Introduction to Organic Chemistry 2 ed William H. Brown

1. Introduction to Organic Chemistry2 edWilliam H. Brown

2. Lipids Chapter 17

3. Lipids • Lipids: a heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties • insoluble in water • soluble in aprotic organic solvents including diethyl ether, methylene chloride, and acetone • Lipids include • triglycerides, phospholipids, prostaglandins, and fat-soluble vitamins • cholesterol, steroid hormones, and bile acids

4. Triglycerides • Triglyceride: an ester of glycerol with three fatty acids

5. Fatty Acids • Fatty acid: a long, unbranched chain carboxylic acid, most commonly of 12 - 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or the phospholipids of biological membranes • In the shorthand notation for fatty acids • the number of carbons and the number of double bonds in the chain are shown by two numbers, separated by a colon

6. Fatty Acids

7. Fatty Acids • Among the fatty acids most abundant in plants and animals • nearly all have an even number of carbon atoms, most between 12 and 20, in an unbranched chain • the three most abundant are palmitic (16:0), stearic acid (18:0), and oleic acid (18:1) • in most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare • unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point

8. Fatty Acids • Stearic and linolenic acids

9. Triglycerides • Physical properties of triglycerides depend on their fatty acid components • melting point increases as the number of carbons in their hydrocarbon chains increases and as the number of double bonds decreases • triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils • triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats • The lower melting points of triglycerides rich in unsaturated fatty acids are related to differences in their three-dimensional shape

10. Triglycerides • A saturated triglyceride

11. Triglycerides • Triglycerides rich in saturated fatty acids: • saturated hydrocarbon chains can lie parallel and there are strong dispersion forces between their chains • they pack into well-ordered, compact crystalline forms and have melting points above room temperature • Triglycerides rich in saturated fatty acids: • because of the cis configuration of their double bonds, their hydrocarbon chains have a less ordered structure • dispersion forces between hydrocarbon chains are weaker; these triglycerides have melting points below room temperature

12. Soaps and Detergents • Natural soaps are prepared by boiling lard or other animal fat with NaOH, in a reaction called saponification (Latin, sapo, soap)

13. Soaps and Detergents • Soaps clean by acting as emulsifying agents • the long hydrophobic hydrocarbon chains of soaps are insoluble in water and tend to cluster in such a way as to minimize their contact with water • the polar hydrophilic carboxylate groups tend to remain in contact with the surrounding water molecules • driven by these two forces, soap molecules spontaneously cluster into micelles

14. Soaps and Detergents • Micelle: a spherical arrangement of organic molecules in water clustered so that their hydrophobic parts are buried inside the sphere and their hydrophilic parts are on the surface of the sphere and in contact with water • When soap is mixed with water-insoluble grease, oil, and fat stains, the nonpolar parts of the soap micelles “dissolve” nonpolar dirt molecules and they are carried away in the polar wash water

15. Soaps and Detergents • Soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water)

16. Synthetic Detergents • The design criteria for a good detergent are • a long hydrocarbon tail of 12 to 20 carbons • a polar head group that does not form insoluble salts with Ca(II), Mg(II), or Fe(III) ions • The most widely used synthetic detergents are the linear alkylbenzene sulfonates (LAS)

17. Soaps and Detergents • Also added to detergent preparations are • foam stabilizers • bleaches • optical brighteners

18. Prostaglandins • Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid

19. Prostaglandins • Prostaglandins are not stored in tissues as such, but are synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers • one such polyunsaturated fatty acid is arachidonic acid

20. Prostaglandins • among the prostaglandins synthesized from arachidonic acid are

21. Prostaglandins • Research on the involvement of PGs in reproductive physiology has produced several clinically useful derivatives • 15-Methyl-PGF2a is used as a therapeutic abortifacient

22. Prostaglandins • the PGE1 analog, misoprostol, is used for prevention of ulceration associated with the use of aspirin-like NSAIDs

23. Steroids • Steroids: a group of plant and animal lipids that have this tetracyclic ring structure

24. Common Features • the fusion of rings is trans and each atom or group of atoms at a ring junction is axial • the pattern of atoms or groups of atoms along the ring junctions is nearly always trans-anti-trans-anti-trans • the steroid system is nearly flat and quite rigid • many have axial methyl groups at C-10 and C-13

25. Cholesterol

26. Androgens • Androgens - male sex hormones • synthesized in the testes • responsible for the development of male secondary sex characteristics

27. Anabolic Steroids • Among the synthetic anabolic steroids are

28. Estrogens • Estrogens - female sex hormones • synthesized in the ovaries • responsible for the development of female secondary sex characteristics and control of the menstrual cycle

29. Synthetic Estrogens • Progesterone-like analogs are used in oral contraceptives

30. Glucorticoid Hormones • synthesized in the adrenal cortex • regulate metabolism of carbohydrates • decrease inflammation • involved in the reaction to stress

31. Mineralocorticoid Horm. • synthesized in the adrenal cortex • regulates blood pressure and volume by stimulating the kidneys to absorb Na+ , Cl-, and HCO3-

32. Bile Acids • synthesized in the liver, stored in the gallbladder, and secreted into the intestine where • their function is emulsify dietary fats and aid in their absorption and digestion

33. Biosynthesis of Steroids • all carbon atoms of cholesterol are derived from the two carbon acetyl group of acetyl-CoA • cholesterol is, in turn, the starting material for the synthesis of these classes of compounds

34. Phospholipids • Phospholipids are the second most abundant group of naturally occurring lipids • they are found almost exclusively in plant and animal membranes, which typically consist of 40% -50% phospholipids and 50% - 60% proteins • the most abundant phospholipids are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid • the three most abundant fatty acids in phosphatidic acids are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1)

35. Phospholipids • A phosphatidic acid • Further esterification with a low-molecular weight alcohol gives a phospholipid • among the most common of these low-molecular-weight alcohols are

36. Phospholipids

37. Phospholipids • A lecithin • in aqueous solution, phospholipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers

38. A Lecithin

39. Biological Membranes • Fluid mosaic model: a biological membrane consists of a phospholipid bilayer with proteins, carbohydrates, and other lipids embedded on the surface and in the bilayer • fluid signifies that the protein components of membranes “float”in the bilayer and can move freely along the plane of the membrane • mosaic signifies that the various components of the membrane exist side-by-side, as discrete units rather than combining to form new molecules and ions

40. Fat-Soluble Vitamins • Vitamins are divided into two broad classes on the basis of their solubility • those that are fat-soluble (and hence classified as lipids • those that are water-soluble • The fat-soluble vitamins include A, D, E, and K

41. Vitamin A • Vitamin A, or retinol, occurs only in the animal world • Vitamin A is found in the plant world in the form of a provitamin in a group of pigments called carotenes (tetraterpenes) • enzyme-catalyzed cleavage of b-carotene followed by reduction gives two molecules of vitamin A

42. Vitamin A

43. Vitamin A • The best understood role of Vitamin A is its participation in the visual cycle in rod cells • the active molecule is retinal (vitamin A aldehyde), which forms an imine with an -NH2 group of the protein opsin to form the visual pigment called rhodopsin • the primary chemical event of vision in rod cells is absorption of light by rhodopsin followed by isomerization of the 11-cis double bond to the 11-trans configuration

44. Vitamin A

45. Vitamin D • A group of structurally related compounds that play a role in the regulation of calcium and phosphorus metabolism • the most abundant form in the circulatory system is vitamin D3

46. Vitamin E • Vitamin E: a group of compounds of similar structure • the most active is a-tocopherol

47. Vitamin E • In the body, vitamin E functions as an antioxidant; it traps peroxy radicals of the type HOO• and ROO• formed as a result of oxidation by O2 of unsaturated hydrocarbon chains in membrane phospholipids

48. Vitamin K • The name of this vitamin comes from the German word Koagulation, signifying its important role in the blood-clotting process

49. Lipids End Chapter 17