Fats and oils have the same basic chemical structure – they are all esters of propane-1,2,3-triol (glycerol) bonded to long chain carboxylic acids (fatty acids). The fat or oil formed is called a triester or triglyceride.
These long chain carboxylic acids are saturated in the case of animal fats (no double bonds in the alkyl chain) and unsaturated in the case of vegetable oils (one or more double bonds in the alkyl chain).
Natural fats and oils contain a mixture of triesters, and this explains their properties – vegetable oils are made up of unsaturated fatty acids and are liquid at room temperature, animal fats are made up of saturated fatty acids and are solid at room temperature (the long alkyl chains have a very regular structure so the molecules pack closely together, increasing the strength of the intermolecular bonding, hence the higher melting points).
The naturally occurring unsaturated fatty acid chains found in oils are almost exclusively Z-isomers. Converting the C=C bond into the E-isomer changes the shape of the molecule and its biological activity.
Vegetable oils can be ‘hardened’ or converted into solid margarine by hydrogenating some or all of the double bonds in the unsaturated fatty acid chains. This is a simple electrophilic addition reaction in which H2(g) is mixed with the oil in the presence of a finely divided nickel catalyst at temperatures above 180°C with pressures between 0.5 and 4 atm.
An unfortunate consequence of the process is that side reactions occur in which the remaining Z (cis) C=C double bonds are converted into E (trans) isomers.
Health studies have shown that trans-fats raise LDL- cholesterol and lower HDL-cholesterol ☹️ potentially leading to heart disease and increasing the risk of strokes. Changing the nickel catalyst for palladium or cobalt-molybdenum may reduce these unwanted side effects as lower temperatures and pressures are required.
The iodine value or iodine number for an unsaturated oil is used to determine the degree of unsaturation in a fat or an oil and can be defined as the mass of iodine (g) that reacts with 100g of the oil or fat.
e.g. olive oil has an iodine value of 75-94 g of I2 per 100g (monounsaturated); sunflower oil has an iodine value of 110-145 g of I2 per 100g (polyunsaturated).
When an excess of aqueous iodine, I2(aq), is shaken with the fat or oil, the iodine reacts with double bonds present and the yellow / brown colour of the iodine solution fades. The amount of unreacted iodine can be found by titration with a standard solution of sodium thiosulfate, Na2S2O3(aq).
Using plant oils as binding media in oil paints
A paint is composed of a coloured pigment and a binding media which binds the pigments together. It must be viscous enough to stop paint running when applied and dry to a durable finish. The earliest examples include egg tempera (yolk) which was widely used in the Middle Ages but by the 16th century, drying oils such as linseed or walnut oil were routinely used as a binding media.
A drying oil forms a tough, solid layer when in contact with air, hardening as a result of a chemical reaction known as autoxidation:
In the modern restoration of oil paintings we need to choose cleaning solvents that won’t dissolve the original binding media. A drying oil can be identified by hydrolysing the ester links with concentrated NaOH to form glycerol and the sodium salts of the fatty acid chains which are then treated with dilute HCl before being separated and analysed using gas-liquid chromatography, GLC.
All plant oils contain a mixture of triesters with both saturated and unsaturated fatty acid chains. Since the unsaturated fatty acid chains have been irreversibly cross-linked into a polymer network when the oil originally hardened, GLC measures the ratio of palmitic acid to stearic acid (both saturated fatty acid chains) in the sample. This ratio differs depending on the drying oil used.
Making soap
Heating animal fats or vegetable oils with NaOH or KOH causes the esters to hydrolyse forming the sodium or potassium salts of the fatty acids, and glycerol.
The salts are soaps, molecules with polar carboxylate groups that can form ion-dipole bonds with water and non-polar fatty acid chains that will dissolve non-polar oils and dirt. The soap molecules arrange themselves into micelles which are easily washed away when clothes etc. are rinsed.
Practice questions
- Oil paints are composed of a pigment suspended in an oil such as linseed oil. Linseed oil is a triester of propane-1,2,3-triol.
(a) Write an equation for the hydrolysis of linseed oil with aqueous sodium hydroxide. You can represent the side chains as ‘R’.
(b) Following hydrolysis, the identification of the side chains can be confirmed using gas-liquid chromatography, but the carboxylate products must first be converted into methyl esters. Draw the full structural formula of the methyl ester formed, representing the side chain as ‘R’.
(c) Suggest why the methyl esters are more suitable for gas-liquid chromatography than the carboxylate salts.
- Butter consists of mainly of triesters of propane-1,2,3-triol. When butter turns rancid the fat molecules are hydrolysed giving three molecules of butanoic acid.
(a) Draw the full structural formula of this triester.
(b) Explain why this triester is an example of a saturated fat.
- Linseed oil is a mixture of triesters of propane-1,2,3-triol bonded to monounsaturated and polyunsaturated fatty acid chains. One such triester is shown below:
(a) Identify, with reasoning, the arrangement around the C=C bonds in this triester as either E or Z isomers.
(b) Repeated heating of linseed oil at high temperatures changes the oil into a trans-fat. The trans-fat is semi-solid at room temperature. Explain why trans-fats have higher melting points than the oils they are made from with reference to any structural changes that happen to the oil at high temperatures.
(c) The iodine value of an oil refers to the number of grams of iodine, I2, that react with 100g of the oil.
0.150g of the linseed oil was shaken with 50 cm3 of 0.255 mol dm-3 I2(aq). Not all of the iodine reacted. The remaining iodine solution was titrated against 0.0200 mol dm-3 sodium thiosulfate, Na2S2O3(aq). The titre was 21.00 cm3.
2S2O32-(aq) + I2(aq) ⇾ S4O62-(aq) + 2I–(aq)
Calculate the iodine value for the linseed oil.
Answers
- (a)
(b)
(c) The methyl esters are more volatile so will be carried by the carrier gas through the column.
2. (a)
(b) This is a saturated fat because the fatty acid chains in the triester have no C=C double bonds.
3. (a) Each of the arrangements around the C=C double bonds is the Z-isomer with both groups / both hydrogen atoms on the same side of the C=C bond.
(b) In trans-fats some of the C=C double bonds are converted from the Z-isomer to the E-isomer so that the hydrogen atoms are on opposite sides of the C=C double bond. This is a more regular arrangement of the fatty acid chain (the chain is less kinked at each C=C bond) so the triesters can pass more closely together. This results is stronger instantaneous dipole – induced dipole intermolecular bonds / London forces which take more energy to break, hence the higher melting point of the trans-fat.
(c) No. of mol of S2O32- used in the titration = 0.0200 mol dm-3 x 0.021dm3 = 4.20 x 10-4 mol
Ratio of S2O32- : I2 from the balanced equation is 2:1 so the no. of mol of I2 remaining must have been 2.10 x 10-4 mol.
The linseed oil was added to 1.275 x 10-3 mol of I2 in the first step (0.0255 mol dm-3 x 0.05dm3).
No. of mol of I2 that reacted with the linseed oil = (1.275 x 10-3) – (2.10 x 10-4) = 1.065 x 10-3 mol.
Mass of iodine that reacted = 1.056 x 10-3 mol x (126.9 x 2 g mol-1) = 0.270g
Iodine value = (100 / 0.150) x 0.270 = 180