Experiment (19): Saponification Saponification Base catalyzed hydrolysis of an ester is known as Saponification, and t he reaction product (the ionized salt) is soap.
O C
R
O
R
NaOH Hydrolysis
Ester
O R
C
O Na
+
Salt
R
OH
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), (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
O C
R2
O C
R3
O C
O O
CH2
R1
C
R2
O C
NaOH O O
CH
Saponification
CH2
Fat or Oil Triacylglycerol
+
C
H O CH
O Na H O CH2
O R3
Ester linkage
H O CH2
O Na
O Na
Salt of fatty acid Soap
Gleycerol Glycerin
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.
1
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 Lauric Palmitic Stearic Oleic Linoleic Linolenic Arachidonic
Structure
No. of C atoms
Melting Point
CH3(CH2)10COOH CH3(CH2)14COOH CH3(CH2)16COOH CH3(CH2)7CH=CH(CH 2)7COOH CH3(CH2)4(CH=CHCH 2)2(CH2)6COOH CH3CH2(CH=CHCH 2)3(CH2)6COOH CH3(CH2)4(CH=CHCH 2)4(CH2)2COOH
12 16 18 18 18 18 20
+44 +63 +70 +16 -5 -11 -50
No. of Double Bonds 0 0 0 1 2 3 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.
2
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.
3
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 2+
2+
metal cations Mg , Ca
3+
and Fe , which are typical components of “hard” water, form
insoluble complexes. It also leaves a hard scum on the surface of sinks and tubs. O CH3 (CH2)16
+
O Na
C
sodium stearate 2+
Ca
O CH3 (CH2)16
C
O O
2+
Ca
O
C
(CH2)16 CH3
calcium stearate
Most detergents, however, do not respond in a similar manner, and can be used in both “hard” and “soft” water.
4
O CH3 (CH2)11
O S O
O +
O Na
sodium n-dodecyl sulfate (detergent)
CH3 (CH2)11
+
S 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 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.
5
