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Saponification of a triglyceride with sodium hydroxide.

Saponification is the hydrolysis of an ester under basic conditions to form an alcohol and the salt of a carboxylic acid (carboxylates). Saponification is commonly used to refer to the reaction of a metallic alkali (base) with a fat or oil to form soap. Saponifiable substances are those that can be converted into soap.

Sodium hydroxide (NaOH) is a caustic base. If NaOH is used a hard soap is formed, whereas when potassium hydroxide (KOH) is used, a soft soap is formed. Vegetable oils and animal fats are fatty esters in the form of triglycerides. The alkali breaks the ester bond and releases the fatty acid salt and glycerol. If necessary, soaps may be precipitated by salting it out with saturated sodium chloride. The saponification value is the amount of base required to saponify a fat sample.

In a classic laboratory procedure the triglyceride trimyristin is obtained by extracting nutmeg with diethyl ether.[1] Saponification to the sodium salt of myristic acid takes place with NaOH in water. The acid itself can be obtained by adding dilute hydrochloric acid.[2]

Contents

Mechanism

The reaction mechanism is based on nucleophilic acyl substitution.[3] Attack of the hydroxyl anion on the carbonyl group of the ester gives an orthoester:

Saponification part I

At this stage, the molecule has a choice: reforming the carbonyl can be accompanied by expulsion of either hydroxide or the alkoxide. The former leads back to the starting materials and is unproductive (explaining why saponification is in fact an equilibrium). On the other hand, expulsion of the alkoxide generates a carboxylic acid:

Saponification part II

The alkoxide is more basic than the conjugate base of the carboxylic acid, and hence proton transfer is rapid:

saponification part III

Corpses

Saponification can also refer to the other soft tissue in a conversion of the fat of a corpse into adipocere, often called "grave wax." This process is more common where the amount of fatty tissue is high, the agents of decomposition are absent or only minutely present, and the burial ground is particularly alkaline.

Fire extinguishers

Fires involving cooking fats and oils (classified as class K or Class F) burn hotter than flammable liquids, rendering a standard class B extinguisher ineffective. Flammable liquids have flashpoints under 100 degres Fahrenheit. Cooking oil is a combustible liquid since it has a flashpoint over 100 degrees. Such fires should be extinguished with a wet chemical extinguisher. Extinguishers of this type are designed to extinguish cooking fats and oils through saponification. The extinguishing agent rapidly converts the burning substance to a non-combustible soap. This process is endothermic, meaning it absorbs thermal energy from its surroundings, decreasing the temperature and eliminating the fire.

Saponification in art conservation

Saponification can occur in oil paintings over time, causing visible damage and deformation. The ground layer or paint layers of oil paintings commonly contain heavy metals in pigments, such as lead white, red lead, or zinc white. If those heavy metals react with free fatty acids in the oil medium that binds the pigments together, soaps may form in a paint layer that can then migrate upwards to the painting's surface. [4]

Saponification in oil paintings was first described in 1997.[5] It is believed to be widespread, having been observed in many works dating from the fifteenth through the twentieth century, works of different geographic origin, and works painted on various supports, such as canvas, paper, wood, and copper. Chemical analysis may reveal saponification occurring in a painting’s deeper layers before any signs are visible on the surface, even in paintings centuries old.[6]

The saponified regions may deform the painting's surface through the formation of visible lumps or protrusions that can scatter light. These soap lumps may be prominent only on certain regions of the painting rather than throughout. In John Singer Sargent’s famous Portrait of Madame X, for example, the lumps only appear on the blackest areas, which may be because of the artist’s use of more medium in those areas to compensate for the tendency of black pigments to soak it up.[7] The process can also form chalky white deposits on a painting’s surface, a deformation often described as "blooming" or "efflorescence," and may also contribute to the increased transparency of certain paint layers within an oil painting over time.[8]

The process is still not fully understood. Saponification does not occur in all oil paintings containing the right materials. It is not yet known what triggers the process, what makes it worse, or whether it can be halted.[9] At present, retouching is the only known restoration method.

See also

References

  1. ^ Organic Syntheses 1:538 Link
  2. ^ Organic Syntheses 1:379 Link
  3. ^ John McMurry, Organic Chemistry (2nd Edition).
  4. ^ Silvia A. Centeno and Dorothy Mahon, "The Chemistry of Aging in Oil Paintings: Metal Soaps and Visual Changes." The Metropolitan Museum of Art Bulletin’’, Summer 2009, pp. 12-19.
  5. ^ Researchers in the Netherlands discovered it while analyzing Rembrandt's The Anatomy Lesson of Dr. Nicolaes Tulp (1632). See Centeno, p. 14.
  6. ^ Centeno, p. 16.
  7. ^ Centeno, pp. 12-13, 15.
  8. ^ Centeno, pp. 16, 19.
  9. ^ Centeno, p. 19.

External links

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