins
• Proteins are natural products formed by many
plants and all animals.
• All proteins contain carbon, hydrogen oxygen &
nitrogen.
• Nitrogen comes from either the air, fertilisers or
from the action of bacteria in nodules on some
plant routes.
• Proteins are condensation polymers made by
linking together many amino acids.
Proteins
• Different proteins are used for different
processes in the body.
• The body cannot produce all the amino
acids required for body proteins.
• Therefore essential amino acids must
come from dietary proteins.
Digestion
• During digestion protein molecules are
broken down or hydrolised to produce a
mixture of amino acids. This process is
catalysed by enzymes.
• The amino acid can then pass through the
gut wall into the bloodstream.
Amino acids..
• As the name
suggests; contain
both an acid group
• And an amino group
(NH2)
H
O
N
H
R
C
OH
Condensation polymerisation
Peptide link
Hydrolysis of proteins
• The structural formula
of the amino acid
released from the
protein can be
identified by looking
at the corresponding
section of the protein
chain.
• In nature, hydrolysis of proteins is catalysed
by enzymes.
• In the lab, hydrolysis of proteins is catalysed
by dilute acid or alkali.
Helix (coils)
Sheets
Protein classification:
Fibrous
• Fibrous proteins are long and thin. They
are the major structural materials for
animal tissue; keratin, (hair, nails &
feathers) collagen (muscles), fibroin (in
silk).
• Generally insoluble in water & resistant to
acids & alkalis.
Protein classification:
Globular
• Globular proteins have spiral chains folded
into compact units.
• Globular proteins are involved in
maintenance and regulation processes
due to their solubility in water.
• Examples include all enzymes, many
hormones, haemoglobin and antibodies.
Enzymes
• Enzyme function is related to the molecular
shapes of proteins.
• Denaturing of a protein involves physical
alteration of the molecules as a result of
temperature change or pH change.
• The ease with which a protein is denatured is
related to the susceptibility of enzymes to
changes in temperature and pH.
• Enzymes are most efficient within a narrow
range of temperature and pH.
Enzymes
• Enzymes are all globular proteins.
Enzymes catalyse specific reactions
because of their specific active site.
• Only molecules with the correct shape will
fit onto the surface and it is only these
molecules which will react using that
particular enzyme as a catalyst.
• When the enzyme is heated or the pH
altered the shape of the molecule may
change because the pattern of
intermolecular bonding between chains is
altered.
• Chemists and biologists call this change
to the enzyme denaturing since it can no
longer carry out the specific reaction it
was originally used for.
Fats & oils
Natural fats & oils
• Depending on their origin can be classified
as:
Vegetable
Animal
Marine
Fatty acids
• Can be saturated or unsaturated straight
chain carboxylic acids containing even
numbers of carbon atoms. (C4-C24)
• Fats and oils consist largely of mixtures of
triglycerides in which the 3 fatty acids
attached to the glycerol may be different.
Hydrolysis of fats & oils
• Fats & oils are esters.
• When fats and oils are heated with
superheated steam they break up or
hydrolyse to form
• Gylcerol
• Fatty acids
Ratio of 3 fatty acids to 1 glycerol
Soaps
• Soaps are produced by the alkaline
hydrolysis of fats and oils.
• This hydrolysis is usually carried out using
sodium or potassium hydroxide.
• Glycerol is liberated and used as a raw
material in other processes.
• The fatty acids are produced in the form of
their potassium or sodium salts. These
salts are soap!
Soaps
Hydrocarbon tail. Soluble
in oils and grease.
Ionic head is
soluble in
water.
The
hydrocarbon ‘tail’
bonds to greasy
material in
fabrics or on the
skin.
The ionic ‘head’
is attracted to the
polar covalent
water molecules.
An
emulsion of the grease in water is
then formed which can be easily
washed away.
Test for unsaturation?
• How do fats & oils effect bromine water?
What's the difference?
• Fats are solid at room temperature, oils
are liquids.
• The difference in melting point comes from
the higher degree of unsaturation in oils.
Van der Waals
Fat
Low level of unsaturation
Van der Waals
Oil
Higher level of unsaturation
Can we change an oil into a fat?
• The conversion of an oil to a fat involves
removal of unsaturation by addition of
hydrogen.
Nickel
Hydrogenation
Healthy diet?
• Fats and oils are important in a balanced
diet and supply the body with energy in a
more concentrated form than
carbohydrates.
• There is evidence of a link between a high
intake of saturated fat in the diet and heart
disease.
• There is further evidence to suggest that
unsaturated fats may help to lower
cholesterol.