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Material World
Carbon Chemistry
Triglycerides | Fatty Acids | Molecular Models | Fats & Oils | Butter vs Margarine | Omega-3 & Omega-6 Fatty Acids
Allotropes of Carbon
Analytical Chemistry
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Triglycerides
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Cow's milk contains roughly
4% fat. The fat content of cheese and butter prepared from the milk is
very much higher because a lot of the water from the milk has been
eliminated.
A fat molecule is called a triglyceride.
A tryglyceride consists of three fatty-acid molecules linked to a glycerol molecule like this:
Cow's milk will contain a
variety of triglycerides which differ in the type of fatty acids they
contain. There may also be fatty acids that are free and not part of a
triglyceride molecule.
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Fatty acids
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A fatty acid is made up of a carboxylic acid group with a number of -CH2 units attached to it, and ending in a -CH3 group. The CH2
groups are joined together by single bonds. One way of showing this is
in the following diagram, in which ETC stands for the rest of the -CH2 groups in the fatty acid.
Occasionally the carbon chain may include two carbons joined by a double bond:
And very, very occasionally we find two carbons joined by a different sort of double bond:
We give these two types of double bonds different names. The first -
and most common - type is called cis and the second, more unusual
variety is called trans.
Sometimes double bonds are referred to as unsaturations.
We catalogue fatty acids by the number of carbons in the chain and by the number of double bonds.
So 18:0 (stearic acid) is a fatty acid with 18 carbons and no double
bonds, whereas 18:1 (oleic acid) is a fatty acid with 18 carbons and
one double bond. We describe a fatty acid that contains double bonds as
unsaturated, and if it contains several double bonds it is said to be polyunsaturated. Fatty acids with no double bonds are described as saturated.
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Molecular models of fatty acids
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Here are images of
space-filling models of stearic acid and oleic acid. Carbon is black,
hydrogen is white and oxygen is red. (The oxygen is part of the acid
group.)
A single stearic acid molecule
A single oleic acid molecule
You can see the double bond clearly in oleic acid - can you work out whether it is cis or trans?
From the image you can see that the presence of one double bond causes
the oleic acid molecule to be banana-shaped rather than straight. As a
result, oleic acid molecules can't pack together as closely as stearic
molecules - look at the second set of images (below):
Two stearic acid molecules
Two molecules of oleic acid
Molecules that pack closely
together are able to hang onto each other tightly and require more
energy to separate them. In the laboratory this effect is observed as a
raise in melting point. So oleic acid melts at 10.5oC whereas stearic acid melts at 69.6oC.
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If a substance such as
butter contains a high proportion of saturated fatty acids it will in
turn have a reasonably high melting point and be a solid at room
temperature. Substances that contain a high proportion of unsaturated
fatty acids will have low melting points and will be liquids at room
temperature. These substances are called oils e.g. olive oil, canola
oil.
The following pie charts show the proportions of different fatty acids
in cow's milk, olive oil, and cod liver oil. (Remember that the first
number in each ratio is the number of carbon atoms in the fatty acid,
while the second number is the number of double bonds in the molecule.)
The proportions of different fatty acids in cow's milk
The proportions of different fatty acids in olive oil
The proportions of different fatty acids in olive oil
It is easy to see why olive oil is liquid at room temperature while butter, which is made from cow's milk, is a solid.
An Atlantic cod is a large fish found in the cold waters of the North
Atlantic, up around Iceland. Can you explain why the fatty acids of the
cod are so highly unsaturated?
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Butter vs Margarine - the good and the bad?
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For health reasons people are sometimes told to eat margarine instead of butter. Why?
Saturated fats have been shown to increase levels of blood cholesterol
and this is considered to place a person at risk of heart attack.
Margarine is produced from plant oils, such as palm oil, that contain a
high proportion of unsaturated fats, and these do not affect
cholesterol levels.
But hang on a minute - palm oil is liquid at room temperature while margarine is a solid! How is this achieved?
It's done by a process of hydrogenation, in which hydrogen atoms are
added to the double bonds to turn some of them into single bonds and
thus increase the melting point. The same process is used to turn
peanut oil into peanut butter. Next time you eat peanut butter check
out the label and you will see the words "partially hydrogenated..."
But this isn't the whole story. Sometimes when the hydrogenation
reaction is carried out some of the double bonds, instead of turning
into single bonds, just change form from cis to trans. This gives rise to the so-called trans-fatty
acids, and these are not nice at all. Trans-fatty acids have been shown
to increase blood cholesterol even more than saturated fatty acids.
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Omega-3 & Omega-6 fatty acids - a twist in the tail
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Another
classification of fatty acids gives the number of carbons, the number
of double bonds, and the position of the last double bond in relation
to the end -CH3 group. (Omega is the last letter of the
Greek alphabet.) An omega-3 fatty acid has a double bond on the third
carbon from the end and an omega-6 has a double bond on the sixth
carbon from the end. These two fatty acids are essential fatty acids
because they are important for bodily functions, but we can't make them
inside our bodies and so must eat them in our diet.
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Allotropes of carbon
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The element carbon has several allotropes:
different chemical modifications of the pure element. Other elements
to have allotropes are oxygen - where the allotropes are O 2 and O 3,
or ozone - and tin. We'll examine the significance of ozone
elsewhere on the site, but here we will look at the different carbon
allotropes, as a way of explaining the concept.
Buckminsterfullerene, diamond, and graphite are all allotroptes of
carbon: they are all made up of carbon atoms but these atoms are bonded
together differently. Because of this the allotropes have different
properties. For example, buckmisterfullerene forms discrete structures
while diamond and graphite form extended lattices.
Buckminsterfullerene has the formula C 60.
Discovered in 1985, it's nicknamed 'buckyball' because of its
soccer-ball shape, and named after Richard Buckminster Fuller, an
architect who popularised the geodesic come. Buckminsterfullerene is
the only soluble form of carbon, forming deep purple solutions.
Buckminsterfullerene
Graphite consists
of parallel sheets of hexagonal rings. Because the sheets are only
weakly bonded together, they can slide over each other - graphite is a
good lubricant. Because it's soft and flakey, graphite readily leaves
traces on other substances - we use it as the 'lead' in pencils. It's
also a good conductor.
Sheets of carbon atoms in graphite
Diamond is an
extended lattice of tetrahedrally bonded carbon atoms - each atom is
bonded to 4 others. Since it's the hardest substance known, diamond is
used in cutting tools. Pure diamond is clear and colourless: coloured
diamonds are that way because they include traces of other elements
within the lattice.
Image sources:
Buckminsterfullerene: azbiotech blogspot
Graphite: The Scottish Science & Technology Roadshow, scifun
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Analytical Chemistry and 'Bush Sickness'
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In the early 20 th century analysis of metallic elements relied on precipitation
of an insoluble salt (gravimetric analysis), or on the formation of a
coloured compound which could be compared - by eye - with a series of
standards. These techniques could achieve detection limits as low as
1µg (10 -6g), but at great cost in time and labour, and
with poor precision. A major improvement came about with the invention
of flame atomic absorption (a joint development by James Eric Allan of
Ruakura Agricultural Research Station and Alan Walsh of CSIRO
Melbourne) in the 1950s. This technique relies on the absorption of photons
(emitted by atoms of the element in question as they are excited in a
hollow cathode lamp) by atoms of the same element produced when a
solution is sprayed into a flame at 1000 - 2000 oC. Atomic
absorption revolutionised chemical analyses of most metallic elements
with vastly improved precision, and detection limits for most elements
of about 1µg - and its developments were largely driven by the
needs of agricultural research.
A good example of the precision of atomic absorption can be found in
the now infamous Arthur Alan Thomas case. The Crown argued that wire
binding the bodies of the victims had come from the Thomas farm, based
on analyses carried out by the rather imprecise technique of arc
spectroscopy. But the defence had the same wires analysed by atomic
absorption spectroscopy and could show that they were not identical.
Over the years incremental improvements were made to atomic absorption
techniques, but by the 1980s a completely new class of instruments
became available with the development of what are known as Inductively
Coupled Plasma Optical Emission Spectrometers (ICP-OES). These used a
plasma to heat droplets of solution to 6000 - 10,000 oC to produce
excited atoms which emitted photons at characteristic wavelengths, and
measured their intensity. ICP-OES provided much improved sensitivity -
up to 1000x - over atomic absorption (AA) in identifying metallic
elements.
At the beginning of the 21 st
century ICP-OES was itself overtaken by Inductively Coupled Plasma Mass
Spectrometry (ICP-MS). In this technique a mass spectrometer is used to
count the number of ions formed by each element in the plasma, rather
than measuring the light they give off as they are excited. This is a
much more efficient process and results in further thousand-fold
increase in sensitivity. ICP-MS instruments like those used at the
University of Waikato can detect as little as 10 -17g of some elements -
almost a trillion times more sensitive than techniques in use when the
cause of 'Bush sickness'
was discovered. What's more, these instruments can be fully automated
to analyse several hundred samples a day, producing data for up to 50
elements from a few millilitres of solution.
Waikato University's ICP-MS equipment showing its
standard mode of operation: 2ml of solution is sucked up from sample
tubes in the rack to the right and are injected into the plasma (the
pale blue glow in the centre of the image.
The ICP-MS coupled with the laser, which is an
alternative sampling system that vaporises solid samples in to the
plasma. Both images courtesy of Chris Hendy.
The ICP-MS is the only such devide
available in New Zealand. It's used to analyse trace elements in a
diverse range of materials - including the ear bones of fish (thus
telling scientists where the fish have lived and when).
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