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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

 

 



Merilyn Manley-Harris, seen here with Ben Deadman and  Chris Adams, provided content on fats and oils. Information on carbon allotropes is courtesy of Michele Prinsep, and Chris Hendy wrote the item on analytical chemistry.
  merilyn manley-harris & students

Triglycerides

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:

simple tryglceride structure

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

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.
simple fatty acid structure

Occasionally the carbon chain may include two carbons joined by a double bond:

cis double bond

And very, very occasionally we find two carbons joined by a different sort of double bond:

trans 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

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.)

single molecule of stearic acid
A single stearic acid molecule

a single oleic 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 stearic acid molecules

two oleic 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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Fats and Oils

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.)

fatty acids in cows milk
The proportions of different fatty acids in cow's milk

fatty acids in olive oil
The proportions of different fatty acids in olive oil
fatty acids in codliver 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?

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

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


The element carbon has several allotropes: different chemical modifications of the pure element. Other elements to have allotropes are oxygen - where the allotropes are O2 and O3, 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 C60.  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
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
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'

In the early 20th 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 - 2000oC. 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 - 10,000oC 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 21st 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.

ICP-MS_in_standard_operating_mode
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. 

ICP-MS_machine_with_laser
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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