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

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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 63^oC. 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 69^oC (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 13^oC. 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 -5^oC. 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 -16^oC.

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 (> 300^oC) 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

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)CH₂OH^δ+llll^δ-: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

 

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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

 

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