Description
SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Fatty Acids
A fatty acid is a molecule characterized by the presence of a carboxyl group attached to a
long hydrocarbon chain.
Therefore these are molecules with a formula R–COOH where R is a hydrocarbon chain.
Fatty acids can be said to be carboxylic acids, and come in two major varieties.
Saturated fatty acids: Saturated fatty acids do not have any double bonds. Saturated fatty
acids are solids at room temperature. Animal fats are a source of saturated fatty acids.
Unsaturated fatty acids Unsaturated fatty acids can have one or more double bonds along
its hydrocarbon chain. A fatty acid with one double bond is called monounsaturated. If it
contains two or more double bonds, the fatty acid is polyunsaturated. The more double
bonds it contains, the lower the melting point. As the length of the tail increases, the melting
point increases. Plants are the source of unsaturated fatty acids.
Dr. Gulzar Alam Page 1 of 14
SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Geometric Isomerism in unsaturated fatty acid
Unsaturated fatty acids can exist in either the cis or trans form depending on the configuration of
the hydrogen atoms attached to the carbon atoms joined by the double bonds. If the hydrogen
atoms are on the same side of the carbon chain, the arrangement is called cis. If the hydrogen atoms
are on opposite sides of the carbon chain, the arrangement is called trans. cis isomers are those
naturally occurring in food fats and oils. Trans isomers occur naturally in ruminant animals such
as cows, sheep and goats and also result from the partial hydrogenation.
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Reaction of Oils/Fats
1. Hydrolysis. The fats are hydrolyzed by the enzymes lipases to yield fatty acids and glycerol.
The lipases catalyze this reaction at a slightly alkaline pH (7.5-8.5) in a stepwise manner. The fats
first split to produce diglycerides, part of these are then split to monoglycerides. Finally, part of
the monoglycerides split to yield fatty acid and glycerol.
2. Saponification. The hydrolysis of fats by alkali is called saponification. This reaction results in
the formation of glycerol and salts of fatty acids which are called soaps. The soaps are of two
types: hard and soft. Hard soaps such as the common bar soaps are the sodium salts of the higher
fatty acids. Soft soaps are the potassium salts of higher fatty acids & are marketed as semisolids
or pastes.
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
3. Hydrogenation. Unsaturated fatty acids, either free or combined in lipids, react with gaseous
hydrogen to yield the saturated fatty acids. The reaction is catalyzed by platinum, palladium or
nickel. The addition of hydrogen takes place at the C—C double bond (s). Thus, 1 mole of oleic,
linoleic or linolenic acid reacts with 1, 2 or 3 moles of hydrogen respectively to form stearic acid.
4. Halogenation. Unsaturated fatty acids and their esters can take up halogens like Br2 and I2 at
their double bond (s) at room temperature in acetic acid or methanol solution. This reaction is the
basis of the ‘iodine number determination’.
5. Oxidation. Unsaturated fatty acids are susceptible to oxidation at their double bonds. Oxidation
may be carried with ozone or KMnO4.
(a) With ozone – An unstable ozonide is formed which later cleaves by water to give rise to 2
aldehydic groups.
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
(b) With KMnO4 – Under mild conditions, the glycols are formed at the sites of double bonds.
Under vigorous conditions, the same reagent cleaves the molecule at the double bond and
oxidizes the terminal portions to the carboxyl group.
Analytical constants for Oils & Fats
Acid number
Synonym: Neutralization number, Acid value
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Acid Value determination: Principle
Significance of acid value:
Saponification Number
Synonym: Saponification value, Koettstorfer number
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Saponification Number determination: Principle
Significance of Saponification Number:
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Iodine Number:
Synonyms: Iodine Value
Iodine Number determination: Principle
Significance of Iodine Number:
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Ester Number (Ester value)
Ester number is the number of mg of KOH required to saponify the esters present in 1 g
of the substance.
Ester value = Saponification value – Acid value.
Method:
Weigh accurately about 2 g of the sample. Add 25ml of 0.5M ethanolic KOH and boiled
under reflux condenser on a water-bath for1hour.
Add 20ml of water and titrate the excess of alkali with 0.5 M HCl using a further 0.2ml of
phenolphthalein indicator.
Repeat the operation without sample. (Blank reading)
The difference between the titrations represents the alkali required to saponify the esters.
Significance:
Ester value = Saponification value – Acid value
The ester value shows the amount alkali consumed in the saponification of the esters and
is possible identify and differentiate the fats with this value.
Reichert-Meissl (RM) value
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Reichert Meissl (RM) value determination: Principle
Significance of Reichert Meissl (RM) value:
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Acetyl Number (Acetyl value)
Significance of Acetyl value:
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Rancidity of oils
Stability of a fat or fatty food is significant to maintain a fresh taste or odour during storage
and use. Fats with substantial unsaturated fatty acids are usually unstable.
The presence of agents causing oxidation (pro-oxidants) or preventing oxidation (anti-
oxidants) and the methods of storage determine their stability.
Vegetable fats are usually more stable than some of the animal fats, such as lard even
though the degree of un-saturation of the vegetable oil may be greater.
Rancidity
Rancidity is the complete or incomplete oxidation or hydrolysis of fats and oils when exposed to
air, light, moisture or by bacterial action, resulting in unpleasant taste and odor.
The change that a lipid undergoes, leading to an undesirable flavor and odour development
is known as rancidity.
This is brought about in three ways: hydrolysis, oxidation and microbes giving rise to
hydrolytic rancidity, oxidative rancidity and microbial rancidity.
1. Hydrolytic rancidity:
Hydrolytic rancidity occurs when the fat is broken up into free fatty acids and glycerol by
the presence of water.
Hydrolysis is catalyzed by acid, bases, enzymes or thermal effects.
When base is the hydrolyzing agent, the liberated fatty acids are converted into their salts
(soaps) and the hydrolysis is termed as “saponification”.
The other agents release free fatty acids and the reactions brought about by them are
collectively known as lipolysis, or “hydrolytic rancidity”.
The presence of the enzyme lipoprotein lipase (LPL) quickens this process.
2. Oxidative rancidity:
Oxidative rancidity is associated with the degradation of fat by oxygen.
The double bonds of an unsaturated fatty acid can undergo cleavage, through a free radical
process, releasing volatile aldehydes and ketones.
Antioxidants are often used as preservatives in fat-containing foods to delay the onset or
slow the development of rancidity due to oxidation.
Natural antioxidants include ascorbic acid (vitamin C) and tocopherols (vitamin E).
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SHIV BALI SINGH GROUP OF EDUCATIONAL AND TRAINING INSTITUTE
COLLEGE OF PHARMACY
Pharmaceutical Organic Chemistry-II (UNIT III)
Synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene
(BHT), tert-Butylhydroquinone (TBHQ), propyl gallate and ethoxyquin.
Mechanism of oxidative rancidity:
The mechanism of oxidative rancidity has been extensively studied.
Rancidity takes place through auto-oxidation because the rate of oxidation increases as the
reaction proceeds.
Oxidation takes place through a free-radical chain mechanism involving three stages:
3. Microbial: Microbial rancidity refers to a process in which microorganisms, such as
bacteria or molds, use their enzymes such as lipases to break down fat.
