ORGANIC CHEMISTRY

Module 8.Fatty acids and lipids

Lesson 27B
STRUCTURE AND PROPERTIES OF NEUTRAL LIPIDS, PHOSPHOLIPIDS AND CHOLESTEROL

27.B.1 Sterols

• Steroids-class of compounds-have parent nucleus of perhydrocyclopentanophenanthrene. (Fig. 27.12)
• Nucleus- consists of 3 six-membered rings (A,B and C) and 1 five-membered ring (D)
• All 4 rings joined together-give 17 carbon compound
• Most natural steroids- have methyl groups (-CH₃) at positions C₁₀ and C₁₃
• Many natural steroids- have one or more alcoholic hydroxyl (-OH) group in the ring structure-known as- sterols-bile acids, sex hormones etc.
• They usually have side chain at position C₁₇

27.B.1.1 Phytosterols

• Occur in oils of higher plants
• Include stigmasterol and its isomers (sitosterols) (Fig. 27.13)
• Difference between stimasterol and cholesterol – in side chain at C₁₇
• Stigmasterol contains

1. Ethyl group (-C₂H₅) at C₂₄
2. Double bond between C₂₂ and C₂₃

• Serves as a marker for detection of added vegetable oil/fat in milk fat (ghee)

27.B.1.2 Cholesterol

• Animal sterols – therefore zoo sterols- not found in plants
• Several sterols occur in animal kingdom, but cholesterol is the most abundant (Fig. 27.14)

27.B.1.2.1 Properties

• Crystalline as white, shining rhombic plate- M.P. 149-150°C
• Tasteless and odourless
• Optically active, has sp. Rotation of -39.5° in chloroform
• Readily soluble in ether, benzene, chloroform, acetone, carbon disulfide, petroleum ether and hot alcohol and also in oleic acid and liquid fat
• Soluble in cold in cold ethyl acetate, whereas, other lipids are not soluble-used for its separation
• Differentiation between cholesterol and phytosterol- based on difference in m.p. of acetate derivatives

Cholesterol acetate- 114°C
Phytosterol acetate – 128 to 137°C

27.B.1.2.2 Importance

1. Plays important role in cell permeability
2. Protective and insulating agent for nerve tissues
3. Precursors for some hormons, provitamins, bile acids
4. Involved in some disorders- atherosclerosis –leads to heart attack and strock
5. Provides base for detection of added vegetable oil/fat in milk fat (ghee)