University of Salahadin/ Colloege of Science

2011

Saponification

Dotsha J. Raheem

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Saponification

Saponification

Base catalyzed hydrolysis of an ester is known as Saponification, and the reaction product

(the ionized salt) is soap.

R C

O

O R

Ester

NaOH

HydrolysisR C

O

O Na + R OH

Salt Alcohol

A fat consists of fatty acids and glycerin. Adding NaOH separates it to glycerin and the salt

of the fatty acid.

A fatty acid is a carboxylic acid with a long unbranched aliphatic tail (chain), which is either

saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even

number of carbon atoms, from 4 to 28.

R1 C

O

O CH2

Fat or OilTriacylglycerol

NaOHR1 C

O

O Na

+

Salt of fatty acidSoap

R2 CO

O CH

R3 C

O

O CH2

Ester linkage

SaponificationR2 C

O

O Na

R3 C

O

O Na

O CH2

O CH

O CH2

H

H

H

GleycerolGlycerin

Soaps are water soluble sodium or potassium salts of fatty acids (12C- 18C), and they are

made from fats and oils by treating them with strong alkali.

If industrially produced fatty acids are used instead of natural fats and oils, the reaction

will yield soap and water instead of glycerin.

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Fats and fatty acids

The fats and oils used in soap making come from animal or plant sources. Fatty acids can

be attached to other molecules, as in triglycerides. When they are not attached to other

molecules they are known as “free fatty acids”.

Vegetable oil contain more unsaturated fatty acids (e.g. olive oil) while animal fats

contain more saturated fats (e.g. beef fat).

Acid Name Structure No. of C atoms

Melting Point

No. of Double Bonds

Lauric CH3(CH2)10COOH 12 +44 0

Palmitic CH3(CH2)14COOH 16 +63 0

Stearic CH3(CH2)16COOH 18 +70 0

Oleic CH3(CH2)7CH=CH(CH2)7COOH 18 +16 1

Linoleic CH3(CH2)4(CH=CHCH2)2(CH2)6COOH 18 -5 2

Linolenic CH3CH2(CH=CHCH2)3(CH2)6COOH 18 -11 3

Arachidonic CH3(CH2)4(CH=CHCH2)4(CH2)2COOH 20 -50 4

The molecular structure allows many fatty acids to

be stacked together. As a result, close intermolecular

interactions result in relatively high melting points.

The introduction of one or more double bonds in the

hydrocarbon chain in unsaturated fatty acids results in

one or more “bends” in the molecule. The geometry of

the double bond is almost always a cis configuration in

natural fatty acids. These molecules do not “stack”

very well. The intermolecular interactions are much

weaker than in saturated molecules. As a result, the

melting points are much lower for unsaturated fatty

acids.

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Fatty acids are divided into short, medium or long chain

Short chain fatty acids contain fewer than six C atoms.

Medium chain contains 6- 12 C atoms.

Long chains are longer than 12 C atoms.

Very long chain fatty acids are longer than 22 C atoms.

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How soap cleans

A soap molecule is made up of two parts (polar and non-polar)

The hydrophobic (non-polar) part of soap

molecule can dissolve the greasy dirt, and the

hydrophilic (polar or ionic) part of the molecule is

attracted to water molecules and hence forming

small suspended particles called micelles. Thus the

dirt is emulsified, broken into small particles, and

can be rinsed a way.

Hard and soft water (soaps vs. detergents)

Although the sodium and potassium salts of common soaps are soluble in water, the

metal cations Mg2+, Ca2+ and Fe3+, which are typical components of “hard” water, form

insoluble complexes. It also leaves a hard scum on the surface of sinks and tubs.

CH3 (CH2)16 C

O

O (CH2)16 CH3C

O

OCa2+

CH3 (CH2)16 C

O

O Na+

sodium stearate

calcium stearate

Ca2+

Most detergents, however, do not respond in a similar manner, and can be used in both

“hard” and “soft” water.

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CH3 (CH2)11 S

O

O Na+

OO

sodium n-dodecyl sulfate (detergent)

CH3 (CH2)11 S

O

O Na+

O

sodium n-dodecyl benzene sulfonate (detergent)

Types of soaps

The properties of the soap depend upon the type of the fat or oil used, including number

of carbons in the chain and the degree of unsaturation in the fatty acid, and the metal ion:

soaps made from KOH have softer consistency than soaps made from NaOH.

Procedure:

1. Place 5 g of oil in a round bottom flask and add to it a solution of 5 g NaOH in 30 ml

H2O and 5 ml ethanol.

2. Add several boiling chips, connect the flask to a condenser and gently reflux the

mixture over a small flame.

3. Saponification is complete when you obtain a homogenous solution (in 30-45

minutes).

4. Prepare a concentrated salt solution by dissolving 25 g of NaCl in 75 ml of water.

5. When Saponification is complete, extinguish the flame and pour the mixture quickly

into the saturated salt solution.

6. Stir the mixture thoroughly for several minutes.

7. Collect the formed soap using a Buchner funnel.

8. Leave it to dry in the laboratory, and then record its weight.