Download Fats as Natural Esters

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
•
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.
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Week 5
Structure of propan-1,2, 3-triol, commonly known as glycerol
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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 abovehexadecanoic 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
Formation of a triglyceride
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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
Octadec-9-enoc acid- oleic acid
Using the shorthand naming system name this
acid.
Name the fatty acid shown below both
systematically AND using shorthand.
Linoleic acid
Octa-9,12-dienoic acid
18,2(9, 1 2)
Week 5
•
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.
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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
Cis and trans configurations around a double bond
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Week 5
Structures of cis and trans-octadec-9-enoic acid.
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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
Transesterification for the production of biodiesel
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This document may have been altered from the original