Part 6.
The Chemistry of Carboxylic Acids and their Derivatives
Doc
Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US
K12 grade 11 grade 12 organic chemistry
6.9
Natural esters - glyceride esters - fats and oils, margarine and biodiesel
Sub-index for this page
6.9.1
Structure
of long chain saturated/unsaturated fatty acids
6.9.2
Structure and function of glyceride esters from animal fats/plant oils
6.9.3
Saponification to obtain fatty acids and soap making
6.9.4
Margarine manufacture from vegetable oils
6.9.5
Biodiesel and the use of the transesterification reaction
See also Part
6.8
Esters
- preparation, reactions including hydrolysis and
transesterification, uses
INDEX of all carboxylic acids
and derivatives notes
All Advanced A Level Organic
Chemistry Notes
[SEARCH
BOX]
BASIC
GCSE NOTES
Biofuels & alternative fuels,
hydrogen, biogas, biodiesel
Carboxylic acids - molecular
structure, chemistry and
uses
Esters, chemistry and uses
including perfumes, solvents
6.9.1 The structure of long chain saturated and unsaturated fatty acids
The structure of four fatty acids are shown below in
Fig 4 below.
All
are obtained from animal fats of vegetable plant oils and all are
monocarboxylic acids.
Fatty acids are
long chain linear monocarboxylic acids which can be saturated (no C=C
bonds) or unsaturated (with at least one C=C bond).
(a)
Palmitic acid C15H31COOH
CH3(CH2)14COOH and Stearic acid
C17H35COOH CH3(CH2)16COOH
The systematic name for palmitic acid is
hexadecanoic acid (fully saturated fatty acid, shown below), and a white waxy
solid, mpt 63oC. It is obtained from the triglyceride esters
in palm oil.
(Saturated and unsaturated fatty acids are
typically white to very pale yellow solids or viscous oils)
The systematic IUPAC name for stearic acid is
octadecanoic acid, another fully saturated carboxylic acid - no C=C
bonds, a white waxy melting point 69oC (shown above).
Stearic acid is mainly used in the manufacture of
detergents, soaps, and cosmetics such as shampoos and shaving cream
products.
Soaps are not made directly from stearic acid, but
indirectly by saponification (hydrolysis) of triglycerides of stearic
acid esters in fats or oils.
The displayed formulae of stearic acid molecule
and the salt sodium stearate.

(b) Oleic
acid C17H33COOH
CH3(CH2)7CH=CH(CH2)7COOH
Systematic name is octadec-9-enoic acid and a monounsaturated
omega-9 fatty acid - one C=C bond.
Oleic acid is obtained from the triglyceride
esters in olive oil from the many olive groves in Europe and elsewhere.
The abbreviated structural formula of oleic acid
cannot show the possible E/Z stereoisomers.
Melting point 13oC. The richest source
of oleic acid is olive oil.
Note the significantly lower melting than
stearic acid, C17H33COOH, with almost the same
molecular mass, partly due to the cis (Z) orientation about the C=C
double bond.
The 'omega-number' refers to the first
carbon atom of the first double bond from the (left-hand) hydrocarbon
end of the molecule - check out omega-6 and omega-3 fatty acids below -
note stearic acid is fully saturated and therefore is not assigned an
omega number.
Stereoisomerism in unsaturated long chain fatty
acids.
The above diagram shows the Z (cis) and E (trans) forms
of oleic acid, E-octadec-9-enoic acid and Z-octadeca-9-enoic acid.
Note
the kink in the cis form. The trans stereoisomer could form part of a
'trans' fat molecule.
The Z/cis form molecules cannot pack as closely
together as the trans isomer.
The Z/cis form of this monounsaturated fatty acid predominates in nature.
(c)
Linoleic acid C17H31COOH
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
Linoleic acid is a polyunsaturated omega-6
fatty acid with two C=C bonds.
Melting point -5oC. The richest source
of linoleic acid is soybean oil.
The IUPAC name for linolenic acid is
9Z,12Z-octadeca-9,12-dienoic acid.
The Z/cis form molecules cannot pack as closely
together as the trans isomer.
The Z/cis form predominates in nature.
(d)
Linolenic acid C17H29COOH
CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH
Linolenic acid is a polyunsaturated omega-3
fatty acid with three C=C bonds.
The linolenic acid group of molecules,
octadecatrienoic acids, are obtained from triglyceride esters of
linolenic acids in vegetable oil, but the richest source of omega-3
acids is fish oils.
Melting point -16oC.
The IUPAC name for linolenic acid is 9Z,12Z,
15Z-octadeca-9,12,15-trienoic acid.
Notes on
molecules (a) to (d):
(i) The molecular formula goes down by H2
for every alkene group, the C=C double bond.
(ii) They are all white waxy solids, often with an
oily odour, and all insoluble in water.
(iii) They have high boiling points (> 300oC)
and tend to decompose on boiling.
(iv) For (b) to (c), the stereoisomeric double
bonds are all of the Z- configuration (the cis isomers in old notation).
(v) They contain an even number of carbon atoms
e.g. these four are based on a linear C18 carbon chain.
(vi) For the same carbon number e.g. C18,
the melting point of the fatty acid decreases with increase in
unsaturation and this is an important consideration in margarine
production.
Mpt sequence for (a) to (d):
CH3(CH2)16COOH
> CH3(CH2)7CH=CH(CH2)7COOH
> CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
> CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH
Structure of propane-1,2,3-triol ('glycerol',
'glycerine', 'glycerine')
Triglyceride esters in animal fats and vegetable oils are synthesised
from the long chain fatty acids, like those described above, esterified with
the triol alcohol molecule
glycerol, whose structure is shown below.
Propane-1,2,3-triol
(glycerol),
,
, two primary and one secondary alcohol groups - one hydroxy group
on each of the three carbon atoms is available to link to a fatty carboxylic
acid via an ester linkage.
With complete esterification you form a triester,
known as a triglyceride from the trivial name of the alcohol (the
'triol' glycerol).
6.9.2 The structure of glyceride esters from animal fats and
plant oils
Fats have many important biochemical roles in
the bodies of mammals including protecting organs, layers of insulation and
an energy store and therefore an important component of a healthy balanced
diet.
Vegetable oils are also considered another
important component in a balanced diet.
Therefore, fats and oils (chemically they are
triglyceride esters) are very important in the food industry.
Animal fats and vegetable oils are examples of
triglyceride esters made from the 'triol' alcohol propane-1,2,3-triol (glycerol)
and linear long hydrocarbon chain fatty carboxylic acids (both saturated
and unsaturated and illustrated below using skeletal formulae).
Comparing a saturated triglyceride ester fat
molecule with an unsaturated triglyceride vegetable oil ester molecule.
The C=C double bonds in the unsaturated
molecule produce significant 'kinks' in the carbon chain compared to the
saturated molecule with no C=C bonds, so they cannot pack together as
closely as the saturated fat molecules.
Note the original glycerol
(propane-1,2,3-triol) molecule to connect with an ester structure
Look for connections with the skeletal
formulae of the long chain saturated and unsaturated fatty acids described above
in 6.9.1
Fats and oils are similar molecules in that they
are all derived from long chain fatty acids and glycerol, but there are
several important differences e.g.
(i) Generally speaking, vegetable oils
consist of much more unsaturated molecules (more C=C double bonds in
carbon chain) than animal fats glycerides.
(ii) The more unsaturated vegetable oils tend to have lower melting points
than more saturated animal fats for a similar molecular mass, hence animal fat glycerides
are solid and vegetable glycerides are liquid oils at room
temperature.
The more saturated triglyceride ester molecules
can pack together more closely increasing, and maximising the
intermolecular bonding forces compared to unsaturated triglyceride
esters (diagram above).
The intermolecular forces between the
triglyceride esters in oils and fats are a combination of (i)
instantaneous dipole - induced dipole forces and (ii) permanent
dipole - permanent dipole forces.
The unsaturated triglyceride esters cannot pack as
closely due to the double bonds which create a 'kink' in the molecules
making them less flexible and able to line up. So they are a bit more
spaced out, lowering the net intermolecular bonding between the
molecules (comparison diagram above).
The above diagram gives a simplified view of
the spacing of typical solid saturated animal fat and the liquid
vegetable oil molecules.
The greater the proportion of unsaturation in
the triglyceride ester, the more likely it is to be an oil at room
temperature - you can see most animal fats are soft solids and
vegetable oils viscous liquids at room temperature.
Their formation and structure of triglyceride esters are illustrated below
using abbreviated structural formula.
In each case I've assumed all three saturated or
unsaturated acids are the same, but in reality there are usually 2 or 3
different fatty acids incorporated in the glyceride.
Using abbreviated structural formulae, above is the
formation of a saturated animal fat triglyceride and below the formation of
an unsaturated vegetable oil triglyceride molecule.
Whether they are saturated or
unsaturated, these fatty acids all have an even number of carbon atoms e.g.
based on C18.
Some examples
of the fatty composition of triglyceride animal fat and plant oil esters
|
Saturated |
Monounsaturated |
Polyunsaturated |
|
g/100g |
g/100g |
g/100g |
Animal fats
|
Lard |
40.8 |
43.8 |
9.6 |
Butter |
54.0 |
19.8 |
2.6 |
Vegetable oils |
Coconut oil |
85.2 |
6.6 |
1.7 |
Palm oil |
45.3 |
41.6 |
8.3 |
Wheat germ oil |
18.8 |
15.9 |
60.7 |
Soybean oil |
14.5 |
23.2 |
56.5 |
Olive oil |
14.0 |
69.7 |
11.2 |
Corn oil |
12.7 |
24.7 |
57.8 |
Sunflower oil |
11.9 |
20.2 |
63.0 |
Animal fats tend to be higher in saturated fats than
vegetable oils.
Vegetable oils tend to be higher in unsaturated fats than
animal fats.
However, there is quite a variation e.g. coconut oil has
the highest saturated fat content of any fat listed.
Source
Wikipedia
Below are more skeletal formulae
which are more realistic in terms of the fatty acid components in
glycerides.
Fig
2
Fig 2 shows the three types of fatty acid component
you can find in the molecular structure of triglyceride esters.
You can have saturated fatty acid, a monounsaturated
fatty acid with one C=C bond, and polyunsaturated fatty acids with at least
two C=C double bonds in the linear hydrocarbon chain.
Note that in the unsaturated fatty acids, you have
E/Z stereoisomerism and all the >C=C< linkages adopt the Z isomer (cis form)
orientation.
Fig 3
Fig 3 emphasises the difference between a
polyunsaturated vegetable oil triglyceride and a fully saturated one from an
animal fat.
Trans fats
and cholesterol levels in our bloodstream
(refer to previous diagram)
Lipoproteins are large molecules that carry lipids
like cholesterol and fats in the blood stream.
High-density lipoproteins (HDLs) are responsible
for transporting cholesterol out of the blood and eventually out of the
body. HDLs are usually referred to as 'good' lipoproteins.
Low-density lipoproteins (LDLs) are carry a high
percentage of the cholesterol in the blood. Unfortunately LDLs can
deposit lipids like cholesterol onto the walls of arteries. If the
deposits build up, they restrict blood flow causing various heart
conditions including high blood pressure leading to heart attacks. LDLs
are usually referred to as 'bad' lipoproteins.
It is thought that trans fats behave like
saturated fats in the body and raise LDL levels increasing the risk of
cardiovascular problems. Trans fats also lower HDL levels exacerbating
the problem.
6.9.3 Saponification to obtain fatty acids and soap making
Hydrolysing an ester with strong alkali e.g.
aqueous or ethanolic sodium/potassium hydroxide is called saponification,
i.e. its a specific name for a particular type of hydrolysis reaction.
If you heat any oil or fat concentrated
sodium/potassium solution, the triglyceride ester is hydrolysed to
give a mixture of three sodium/potassium salts of fatty acids and a molecule
of glycerol.
The saponification (hydrolysis) reaction is illustrated below
in Fig 5 with skeletal
formulae.
Fig 5
A general abbreviated structural formula equation is
given below.
RCOOCH2CH(OOCR')CH2OOCR"
+ 3KOH ==> RCOO-K+ + R'COO-K+
+ R"COO-K+ + HOCH2CH(OH)CH2OH
R, R' and R" represent the hydrocarbon chain and can
be saturated, monounsaturated or polyunsaturated.
R, R' and R" can be all the same, but frequently they
are two or three different structures.
Glycerol (propane-1,2,3-triol, glycerine)
is a colourless odourless viscous liquid with a sweet taste. It is harmless
and used in processed food and formulations of cosmetics and toothpaste and
also used as an antifreeze in cooling systems of road vehicles.
It is very soluble in water because of hydrogen
bonding between water and glycerol molecules allowing solvation to take
place. Several hydrogen bonds can be formed between each molecule of
glycerol and water.
Example of a glycerol - water hydrogen bond:
HOCH2CH(OH)CH2OHδ+llllδ-:OH2
Fig 6
Fig 6 shows the addition of mineral acid (e.g.
dilute hydrochloric or sulfuric acid) to free the fatty acids from their
sodium/potassium salts.
Note that the saponification hydrolysis is a means of making
soap - one type of which consists of the sodium or potassium salts of a fatty
acid.

This is a simplified equation assuming all three acids
are saturated and the same (in this case palmitic acid).
Palmitic acid is used in soaps and is obtained, not
surprisingly from its name, in vegetable palm oil.
How do soaps work?
Diagrams E1 and E2 show the basic
structure of a soap 'molecule' or other 'surface-active agents', known
as surfactants. Soaps and detergents enable surfaces to be 'wetted' by
lowering the surface tension, essential to getting a cleaning action to
remove grease or oil stains from clothes or plates etc. This effect
keeps the particles of dirt, grease, oil etc. in a dispersed state so
they are washed away - effectively an emulsion is formed.
Diagram D also illustrates the mechanism
by which soaps interact with oily/greasy particles.
You can see one end of the soap
molecule is attracted to water (hydrophilic end) and the other end
attracted to oil or fat (hydrophobic end). Therefore they can interact
with the different components and keep the different types of molecules
dispersed in each other.

Soap molecules have a negative ionic hydrophilic 'head' ('water liking'/'oil
hating' end of molecule) and a
hydrophobic 'tail' ('water hating'/'oil liking' end of molecule').
eg the stearate ion from the soap sodium
stearate shown above in diagram S3.
Being 'ionic' the carboxylate hydrophilic
head readily solvates with water, but the long chain hydrocarbon
hydrophobic 'tail' of the soap molecule cannot interact with water,
but does interact with particles of oil/fat/grease etc.
In effect, the head dissolves in water and
the tail dissolves in the fat/oil/grease etc.
When you shake soap with an oily/greasy
material (washing clothes or scrubbing a surface), the oil/grease breaks
up into tiny droplets or globules which can be washed away in the waste
cleaning water. Why? ...
The hydrocarbon hydrophobic tail of the
soap dissolves in the oil or grease globule and the negative head is on
the surface of the globules/droplets.
The hydrophobic tail can only
interact with oil/grease i.e. is attracted to oil and grease.
The ionic negatively charged hydrophilic head can only
interact with water i.e. is attracted to water and weakly bonds with
water molecules.
Two hydrophilic heads cannot interact
with each other and tend to repel each other especially if the
hydrophilic head carries a negative charge (ionic), therefore you get
repulsion between the oil/fat globules - though this argument is only
part of the 'mechanism story' - read on!
In effect, the globules of oil/fat get a
surface coating of the soap inhibiting them coming together.
So, the oil/fat/grease particles cannot
re-clump together to form a separate layer on the clothes or surface
being cleaned, and in the context of washing, the dirt/oil/grease
particles remain
dispersed in the soapy washing water and hence washed away and off the
surface of a fabric or a greasy plate!
6.9.4 Margarine manufacture from vegetable oils
Vegetable oils are too liquid for consumers who like
their butter/margarine in a soft solid spreadable form.
We also generally like a lower 'fat' spread i.e. more
of the unsaturated fats like vegetable oils - which unfortunately are too
liquid for our use at the dinner table (but in Mediterranean countries, olive oil is readily spread on bread
- its a way of life!).
There are two methods for producing the 'acceptable'
margarine we buy in plastic tubs from the shop.
(1)
Partial hydrogenation of vegetable oils
In partial hydrogenation, a controlled quantity of
hydrogen is added to the vegetable oil in the presence of finely divide
nickel catalyst (large surface area). This is illustrated in Fig 8 above
using skeletal formulae.
The hydrogen adds to some of the C=C double bonds,
increasing saturation and decreasing unsaturation.
In the above case two of the original four C=C
double bonds have been hydrogenated in the triglyceride ester of the
vegetable oil to make margarine with a higher softening point.
The product has is a higher softening/melting
point so that the product is a soft solid ('margarine') at room
temperature. (Check out the melting point trend in section 6.9.1).
The advantage of hydrogenation is that the order
of fatty acids on the glycerol is unchanged.
There is however, one disadvantage to the
hydrogenation process.
The C=C bond is weakened when the glyceride
molecules are adsorbed onto the nickel catalyst surface.
This is necessary for the addition of the
hydrogen molecule.
Unfortunately stereo-isomerisation can happen
and some of the Z C=C linkages (cis) can change to E C=C (trans)
linkages. Trans-fats are considered less healthy in our diet than
cis-fats.
On the left, by the short
vertical magenta arrow. I've shown one
transformation of a C=C double bond with a Z (cis) bond orientation into
a E (trans) orientation - an example of an isomerisation reaction.
An idealised complete hydrogenation of a
triester glyceride formed from a monounsaturated acid.
(2)
Transesterification
(interesterification)
The vegetable oil is mixed with stearic acid, a
fully saturated fatty acid, CH3(CH2)16COOH,
and a catalyst.
The saturated stearic replaces one of the
unsaturated fatty acids in the glyceride.
In a process of controlled crystallisation, the
'harder' less unsaturated glycerides crystallise first and are separated
out to make the margarine.
As in method (1) the result is the
softening/melting point is raised so that the product is a soft solid at
room temperature. (Check out the melting point trend in section
6.9.1).
Stearic acid is used because it doesn't affect the
concentration of 'bad' cholesterol (low-density lipoproteins) in the
bloodstream.
This process has the advantage that no trans fatty
acids are produced.
One possible disadvantage is that the stearic acid
can link to the middle carbon of glycerol, this doesn't happen
naturally, but it is thought it might be slightly harmful to your
health.
The diagram above illustrates the
transesterification of a polyunsaturated vegetable oil to produce a more
saturated fat suitable for margarine production.
In this case the fully saturated fatty
acid, steric acid, has replaced a polyunsaturated fatty acid of the original
triglyceride from the vegetable oil.
(3)
The final product - margarine
The modified oils are then mixed with unmodified
vegetable oils together with lipid soluble additives e.g. colouring
agents, emulsifiers and vitamins A and D.
To complete the process, this mixture is blended
with water-soluble additives like milk proteins, milk whey and salt -
and now you have your 'spreadable' margarine.
For more on
transesterification see
6.8
Esters
- preparation, reactions including hydrolysis and
transesterification
6.9.5 Biodiesel and the use of the transesterification
reaction
Biodiesel is made using natural vegetable oils e.g.
from soybeans, rapeseed, sunflower oils and also animal fats.
It is cleaner burning than purely hydrocarbon
diesel from oil.
The seeds from the Jatropha tree are particularly
useful because the trees grow on poor soil, too poor to be used for food
production.
In the future, 'green' biodiesel might be made via
photosynthesis in algae organisms.
Biodiesel can also be made from waste cooking oil
or animal fats.
Unfortunately, raw vegetable oils are not very good
fuels because do not readily vapourise and clog up the fuel injection
nozzles of a diesel engine.
The triglycerides have very high boiling points
much higher than the methyl esters, which are only 1/3rd the size of the
original triglyceride molecule.
What is needed is more volatile liquid that avoids
this problem.
You can achieve this by using a transesterification
reaction.
The natural vegetable oil (the triglyceride ester)
is mixed with methanol (CH3OH) and sodium
hydroxide as a catalyst.
This process is referred to as
base-catalysed transesterification.
The mixture is heated to make the more volatile
methyl esters of the fatty acids.
The overall reaction is:
1 triglyceride ester + 3 molecules of
methanol ===> 3 methyl ester molecules of fatty acid + 1
molecule glycerol
So the products are a mixture of methyl esters of
long-chain fatty acids propane-1,2,3-triol (glycerol).
Above illustrates the transesterification reaction
converting a big triglyceride ester molecule into three small methyl
esters of three different fatty acids.
The oil from rape seed oil produces a biodiesel
with physical and combustion properties similar to diesel from crude
oil.
The above diagram gives the structural formula
equation for converted a fully saturated fat glyceride into the methyl ester
of the biodiesel with glycerol as the bye-product.
For more on
transesterification see
6.8
Esters
- preparation, reactions including hydrolysis and
transesterification
[SEARCH
BOX]
INDEX of all carboxylic acids
and derivatives notes
All Advanced Organic
Chemistry Notes
BASIC GCSE NOTES
Biofuels & alternative fuels,
hydrogen, biogas, biodiesel
Carboxylic acids - molecular
structure, chemistry and
uses
Esters, chemistry and uses
including perfumes, solvents
|
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries & references to science course specifications
are unofficial. |
|