Organic Chemistry 5 th Edition L G Wade
- Slides: 49
Organic Chemistry, 5 th Edition L. G. Wade, Jr. Chapter 23 Carbohydrates and Nucleic Acids Jo Blackburn Richland College, Dallas, TX Dallas County Community College District Chapter 23 ã 2003, Prentice Hall
Carbohydrates • Synthesized by plants using sunlight to convert CO 2 and H 2 O to glucose and O 2. • Polymers include starch and cellulose. • Starch is storage unit for solar energy. • Most sugars have formula Cn(H 2 O)n, “hydrate of carbon. ” => Chapter 23 2
Classification of Carbohydrates • Monosaccharides or simple sugars Øpolyhydroxyaldehydes or aldoses Øpolyhydroxyketones or ketoses • Disaccharides can be hydrolyzed to two monosaccharides. • Polysaccharides hydrolyze to many monosaccharide units. E. g. , starch and cellulose have > 1000 glucose units. => Chapter 23 3
Monosaccharides • Classified by: Øaldose or ketose Ønumber of carbons in chain Øconfiguration of chiral carbon farthest from the carbonyl group glucose, a D-aldohexose fructose, a D-ketohexose Chapter 23 => 4
D and L Sugars sugars can be degraded to the dextrorotatory (+) form of glyceraldehyde. L sugars can be degraded to the levorotatory (-) form of glyceraldehyde. • D • Chapter 23 5 =>
The D Aldose Family Chapter 23 6 =>
Erythro and Threo • Terms used for diastereomers with two adjacent chiral C’s, without symmetric ends. • For symmetric molecules, use meso or d, l. => Chapter 23 7
Epimers Sugars that differ only in their stereochemistry at a single carbon. => Chapter 23 8
Cyclic Structure for Glucose cyclic hemiacetal formed by reaction of -CHO with -OH on C 5. => Chapter 23 D-glucopyranose 9
Cyclic Structure for Fructose Cyclic hemiacetal formed by reaction of C=O at C 2 with -OH at C 5. D-fructofuranose Chapter 23 10 =>
Anomers Chapter 23 11 =>
Mutarotation Glucose also called dextrose; dextrorotatory. Chapter 23 => 12
Epimerization In base, H on C 2 may be removed to form enolate ion. Reprotonation may change the stereochemistry of C 2. => Chapter 23 13
Enediol Rearrangement In base, the position of the C=O can shift. Chemists use acidic or neutral solutions of sugars to preserve their identity. Chapter 23 14 =>
Reduction of Simple Sugars • C=O of aldoses or ketoses can be reduced to C-OH by Na. BH 4 or H 2/Ni. • Name the sugar alcohol by adding -itol to the root name of the sugar. • Reduction of D-glucose produces D-glucitol, commonly called D-sorbitol. • Reduction of D-fructose produces a mixture of D-glucitol and D-mannitol. =>15 Chapter 23
Oxidation by Bromine water oxidizes aldehyde, but not ketone or alcohol; forms aldonic acid. Chapter 23 16 =>
Oxidation by Nitric Acid Nitric acid oxidizes the aldehyde and the terminal alcohol; forms aldaric acid. Chapter 23 17 =>
Oxidation by Tollens Reagent • Tollens reagent reacts with aldehyde, but the base promotes enediol rearrangements, so ketoses react too. • Sugars that give a silver mirror with Tollens are called reducing sugars. => Chapter 23 18
Nonreducing Sugars • Glycosides are acetals, stable in base, so they do not react with Tollens reagent. • Disaccharides and polysaccharides are also acetals, nonreducing sugars. => Chapter 23 19
Formation of Glycosides • React the sugar with alcohol in acid. • Since the open chain sugar is in equilibrium with its - and -hemiacetal, both anomers of the acetal are formed. • Aglycone is the term used for the group bonded to the anomeric carbon. => Chapter 23 20
Ether Formation • Sugars are difficult to recrystallize from water because of their high solubility. • Convert all -OH groups to -OR, using a modified Williamson synthesis, after converting sugar to acetal, stable in base. Chapter 23 21 =>
Ester Formation Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters. => Chapter 23 22
Osazone Formation Both C 1 and C 2 react with phenylhydrazine. => Chapter 23 23
Ruff Degradation Aldose chain is shortened by oxidizing the aldehyde to -COOH, then decarboxylation. => Chapter 23 24
Kiliani-Fischer Synthesis • This process lengthens the aldose chain. • A mixture of C 2 epimers is formed. Chapter 23 25 =>
Fischer’s Proof • Emil Fischer determined the configuration around each chiral carbon in D-glucose in 1891, using Ruff degradation and oxidation reactions. • He assumed that the -OH is on the right in the Fischer projection for D-glyceraldehyde. • This guess turned out to be correct! => Chapter 23 26
Determination of Ring Size • Haworth determined the pyranose structure of glucose in 1926. • The anomeric carbon can be found by methylation of the -OH’s, then hydrolysis. => Chapter 23 27
Periodic Acid Cleavage • Periodic acid cleaves vicinal diols to give two carbonyl compounds. • Separation and identification of the products determine the size of the ring. Chapter 23 28 =>
Disaccharides • Three naturally occurring glycosidic linkages: • 1 -4’ link: The anomeric carbon is bonded to oxygen on C 4 of second sugar. • 1 -6’ link: The anomeric carbon is bonded to oxygen on C 6 of second sugar. • 1 -1’ link: The anomeric carbons of the two sugars are bonded through an oxygen. => Chapter 23 29
Cellobiose • Two glucose units linked 1 -4’. • Disaccharide of cellulose. • A mutarotating, reducing sugar. Chapter 23 30 =>
Maltose Two glucose units linked 1 -4’. => Chapter 23 31
Lactose • Galactose + glucose linked 1 -4’. • “Milk sugar. ” => Chapter 23 32
Gentiobiose • Two glucose units linked 1 -6’. • Rare for disaccharides, but commonly seen as branch point in carbohydrates. => Chapter 23 33
Sucrose • Glucose + fructose, linked 1 -1’ • Nonreducing sugar => Chapter 23 34
Cellulose • Polymer of D-glucose, found in plants. • Mammals lack the -glycosidase enzyme. => Chapter 23 35
Amylose • Soluble starch, polymer of D-glucose. • Starch-iodide complex, deep blue. => Chapter 23 36
Amylopectin Branched, insoluble fraction of starch. => Chapter 23 37
Glycogen • Glucose polymer, similar to amylopectin, but even more highly branched. • Energy storage in muscle tissue and liver. • The many branched ends provide a quick means of putting glucose into the blood. => Chapter 23 38
Chitin • Polymer of N-acetylglucosamine. • Exoskeleton of insects. => Chapter 23 39
Nucleic Acids • Polymer of ribofuranoside rings linked by phosphate ester groups. • Each ribose is bonded to a base. • Ribonucleic acid (RNA) • Deoxyribonucleic acid (DNA) Chapter 23 => 40
Ribonucleosides A -D-ribofuranoside bonded to a heterocyclic base at the anomeric carbon. => Chapter 23 41
Ribonucleotides Add phosphate at 5’ carbon. Chapter 23 42
Structure of RNA Chapter 23 => 43
Structure of DNA • -D-2 -deoxyribofuranose is the sugar. • Heterocyclic bases are cytosine, thymine (instead of uracil), adenine, and guanine. • Linked by phosphate ester groups to form the primary structure. => Chapter 23 44
Base Pairings Chapter 23 45 =>
Double Helix of DNA • Two complementary polynucleotide chains are coiled into a helix. • Described by Watson and Crick, 1953. => Chapter 23 46
DNA Replication => Chapter 23 47
Additional Nucleotides • Adenosine monophosphate (AMP), a regulatory hormone. • Nicotinamide adenine dinucleotide (NAD), a coenzyme. • Adenosine triphosphate (ATP), an energy source. => Chapter 23 48
End of Chapter 23 49
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