Organic Chemistry

Module 8.Fatty acids and lipids

Lesson 25
DEFINITION AND CLASSIFICATION

25.1 Introduction

• Various terms- used interchangeably for lipids.

Lipids - Lipins - Lipoids- Fats - oils i.e. used synonymously.

• Term ‘lipid’ - coined by Bloor
• Denotes the class of compounds which are insoluble in water but soluble in usual organic solvents like alcohol, ether, chloroform, benzene etc.

25.2 Importance (functions) of Lipids

1. Rich source of energy - 9 kcal/g
2. Serves as a carrier of fat soluble vitamins - A,D, E and K
3. Carrier of essential fatty acids
4. Structural component of cell wall- play important role in cell permeability
5. Structural component of nerve tissues - protect them

25.3 Chemically What is Lipid, Fat and Oil?

• When alcohol and acid react - ester is formed

Fig. 25.1 Formation of ester from alcohol

• Trihydric alcohol (Glycerol) -esterified with carboxylic acids - ester - known as glyceride.

Fig. 25.2 Formation of Glyceride from Glycerol



Fig. 25.3 Forms of Glyceride

• Fats or oils - chemically glycerides- an ester of fatty acids with glycerol.
• The difference between them - physical state - solid or liquid - at room temperature
• The difference - due to difference in their fatty acids make up

• relatively more unsaturated fatty acids content in oil - therefore low melting point – therefore liquid
• relatively less unsaturated fatty acids content in fat - therefore high melting point - therefore solid

• Oils and fats - synthesize - plants or animals

• Some other organic compounds also synthesize and those which are oil/fat soluble get associated with them (become soluble in oil/fat)

• Include- sterols – cholesterol (animal) and phytosterol (plant)
• Vitamins – A,D,E,K
• Hydrocarbons- squelene
• Pigments-carotenoids- carotenes- β-carotene
• Waxes, long chain alcohols and acids etc

• Glycerides (fats/oils) with associated compounds- collectively known as ‘lipid’

Therefore Lipid = Glyceride + Glyceride soluble compounds
(fat/oil)

25.4 Classification of Lipids

• Lipids – classified on several basis- complexity, saponification, polarity etc.
  

25.4.1 Based on complexity

Three groups - simple, complex and derived lipids

Table 25.1 Three groups - simple, complex and derived lipids

25.4.2 Based on polarity

Two groups - polar and non-polar

1. Polar lipids

• They are charged molecules
• Soluble in polar solvents - alcohol, chloroform etc
• e.g. phospholipids, fatty acids etc

2. Non-polar lipids

• They are uncharged molecules
• Soluble in non-polar solvents- ether, benzene, hexane, etc
• e.g. Glycerides, cholesterol esters, vitamins

25.4.3 Based on saponification

Ability to react with alkality to give soap
Two groups - saponifiable and unsaponifiable

1. Saponifiable lipids

• React with alkali and form soap
• Present in large amount
• e.g. Glycerides, phospholipids, fatty acids, cholesterol

2. Unsaponifiable lipids

• Do not react with alkali to form soap
• Present in relatively small amount
• e.g. vitamins, sterols, hydrocarbons, carbonyls etc.

25.5 Traditional Classification of Edible Fats/Oils

Based on source of fat/oil and constituent fatty acids

25.5.1 Milk fat

• Fats of this group are derived from milk of mammals, particularly from cows and buffalo
• Majority of fatty acids of milk fat are palmitic (C_16:0), stearic (C_18:0) & oleic (C_18:1)
• This fat is unique in that it contains appreciable amounts of short chain fatty acids (C_4:0, C_6:0, C_8:0 ,C_10:0)
• Butyric acid (C_4:0) is unique to milk fat

25.5.2 Lauryl or lauric acid fat

• Fat of this group are derived from certain species of palm - such as coconut and babasu
• These fats are characterized by their high amounts (content) of lauric acid - 40 to 50%
• Contain moderate amount of C_6:0, C_8:0 ,C_10:0 fatty acids
• Content of unsaturated fatty acids is low
• Even though melting point is low- due to short chain fatty acids

25.5.3 Vegetable butters

• Fats of this groups are derived from the seeds of various tropical trees.
• They are distinguished by their narrow melting range - which is mainly due to arrangement of fatty acids in the triacylglycerol molecules
• They are extensively used in the manufacture of confections.
• Cocoa butter being the most important member of this group

25.5.4 Oleic –linoleic acids fats

• Fat in this group are the most abundant
• Vegetable origin and contain large amounts of oleic and linoleic acids and less than 20% saturated fatty acids
• The most important members of this group are cottonseed, corn, peanut, sunflower, palm olive and sesame oils

25.5.5 Linolenic acid fats

• Fat in this group contain substantial amount of linolenic acid
• Examples are soybean, rapeseed and flexseed, wheat germ, hempseed and perilla oils; with soybean being the most important.
• The abundance of linolenic acid in soybean oils is responsible for the development of an off-flavour problem known as flavour reversion

25.5.6 Animal body fats

• This group consists of depot fats from domestic land animals e.g. lard and tallow.
• All contain large amounts of C₁₆>C₁₈ fatty acids, medium amounts of unsaturated fatty acids (mostly C_18:1> C_18:2) and small amount of odd-numbered acids
• Also contain appreciable amounts of fully saturated triacylglycerol and exhibits relatively high melting points
• Egg lipids are of particular importance because of their emulsifying properties and their high content of cholesterol
• This lipid content whole egg is approx. 12%, most exclusively present in the yolk
• The yolk contains 32-36 % lipids
• Yolk lipids consist of about 66 % triacylglycerol and 28% phospholipids and 5% cholesterol
• The major phospholipids of egg yolk are phosphoatidylcholine (73%) and phosphatidylethanolamine (18%)

25.5.7 Marine oils

• These oils typically contain large amounts of long-chain omega-3-polyunsaturated fatty acids, with upto six double bonds
• They usually rich in vitamins A & D.