Chapter 12 Carbohydrates

Chapter 12Carbohydrates

Chapter 13 2

Carbohydrates

• Synthesized by plants using sunlight to convert CO2 and H2O to glucose and O2.

• Polymers include starch and cellulose.• Starch is storage unit for solar energy.• Most sugars have formula Cn(H2O)n, “hydrate

of carbon.”

Chapter 12 3

Classification of Carbohydrates

• Monosaccharides or simple sugars– polyhydroxyaldehydes or aldoses– polyhydroxyketones 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 12 4

Monosaccharides• Classified by:– aldose or ketose– number of carbons in chain– configuration of chiral carbon farthest from the

carbonyl group

glucose, a D-aldohexose

fructose, a D-ketohexose =>

Chapter 12 5

D and L Sugars• D sugars can be degraded to the

dextrorotatory (+) form of glyceraldehyde.• L sugars can be degraded to the levorotatory

(-) form of glyceraldehyde.

Chapter 23 6

The D Aldose Family

Chapter 12 7

EpimersSugars that differ only in their

stereochemistry at a single carbon.

Chapter 12 8

Cyclic Structure for GlucoseGlucose cyclic hemiacetal formed by reaction

of -CHO with -OH on C5.

D-glucopyranose

Chapter 12 9

Cyclic Structure for FructoseCyclic hemiacetal formed by reaction of C=O

at C2 with -OH at C5.

D-fructofuranose

Chapter 12 10

Anomers

Chapter 12 11

Mutarotation

Glucose also called dextrose; dextrorotatory.

Chapter 12 12

EpimerizationIn base, H on C2 may be removed to form

enolate ion. Reprotonation may change the stereochemistry of C2.

Chapter 12 13

Reduction of Simple Sugars

• C=O of aldoses or ketoses can be reduced to C-OH by NaBH4 or H2/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.

Chapter 12 14

Oxidation by Nitric AcidNitric acid oxidizes the aldehyde and the

terminal alcohol; forms aldaric acid.

Chapter 12 15

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 16

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 17

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 18

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 12 19

Ester Formation

Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters.

Chapter 23 20

Osazone FormationBoth C1 and C2 react with phenylhydrazine.

Chapter 12 21

Kiliani-Fischer Synthesis• This process lengthens the aldose chain.• A mixture of C2 epimers is formed.

Chapter 12 22

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 12 23

Disaccharides

• Three naturally occurring glycosidic linkages:• 1-4’ link: The anomeric carbon is bonded

to oxygen on C4 of second sugar.• 1-6’ link: The anomeric carbon is bonded

to oxygen on C6 of second sugar.• 1-1’ link: The anomeric carbons of the two

sugars are bonded through an oxygen.

Chapter 12 24

Cellobiose• Two glucose units linked 1-4’.• Disaccharide of cellulose.• A mutarotating, reducing sugar.

Chapter 12 25

MaltoseTwo glucose units linked 1-4’.

Chapter 12 26

Lactose• Galactose + glucose linked 1-4’.• “Milk sugar.”

Chapter 12 27

Sucrose• Glucose + fructose, linked 1-1’• Nonreducing sugar

Chapter 12 28

Cellulose

• Polymer of D-glucose, found in plants.• Mammals lack the -glycosidase enzyme.

Chapter 12 29

Amylose• Soluble starch, polymer of D-glucose.• Starch-iodide complex, deep blue.

Chapter 12 30

AmylopectinBranched, insoluble fraction of starch.

Chapter 12 31

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.

32

End of Chapter 12