Fats as Natural Esters

• Fats and oils are naturally occurring esters used as energy storage molecules by both plants and animals.

• Fats are solid at RT, oils are liquids – this is the only significant difference between them.

• They are all derived from glycerol, propan-1,2,3-triol also known as glycerine.

• It has 3 OH groups, each of which can be esterified by carboxylic acids.

• In fats and oils these are long chain molecules also described as fatty acids because they are insoluble in water. (Why?)

Week 5

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• Describe a triglyceride as a triester of glycerol (propane-1,2,3-triol) and fatty acids.

• Compare the structures of saturated fats, unsaturated fats and fatty acids.

Week 5

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Structure of propan-1,2, 3-triol, commonly known as glycerol

Drawing Fatty Acids

• By definition these are long chain molecules so convention suggests that the easiest way to represent them is by drawing their skeletal formulae.

• Draw the skeletal formula for the above- hexadecanoic acid – a saturated fatty acid.

Fats and Oils as Triesters

• Each of the OH groups on the glycerol molecule can be esterified by fatty acids.

• This leads to mono, di and triglycerides respectively.

• Each OH group can react with a different fatty acid. Most natural triglycerides are derived from 2 or 3 different fatty acids.

Exercise

• What is the other product when octadecanoic acid and propane-1,2,3-triol form a trigylceride?

• What type of reaction has taken place?• How many moles of the second product are

formed per mole of propane-1,2,3-triol?• Write a balanced equation for the reaction

using condensed structural formulae as required.

• Lay it out neatly!

Week 5

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Formation of a triglyceride

Saturated and Unsaturated Fats

• Fatty acids can be either saturated or unsaturated (containing 1 or more double bonds).

• This leads to the triglycerides being identified as saturated or unsaturated fats.

• Unsaturated fats tend to have low melting points and many are oils.

• Saturated fats are more likely to be solid fats.

Fatty Acids Shorthand

• Systematic naming for fatty acids is straightforward but cumbersome because the chains are so long.

• The shorthand system which involves numbers only is often used.

• Use 2 numbers only, separated by a colon.• The first number indicates the number of carbon

atoms.• The second number indicates the number of

double bonds.• The number in brackets indicates the position of

the double bond.

Saturated and Unsaturated Fats

Using the shorthand naming system name this acid.

Name the fatty acid shown below both systematically AND using shorthand.

Octadec-9-enoc acid- oleic acid

Linoleic acid

Octa-9,12-dienoic acid

18,2(9, 1 2)

Week 5

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• Compare the structures of cis and trans isomers of unsaturated fatty acids.

• Compare the link between trans fatty acids, the possible increase in ‘bad’ cholesterol and the resultant increased risk of coronary heart disease and strokes.

• Describe and explain the increased use of esters of fatty acids as biodiesel fuels.

Cis-Trans Isomers

• Special case of EZ isomerism where there is restricted rotation about the double bond and each C of the double bond has one atom or group that is the same.

• While saturated fats are associated with increased cholesterol and heart disease there are issues with unsaturated fats also.

• Unsaturated fats are more easily digested but trans fats are now also believed to increase the risk of heart disease.

‘Bad Fats’

Digestion breaks fats down to produce fatty acids and glycerol which pass into the blood stream.

If the fatty acids are saturated they are resistant to further use by the body because this occurs by oxidation.

Saturated fatty acids have no reactive group susceptible to attack by oxidative reactants.

They tend to stay in the veins and arteries and since they pack together easily they form plaques which clog up the vessels.

Cis vs trans fatty acids

• Cis fatty acids are more readily oxidised and used in the cells.

• Those which remain in the bloodstream are not likely to deposit as plaques in the vessels since they do not pack well.

• Trans fatty acids may be removed by oxidative reactions but if they are not they too will pack and so can form plaques in the vessels apart from any other problems they may cause.

Trans Fats and Cholesterol

• Hardening of fats by hydrogenation also changes some cis double bonds into trans isomers.

• Trans fatty acids increase LDL levels which carry 65% of the cholesterol in the blood and deposit cholesterol onto artery walls.

• LDLs lower HDL levels.• Increased risk of heart disease.

Week 5

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Cis and trans configurations around a double bond

Week 5

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Structures of cis and trans-octadec-9-enoic acid.

Biodiesel

• Vegetable oils have long alkane chains but the chains cannot be burnt as fuels because the glycerol residues formed during the combustion clog up the engine.

• So the oil is hydrolysed then acidified and the carboxylic acids isolated.

• They are then esterified with methanol and the methyl esters are used as substitutes for diesel.

• Biodiesel doesn’t produce SO2 and emits fewer particulates during combustion.

Week 5

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Transesterification for the production of biodiesel