Hydrolysis of an Ester

Triglycerides

Triglycerides are triester molecules composed of a glycerol backbone that is bound to 3 fatty acids. These form the basis of many biological lipids that are found in vegetable oils and fats. Lipids that are isolated from plant material are the basis of vegetable oils, whereas lipids extracted from animals are used as lard or general sources of fats.

A fatty acid molecule has a long carbon chain — between 12 and 20 carbons — with a weak organic acid at one end. Some fatty acids have double bonds in the carbon chain. These are ‘unsaturated’ fats. Because of the double bond in the carbon chain, these fatty acids typically have a bent chain shape, as the bond tends to adopt the cis conformation rather than the trans conformation.

The triglyceride molecules in an unsaturated fatty acid are unable to stack uniformly, leading to less attractive forces between the molecules. Thus, unsaturated fats are usually liquid at room temperature. In contrast, saturated fats, meaning those with only single bonds in their carbon chain, are able to stack because their chain is more regular in shape. Saturated fats, therefore, have stronger attractive forces between the molecules and, as a result, higher melting points. Thus, saturated fats tend to be solid at room temperature.

Hydrolysis Reaction

Triglycerides, like other molecules possessing ester groups, are able to undergo hydrolysis. Hydrolysis is a type of chemical reaction where a water molecule breaks a bond. In the case of an ester hydrolysis, the nucleophile — water or a hydroxide ion — attacks the carbonyl carbon of the ester group to break the ester bond. One fragment of the original molecule gains the hydrogen ion from water to form an alcohol, while the other fragment gains the hydroxide group to form a salt of the carboxylic acid.

Under normal conditions, only a few hydrolysis reactions occur without the addition of a strong acid or base. The strong acid or base acts as a catalyst to speed up the reaction, and it is recovered at the end. The hydrolysis reaction is reversible. It is called a condensation reaction because it produces water as a side product.

The hydrolysis of a triglyceride is one of the oldest examples of a hydrolysis reaction, as it has been used for centuries to make soap. The reaction is called saponification.

Saponification

During saponification, hydroxide ions attack the carbonyl groups in the three esters of the triglyceride. Therefore, the stoichiometric addition of three moles of sodium hydroxide to one mole of triglyceride will result in the hydrolysis of the three ester groups. This reaction produces three moles of soap molecules and one mole of glycerol. Often, the reaction will be performed with sodium hydroxide as the limiting reagent to minimize the amount of unreacted triglyceride, which can reduce the drying effect of soap.

A soap molecule has a long hydrophobic carbon chain with a polar hydrophilic carboxylate anion on the other end. The difference in solubility along the molecule enables its cleaning ability. In water, the long carbon chains of the soap molecules will be attracted to one another and reorganize to avoid the polar water molecules. This orients the polar carboxylate heads towards the water molecules, forming a spherical structure known as a micelle. The center is hydrophobic, and the exterior is hydrophilic, allowing micelles to be soluble in aqueous solutions.

The micelle properties of soap are essential to its cleaning ability. Simply rinsing grease or oil off a surface cannot remove it. This is because grease is hydrophobic and not water-soluble. However, when soap is used, the grease enters the interior of the micelle. The exterior of the micelle is hydrophilic, so it can be rinsed away with water.

A significant drawback to soap is its intolerance to hard water. Hard water is an aqueous solution containing magnesium, iron, and calcium ions in the form of minerals. The “hardness” of water is dependent on the concentration of these ions. In the presence of metallic cations, soap molecules will interact with the molecules and cause them to fall out of solution, forming a salt precipitate more commonly referred to as soap scum.

References

1. Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chemistry and Chemical Reactivity. Belmont, CA: Brooks/Cole, Cengage Learning.
2. Silberg, M.S. (2015). Chemistry: The Molecular Nature of Matter and Change. Boston, MA: McGraw Hill.
3. Streiwiester, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction to Organic Chemistry. Upper Saddle River, NJ: Prentice Hall.