Saponification & the Chemistry of

INTRODUCTION:

The production of soap starts with an understanding of . All fats & oils share a common molecular functional group. Whether it’s an like lard or butter, or a like olive oil or coconut oil, all of these substances can be classified as . Triglycerides are composed of three esters linked together through a common backbone (see figure below). And just as esters can be made by the combination of an alcohol and a , the reverse reaction can also occur. In a process known as saponification, triglycerides can be cleaved along the functional groups to produce (a molecule containing three alcohol functional groups) and the basic form of fatty acids (long chain carboxylic acids which have been deprotonated).

This is the fundamental reaction behind the production of soap. Starting with a fat or oil, a basic reagent (like sodium , NaOH) is added to fragment the into its components. The resulting exists as a —what we call soap. It has two elements:

1. A polar, hydrophilic (water “loving”) head that is composed of sodium ions (or other cations) & carboxylate anions (from the deprotonated fatty acid)

2. A nonpolar, hydrophobic (water “hating”) tail that consists of a chain of carbon atoms (the exact structure of this chain depends on the origin of the fat or oil)

It’s the presence of these two different polarities in the same molecule that allows soap to function as it does. The charged head of the soapy molecule strongly interacts with water molecules. On the other hand, the hydrophobic chains of carbon atoms aggregate to avoid water contact. This structure, called a micelle, generates a sphere with a nonpolar interior capable of dissolving dirts and oils. In this way, soap can be used to wash away grease and grim (nonpolar substances) using an aqueous solution. Micelle After saponification is complete, the soap is separated from the glycerol by “salting out.” The solution is mixed with a concentrated sodium chloride solution. This electrolyte causes the dispersed soap to coagulate. The soap is then washed several times with concentrated sodium chloride to remove the excess (aka lye). Such lye soap can be made from many different animal fats or vegetable oils, each giving slightly different cleaning qualities.

PROCEDURE:

Experiment 8A—Day 1

There are many different methods for performing the saponification chemistry to make soap from fats and oils. In this lab period, we will use the hot process, which will allow us to make and test our soap by next week.

CAUTION: Sodium hydroxide (lye) is CORROSIVE—avoid all contact, and be careful not to overheat solutions to avoid splattering.

1. Place 20 mL of and 30 mL of 20% sodium hydroxide in a 300 mL beaker. 2. Heat the mixture with vigorous stirring. Keep your head and hand far from the beaker. The stirring is necessary two mix the two layers and avoid charring the soap. a. The mixture will begin to foam and rise in the beaker. Continue to heat and stir. b. When the foam dies down, the mixture will appear syrupy. Continue to heat and stir. Vigorous stirring should prevent spattering and frothing as long as a slow and controlled boiling is maintained. c. When the mixture is waxy, discontinue heating. Waxy means that it sticks to and builds up on the stirring rod. The mixture should appear moist or it has overcooked (in which case boiling has removed all of the water from the mixture). 3. STOP HEATING: When the water layer is no longer visible, let the mixture cool slightly. If a waxy solid forms, the process is complete. Let the mixture cool. If a syrupy liquid results, the reaction is not complete, and heating must be resumed. 4. “Salting out”: Pour ~40 mL of concentrated sodium chloride solution into the beaker containing the soap. a. Break up the soap into small pieces with the stirring rod so that all of the soap is washed free of glycerol and sodium hydroxide. 5. Filter the soap with a piece of filter paper in a funnel. Moisten the filter paper with water so the filter paper flings to the funnel. Pour the soap mixture through the filter paper, collecting the filtrate in a second beaker. 6. Scrape the soap back into the original beaker and repeat the washing with two more 40 mL portions of the concentrated sodium chloride solution. 7. Work the soap on a paper towel to remove the last part of the wash water, and store this block for next week’s analysis.