24.1 Introduction to Carbohydrates - · PDF file•Carbohydrates are NOT true hydrates. WHY? ... at the chiral carbon farthest from the carbonyl. ... •The equilibrium greatly favors

• Carbohydrates (sugars) are abundant in nature:

– They are high energy biomolecules.

– They provide structural rigidity for organisms (plants, crustaceans, etc.).

– The polymer backbone on which DNA and RNA are assembled contains sugars.

• The term, carbohydrate, evolved to describe the formula for such molecules: Cx(H2O)x.

• Carbohydrates are NOT true hydrates. WHY?

24.1 Introduction to Carbohydrates

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-1

• Carbohydrates (sugars) are polyhydroxy aldehydes or ketones.

– Consider glucose, which is made by plants:

– Describe the potential energy change that occurs during glucose photosynthesis.

– Is glucose a polyhydroxy aldehyde or ketone?


• Saccharides have multiple chiral centers, and they are often drawn as Fischer projections.

– Designate each chirality center in glucose as either R or S.

24.2 Classification of Monosaccharides


• Saccharides have multiple chiral centers, and they are often drawn as Fischer projections.

• What does the suffix, “ose” mean?

• Define the following terms:

– Aldose and ketose

– Pentose and hexose


• Glyceraldehyde is a monosaccharide with one chirality center.

– Natural glyceraldehyde is dextrorotatory (D): it rotates plane polarized light in the clockwise direction.


• Naturally occurring larger sugars can be broken down into glyceraldehyde by degradation.

• Such sugars are often called D-sugars.



• Recall that dextrorotatory versus levorotatory rotation cannot be predicted by the R or S configuration.

• Here, D no longer refers to dextrorotatory. Rather it refers to the R configuration at the chiral carbon farthest from the carbonyl.



• There are four aldotetroses. Two are shown below.

• What are the other two structures?

24.3 Configuration of Aldoses


• Aldopentoses have three chirality centers. The number of isomers will be 23.

• Recall the 2n rule from Section 5.5.

• The D-sugars are naturally occurring.



• Ribose is a key building block of RNA.

– WHAT is RNA? More detail to come in Section 24.10.

• Arabinose is found in plants.

• Xylose is found in wood.



• Based on the 2n rule, how many aldohexoses are there?

• How many of the aldohexoses are D isomers.

• Glucose is the most common aldohexose.

• Mannose and galactose are also common.



• Relevant ketoses have between three and six carbons.

• For each naturally occurring D isomer, there is an L enantiomer.

24.4 Configuration of Ketoses


24.4 Configuration of Ketoses


• Recall from Section 20.5 that carbonyls can be attacked by alcohols to form hemiacetals.

– The intramolecular reaction is generally favored for 5 and 6-membered rings. WHY?

24.5 Cyclic Structures of Monosaccharides


• For the following compound, draw the mechanism and resulting product that results from acid catalyzed ring-closing hemiacetal formation.



• Monosaccharides, like glucose, can also undergo ring-closing hemiacetal formation.

• The equilibrium greatly favors the closed form called pyranose.




• Distinguish between the α and β anomers.


Anomeric effect

• Which would you predict to be more stable?

– Beta 67% , alpha 33%, open 0.01%

19

• Ketoses form both furanose (5-membered) and pyranose (6-membered) rings:





70% β 0.7% 23%-β

2% α 5%-α

• The equilibrium concentrations in water are above.

• The furanose form takes part in most biochemical reactions.



• Monosaccharides are generally soluble in water. WHY?

• To improve their solubility in organic solvents, the hydroxyl groups can be acetylated.

• WHY is pyridine added to the reaction?

• How might acetylation help in purification efforts.

24.6 Reactions of Monosaccharides


• Monosaccharides can also be converted to ethers via the Williamson ether synthesis.

• Ether linkages are more robust than ester linkages. WHY?



• When treated with an excess of an alcohol, the hemiacetal equilibrium can be shifted to give an acetal.

• When a sugar is used, alpha and beta glycosides are formed.



Copyright 2012 John Wiley & Sons, Inc.

• The mechanism of glycoside formation is analogous to the acetal formation mechanism.

• Only the anomeric hydroxyl group is replaced.


Klein, Organic Chemistry 1e 24-30

• The mechanism of glycoside formation is analogous to the acetal formation mechanism.

• What factors would you consider when trying to predict whether the alpha or beta anomer will be the major product?

• Practice with CONCEPTUAL CHECKPOINTs 24.28 and 24.29.



• Under strongly basic conditions, glucose and mannose interconvert.

• Mannose and glucose are epimers because they only differ in the configuration of one carbon center.

• Practice with CONCEPTUAL CHECKPOINT 24.30.



• Monosaccharides can be reduced to ALDITOLs shifting the equilibrium to the right. HOW?

– D-sorbitol or D-glucitol are sugar substitutes.



• If the sugar has an –OH attached to the anomeric carbon, then the sugar is a reducing sugar

• If it has –OR, then it is not a reducing sugar

Reducing sugars

Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24 -34

• Practice with SKILLBUILDER 24.4.



• Disaccharides form when two sugars connect through a glycosidic linkage.

– The 1 4 glycosidic linkage is most common.

– The bottom ring is capable of mutarotation at its anomeric position.

– Because the anomeric position of the bottom ring is a HEMIACETAL rather than an acetal, it is in equilibrium with the open form. Thus, maltose is a reducing sugar.

24.7 Disaccharides


• Cellobiose is similar to maltose. WHAT are the differences?

• Will cellobiose be a reducing sugar?

24.7 Disaccharides


• Lactose is another disaccharide.

• Some people have trouble digesting lactose.

24.7 Disaccharides


• Sucrose (table sugar) is also a disaccharide.

– Honey bees can convert sucrose into a mixture of sucrose, fructose, and glucose.

– Fructose is very sweet.

• Sucrose is not a reducing sugar. WHY?

24.7 Disaccharides


• Cellulose is a polysaccharide containing 7000–12000 glucose units connected through glycosidic bonds.

• How is cellulose capable of giving plants like trees their rigidity and strength?

24.8 Polysaccharides


• Starch is a major components of grains and other foods, like potatoes.

• What is the difference between molecules of starch and molecules of cellulose?

• Starch is made of amylose and amylopectin.



• Amylopectin has some 16-α-glycoside branches.

• We can eat corn and potatoes, but not grass or trees. WHY?



• Amino sugars like glucosamine are important biomolecules.

• Acetylated glucosamine can form an

important polysaccharide called chitin.

24.9 Amino Sugars


• The carbonyl groups in chitin allow for even stronger H-bonding between neighboring chains.

• Chitin is used in insect and arthropod exoskeletons. WHY?

24.9 Amino Sugars


• N-glycosides can be formed when sugars are treated with an amine and an acid catalyst.

• RNA and DNA incorporate important N-glycosides called nucleosides.

24.10 N-Glycosides


• Ribose forms ribonucleosides in RNA.

• Deoxyribose forms deoxyribonucleosides in DNA.

24.10 N-Glycosides


• There are four different heterocyclic amines that attach to deoxyribose molecules to form DNA nucleosides.

24.10 N-Glycosides


• In DNA, the nucleosides are attached to phosphate groups forming nucleotides.

24.10 N-Glycosides


• The phosphate groups of the nucleotides are connected together to make the DNA strand or POLYNUCLEOTIDE.

24.10 N-Glycosides


• The nucleotides in DNA can attract one another through H-bonding of the DNA base pairs.

24.10 N-Glycosides


• WHY does DNA form a double helix?

24.10 N-Glycosides


• RNA is structurally different from DNA :

– The sugar in RNA is ribose. WHAT is the sugar in DNA?

– RNA contains uracil instead of thymine.

• RNA translates the information stored in DNA into working molecules (proteins and enzymes).

24.10 N-Glycosides


• RNA strands generally do not form double helices like DNA.

• RNA strands can fold into many different shapes, and some even act as catalysts called ribozymes.

• It is possible that RNA evolved self-replication as an early step in the evolution of life from small molecules.

24.10 N-Glycosides


24.1 Introduction to Carbohydrates - · PDF file•Carbohydrates are NOT true hydrates. WHY? ... at the chiral carbon farthest from the carbonyl. ... •The equilibrium greatly favors

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