JoVE Lab Manual
Lab: Chemistry
Lab: Chemistry
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Vegetable oils and animal fats are naturally occurring lipids in plants or animals. Oils and fats are often in the form of triglycerides, which are formed from a molecule of glycerol and three fatty acids. A fatty acid is a weak carboxylic acid with a hydrocarbon chain as the functional group. In the triglyceride, the three fatty acids are linked to the molecule of glycerol by ester bonds. A triglyceride, therefore, has three ester groups, making the molecule a triester.
An ester is a type of organic molecule that is like a carboxylic acid, with the hydrogen of the hydroxyl replaced by an alkyl or aryl group. This group is labeled R-prime to show that it may be different from the R-group on the other side. In a triglyceride, R-prime is the glycerol backbone, and each R-group is the chain of a fatty acid.
Fatty acids with double bonds in their carbon chains are known as unsaturated fatty acids. These double bonds tend to be in the cis conformation, which puts a bend in the chain. Triglycerides with these bent chains can't fit closely together, so the attractive forces between them are typically long-range and weak.
Fatty acids with only single bonds between the carbon atoms are saturated. These unbent chains allow triglycerides to pack closely together, enabling stronger, short-range intermolecular interactions. This gives saturated triglycerides a higher melting point than unsaturated triglycerides, which are usually liquid at room temperature.
Triglycerides can be converted back to fatty acids and glycerol by hydrolyzing the three esters. Ester hydrolysis is a reaction that breaks an ester bond with a molecule of water or a hydroxide ion to form a carboxylic acid and an alcohol. One common use of ester hydrolysis is to create soaps, which are the salts of fatty acids from triglycerides. This process is called saponification.
In this reaction, hydroxide ions attack each of the three ester carbonyls in the triglyceride, yielding three fatty acid molecules and one molecule of glycerol. The fatty acid carboxylates associate with the counterion from the base, which is usually sodium or potassium.
But how does soap work? The long carbon chain is lipophilic and hydrophobic, meaning that it is attracted to fats but not to water. The carboxylate group is hydrophilic, meaning that it is attracted to water. When you mix soap and water, the long carbon chains tend to interact with each other and avoid interaction with water molecules, while the carboxylate groups prefer to interact with water. Ultimately, the soap molecules form clusters called micelles, with the hydrophilic carboxylates facing outward.
Now, let's assume there is hydrophobic grease on your hands. Simply rinsing your hands with water would not remove the grease. When you wash with soap, the grease interacts with the soap's carbon chains. As the soap molecules form micelles, they bring the grease into the hydrophobic core enclosing it in a hydrophilic shell that can then be rinsed away with water.
In this lab, you will perform as a saponification reaction to create soap from coconut oil and sodium hydroxide. You will then test the tolerance of your soap for hard water by comparing the foaming ability in deionized water, tap water, and a calcium chloride solution, which mimics extremely hard water.
酯水解
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
- Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chemistry and Chemical Reactivity. Belmont, CA: Brooks/Cole, Cengage Learning.
- Silberg, M.S. (2015). Chemistry: The Molecular Nature of Matter and Change. Boston, MA: McGraw Hill.
- Streiwiester, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction to Organic Chemistry. Upper Saddle River, NJ: Prentice Hall.
