L6 - Triglycerides

objectives

Fatty Acids

• Fatty acids are carboxylic acids that generally contain 4-28 carbons in length.
• The length of the fatty acid can be classified as short-chain (less than six carbons), medium-chain (6-12 carbons), long-chain (13-21 carbons) and very long chain (over 22 carbons).
• Most naturally occurring fatty acids contain a straight chain (no branching) and an even number of carbons.

• The hydrocarbon chain can be saturated, containing only carbon-carbon single bonds, or unsaturated, containing one or more carbon-carbon double bonds.
• The palmitic acid shown in Figure 1 contains only carbon-carbon single bonds, making it a saturated fatty acid.
• Figure 2 contains one carbon-carbon double bond, making it an unsaturated fatty acid.
• Some fatty acids contain only one C=C, known as a monounsaturated fatty acid, while others contain more than one carbon-carbon double bond, known as a polyunsaturated fatty acid.
• When the carbon-carbon double bond forms, it generally takes on the CIS Formation (HL only).

• Although the carboxyl group is highly polar and capable of hydrogen bonding, the long, nonpolar hydrocarbon chain results in a molecule that has greater nonpolar characteristics.
• As a result, fatty acids are not soluble in water.
• The strength of the intermolecular forces between fatty acids depends on the length of the chain and whether it is saturated or unsaturated.
• Longer chain lengths result in stronger London dispersion forces, resulting in higher melting points.
• Saturated fatty acids are very straight, while unsaturated fatty acids have a bend or 'kink' in the chain.
• Straighter chains allow for better contact between fatty acid chains, increasing the strength of the London dispersion forces and the melting point.

Triglycerides

• Fatty acids bond to a molecule of glycerol through a condensation reaction to form a triglyceride.
• Glycerol, or propane-1,2,3-triol, contains three hydroxyl groups that are capable of reacting with the carboxyl functional group in the fatty acid to eliminate three water molecules.
• The three fatty acids that bond with glycerol can be the same type or they can be different.
• A condensation reaction between an alcohol and a carboxylic acid forms a class of compounds known as an ester, so the bond that forms is referred to as an ester linkage.
• The reverse esterification reaction can occur to liberate the three fatty acids from the glycerol backbone through a hydrolysis reaction.

Melting Point of Triglycerides

• Triglycerides are commonly referred to as 'fats' or 'oils'.
• The difference between a fat and an oil is the state at room temperature and its source.
• Lipids from animal sources, such as meat and dairy, are generally referred to as fats and tend to be solid at room temperature, while lipids from plant sources are usually referred to as oils and tend to be liquid at room temperature.
• There are some exceptions, such as coconut oil, which is from a plant source, but has a melting point of 24°C, making it often solid at room temperature, although it is referred to as an oil, rather than a fat.
• When fatty acid chains contain carbon-carbon double bonds, making them unsaturated, the chain is no longer linear.
• This causes weaker London dispersion forces between triglyceride molecules, lowering the melting point and making the triglyceride more likely to be a liquid at room temperature.
• Saturated fatty acid chains are more linear, so there is more surface area between triglycerides, increasing the strength of the London dispersion forces and the melting point.

Iodine Number

• A quantitative measurement of unsaturation used for fats is the iodine number, also known as the iodine value or iodine index.
• The iodine number is the mass of iodine that is absorbed by 100 g of a chemical substance.
• Since unsaturated carbon-carbon double bonds react with halogens, such as iodine, in an addition reaction (HL only), having more sites of unsaturation would result in a greater iodine number.
• Iodine is yellow-brown in solution and as it reacts with carbon-carbon double bonds, the iodine decolourises.
• The addition reaction involves the iodine being added to the molecule and no longer in its free form, I2, which is coloured.
• Iodine can be slowly added to a sample of a lipid until no further colour change is observed and the yellow-brown colour remains, indicating that there are no more carbon-carbon double bonds present.

• Vegetable oils tend to have more unsaturation than fats from animal sources, so the iodine number of vegetable oils tends to be higher than the iodine number from animal fats.
• Vegetable oils tend to have iodine numbers in the range of 80-200, while animal fats tend to be in the range of 40-70.
• Recall that 'fats' are solid at room temperature, indicating a higher degree of saturation and thus a lower iodine number, while 'oils' are liquid at room temperature, indicating greater unsaturation and a higher iodine number.

Calculations involving Iodine Number

• The iodine number is expressed as a percentage of the mass of the iodine reacted by the mass of the lipid reacted.

Calculating the Number of C=C

• The number of carbon-carbon double bonds can also be determined using the iodine number and the relative formula mass of the lipid and some stoichiometry to determine the number of moles of iodine that reacted.
• Each mole of iodine reacted represents one carbon-carbon double bond.