7. Saponification Flashcards
We are able to manufacture soaps from what type of reaction?
Saponification reaction.
The word saponification literally means what?
“Soap making”.
Soaps are made by a process that has been essentially unchanged over the last 2000 years, in which animal or vegetable fats 1 are boiled with a strong base, usually what?
Sodium hydroxide.
Animal fats used for soap production include tallow, lard and fish oil. Vegetable fats include palm, soybean, groundnut, coconut, canola and olive oils.
Most naturally occurring fatty acids have an even number of carbons, between how many?
6 and 28.
Fats and oils contain a special kind of ester known as a what?
Triglyceride.
This makes saponification reactions possible. These esters are much larger than other esters.
A triglyceride (a type of lipid) is an ester derived from glycerol combined with three what?
Fatty acid molecules.
Each of these three fatty acid molecules undergoes an esterification reaction with one of the hydroxyl groups of the glycerol molecule. The result of this esterification reaction is a large triester molecule referred to as a triglyceride.
So how do triglycerides form soap? Soaps are essentially sodium or potassium salts of what?
Fatty acids.
The preparation of soaps begins by boiling triglycerides with sodium hydroxide (NaOH) in the reaction known as what?
Saponification (it is an example of a hydrolysis reaction, the complete opposite reaction of an esterification reaction).
When heated with NaOH triglycerides are hydrolysed to fatty acids in the form of sodium salts of carboxylic acids and a molecule that is commonly called what?
Glycerol (but has the systematic name propan-1,2,3-triol). These sodium salts are a key ingredient in the manufacturing of soaps.
After hydrolysis is complete, what is added to precipitate the soap as thick curds?
Sodium chloride.
The water layer is then drawn off, and glycerol is removed by vacuum distillation. The crude soap contains sodium chloride, sodium hydroxide and other impurities that are removed by boiling the curd in water and precipitating again with more sodium chloride.
The carboxylic acid salt formed from the saponification of beef fat is found in many types of soaps and even deodorants. This important carboxylic acid salt is called what?
Sodium stearate.
The stearate ion has a a negatively charged carboxylate ion “head” and a long hydrocarbon “tail”. The typical length of a carboxylic acid salt hydrocarbon tail is 12-18 carbons.
Soap owes its remarkable cleaning properties to its ability to act as what?
An emulsifying agent.
Because the long hydrocarbon chains of natural soaps are insoluble in water, they tend to cluster to minimise their contact with surrounding water molecules.
The polar carboxylate groups, by contrast, tend to remain in contact with the surrounding water molecules. Thus, in water, soap molecules spontaneously cluster into what?
Micelles.
A micelle is a spherical arrangement of organic molecules in a water solution. These molecules are clustered in a way that their hydrophobic parts are buried inside the sphere and their hydrophilic parts are on the surface of the sphere and in contact with water.
Most of the things we commonly think of as dirt (such as grease, oil and fat stains) are nonpolar and insoluble in water.
When soap and this type of dirt are mixed together, as in a washing machine, the nonpolar hydrocarbon inner parts of the soap micelles do what?
“Dissolve” the nonpolar dirt molecules.
In effect, new soap micelles are formed, this time with nonpolar dirt molecules in the centre. In this way, nonpolar organic grease, oil and fat are “dissolved” and washed away in the polar wash water.
When soapy water comes in contact with dirt (i.e. oil or grease), what series of events happen?
- The non-polar tails of the soap molecules adsorb onto the dirt.
- The charged heads of the surfactant form a layer around the dirt, giving it a hydrophilic surface.
- The water penetrates between the dirt and the surface below it, causing a dirt droplet to form.
- The dirt droplet is held in solution and attracted to the water molecules by anions on the droplet’s surface.
- These soap water molecules then lifts the dirt off the substance, leaving the underlying surface clean.
While soaps are able to remove dirt, they do have their disadvantages.
The main disadvantage of soaps is that they form water-insoluble salts when used in “hard” water, which contains what?
Ca(II), Mg(II) or Fe(III) ions.
Water systems that use groundwater as a source are concerned with water hardness, as the water moves through soil and rock, dissolving small amounts of naturally-occurring minerals.
You may have felt the effects of hard water, such as needing to use more soap or detergent in order to get things clean (i.e. your hands, hair or laundry).
These calcium, magnesium and iron salts of fatty acids create problems, including rings around the bathtub, films that spoil the lustre of hair and greyness/roughness that builds up on textiles after repeated washings.
Following the discovery of the cleaning action of soaps, chemists were in a good position to design a what that did not have the same problems as soaps?
Synthetic detergent.
Scientists discovered these essential characteristics of a good detergent could be produced in a molecule containing which group, instead of a carboxylate (COO−) group?
A sulfonate (SO3−) group.
Calcium, magnesium and iron salts of monoalkylsulfuric and sulfonic acids are what?
Much more soluble in water than comparable salts of fatty acids (thereby preventing the problematic rings around the bathtub, etc.).
