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 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

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).

Fig 4

(a) Stearic acid   C₁₇H₃₅COOH   CH₃(CH₂)₁₆COOH

Systematic IUPAC name for stearic acid is octadecanoic acid, a fully saturated carboxylic acid - no C=C bonds, melting point 69^oC.

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 ? acid and a monounsaturated omega-9 fatty acid - one C=C bond.

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.

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 omiga number.

(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.

(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

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.

Their formation and structure 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 several 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 glycerides.

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.

 

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.

 

Fig 6

Fig 6 shows the addition of mineral acid (e.g. hydrochloric or sulfuric) 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.

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

Fig 8

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.

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.

Fig 1

In Fig 1, 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 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.

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.

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.

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 a catalyst.

The mixture is heated to make the more volatile methyl esters of the fatty acids.

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.

 

The overall reaction is:

1 triglyceride ester  + 3 molecules of methanol  ===> 3 methyl ester molecules of fatty acid  + 1 molecule glycerol

Fig 7

Fig 7 illustrates the transesterification reaction converting a big triglyceride ester molecule into three small methyl esters of three different fatty acids.

For more on transesterification see

6.8 Esters - preparation, reactions including hydrolysis and transesterification

 

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Doc Brown's Advanced Level Chemistry Revision Notes

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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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