Organic Chemistry, 9th Edition L. G. Wade, Jr. Chapter 25 Lecture Lipids Chad Snyder, PhD Grace College
2017 Pearson Education, Inc. 2017 Pearson Education, Inc. Introduction Lipids are compounds that can be extracted from cells by nonpolar organic solvents. Complex lipids are easily hydrolyzed. Long-chain esters are called fatty acids.
Simple lipids are not easily hydrolyzed in acid or base. Steroids Prostaglandins Terpenes 2017 Pearson Education, Inc.
Examples of Lipids 2017 Pearson Education, Inc. Waxes Esters of long-chain fatty acids with long-chain alcohols Commonly found in nature Spermaceti is found in the head of the sperm whale.
Beeswax is a mixture of waxes, hydrocarbons, and alcohols that bees use to make their honeycomb. Carnauba wax is a mixture of waxes of high molecular weights. 2017 Pearson Education, Inc. Fatty Acids Unbranched carboxylic acids with 1220 carbons
Most contain an even number of carbons because they are built from acetic acid units. Melting points increase with increasing molecular weights. Unsaturation greatly lowers the melting point. 2017 Pearson Education, Inc. Glycerides
Fatty acid esters of the triol glycerol Triglycerides are the most common glycerides, and they are used for long-term energy storage in plants and animals. Fats Solid at room temperature Most are derived from mammals. Oils
Liquid at room temperature Most are derived from plants or cold-blooded animals. 2017 Pearson Education, Inc. Melting Points of Fatty Acids 2017 Pearson Education, Inc.
Melting Points The cis double bond in oleic acid lowers the melting point by 66 C. A cis double bond bends the molecule, so it cannot pack efficiently. A trans double bond has less effect. 2017 Pearson Education, Inc.
Fats and Oils Unsaturated triglycerides have lower melting points because their unsaturated fatty acids do not pack as well in a solid lattice. 2017 Pearson Education, Inc. 2017 Pearson Education, Inc.
Trans Fats Consumers are concerned with the presence of unnatural trans fatty acids in partially hydrogenated vegetable oil. The hydrogenation process can produce trans fats, which are associated with an increase in coronary heart disease. 2017 Pearson Education, Inc.
Biodiesel Most diesel engines can run on cooking oil once they are warm, but cooking oil is not sufficiently volatile to start a cold diesel engine. 2017 Pearson Education, Inc. Saponification
Base-catalyzed hydrolysis of ester linkages in fats and oils Long-chain carboxylate salts are known as soap. 2017 Pearson Education, Inc. Soap Structure: Micelles 2017 Pearson Education, Inc.
Grease in Soap Solution In water, soap forms a cloudy solution of micelles, with the hydrophilic heads in contact with water and the hydrophobic tails clustered in the interior.
The Na+ ions (not shown) are dissolved in the water surrounding the micelle. 2017 Pearson Education, Inc. Hard Water and Acid Scum Hard water is water that is acidic or that contains
ions of calcium, magnesium, or iron. Although these mineral-rich waters can be healthful for drinking, the ions react with soaps to form insoluble salts called hard-water scum. 2017 Pearson Education, Inc. Synthetic Detergents Synthetic detergents may have anionic, cationic,
or nonionic hydrophilic functional groups. 2017 Pearson Education, Inc. Phospholipids Phospholipids are lipids that contain groups derived from phosphoric acid. Phosphoglycerides usually have one phosphoric acid group and two fatty acids.
The phosphate may have an additional alcohol attached by an ester linkage. 2017 Pearson Education, Inc. Phosphatidic Acids Although it is often drawn in its acid form, a phosphatidic acid is actually deprotonated at neutral pH.
2017 Pearson Education, Inc. Lipid Bilayer Phosphoglycerides can aggregate into a bilayer membrane with their polar heads exposed to the aqueous solution and the hydrocarbon tails protected within. This lipid bilayer is an important part of the cell membrane. It restricts the flow of water and dissolved substances.
2017 Pearson Education, Inc. Steroids Steroids are classified as simple lipids because they do not undergo hydrolysis. Common structural features:
O or OH at C3 Side chain at C17 May also have a double bond from C5 to either C4 or C6 2017 Pearson Education, Inc. Cis- and Trans-Decalins
In trans-decalin, the two bonds to the second ring are trans to one another, and the hydrogens on the junction are also trans. In cis-decalin, the bonds to the second ring are cis, and the junction hydrogens are also cis. 2017 Pearson Education, Inc. Cis and Trans Steroids
Common steroids may have either a cis or a trans A-B ring junction. The other junctions are normally trans. 2017 Pearson Education, Inc. Cholesterol Common biological intermediate Probably a precursor to other steroids
Side chain at C17 and double bond at C5C6 2017 Pearson Education, Inc. Sex Hormones Testosterone is the most potent of the natural male sex hormones, and estradiol is the most potent natural female hormone. Notice that the female sex hormone differs from
the male hormone by its aromatic A ring. 2017 Pearson Education, Inc. Examples of Steroids Cortisol is the major natural hormone of the adrenal cortex. Fluocinolone acetonide is more potent for treating skin inflammation.
Beclomethasone is more potent for treating asthma. 2017 Pearson Education, Inc. Prostaglandins Biochemical regulators more powerful than steroids Cyclopentane ring with two long side chains trans to each other
Most have 20 carbon atoms. Derived from arachidonic acid 2017 Pearson Education, Inc. Biosynthesis of Prostaglandins Biosynthesis of prostaglandins begins with an enzyme-catalyzed oxidative cyclization of
arachidonic acid. 2017 Pearson Education, Inc. Terpenes Composed of five carbon isopentyl (isoprene) groups Isolated from the essential oils of plants Pleasant taste or fragrant aroma and used as
flavorings 2017 Pearson Education, Inc. Terpenes Terpenes are composed of two or more isoprene units. The isoprene units will maintain their isopentyl, usually with modification of the isoprene double bonds. 2017 Pearson Education, Inc.
Structure of Terpenes The isoprene molecule and the isoprene unit are said to have a head (the branched end) and a tail (the unbranched ethyl group). 2017 Pearson Education, Inc. Classification
Terpenes are classified by the number of carbons they contain in groups of 10. A monoterpene has 10 carbons and 2 isoprene units. A sesquiterpene has 15 carbons and 3 isoprene units. A diterpene has 20 carbons and 4 isoprene units. 2017 Pearson Education, Inc. Cholesterol
2017 Pearson Education, Inc. Terpenoids Many natural products are derived from terpenes, even though they do not have carbon skeletons composed exclusively of C5 isoprene units. These terpene-like compounds are called terpenoids.
Terpenoids may have been altered through rearrangements, loss of carbon atoms, or introduction of carbon atoms (example: cholesterol). 2017 Pearson Education, Inc.
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