Organic Chemistry Second Edition Chapter 24 David Klein Carbohydrates Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.1 Introduction to Carbohydrates Carbohydrates (sugars) are abundant in nature High energy biomolecules Provide structural rigidity for organisms (plants, crustaceans, etc.) The polymer backbone on which DNA and RNA are assembled Expressed on cells so they can recognize one another The term, carbohydrate, evolved to describe the formula for such molecules: Cx(H2O)x Carbohydrates are NOT true hydrates. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.1 Introduction to Carbohydrates 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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.2 Classification of Monosaccharides Saccharides have multiple chiral centers, and they are often expressed as Fischer projections checkpoints 24.1 and 24.2 What does the suffix, “ose” mean? Define the following terms: Aldose and ketose Pentose and hexose Practice conceptual Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.2 Classification of Monosaccharides Glyceraldehyde is a monosaccharide with one chirality center Natural glyceraldehyde is dextrorotatory (D) – it rotates plane polarized light in the clockwise direction Does the direction plane polarized light is rotated necessarily tell us whether it is (R) or (S)? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.2 Classification of Monosaccharides Naturally occurring larger sugars can be broken down into glyceraldehyde by degradation Such sugars are often called D sugars Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.2 Classification of Monosaccharides Recall that dextrorotatory versus levorotatory rotation can not be predicted by the R or S configuration Practice conceptual checkpoints 24.3 through 24.6 Here, D no longer refers to dextrorotatory. Rather it refers to the R configuration at the chiral carbon farthest from the carbonyl Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.3 Configuration of Aldoses There are four aldotetroses. Two are shown below What are the other two structures? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.3 Configuration of Aldoses 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.3 Configuration of Aldoses Based on the 2n rule, how many aldohexoses are there? How many of the aldohexoses are D isomers Glucose is the most common aldohexose Manose and Galactose are also common Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.4 Configuration of Ketoses Relevant ketoses have between 3 and 6 carbons For each naturally occurring D isomer, there is an L enantiomer Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.4 Configuration of Ketoses Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.5 Cyclic Structures of Monosaccharides Recall from section 20.5 that carbonyls can be attacked by alcohols to form hemiacetals The intermolecular reaction is not favored. WHY? The intramolecular reaction is generally favored for 5 and 6-membered rings. WHY? Practice with SkillBuilder 24.1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.5 Cyclic Structures of Monosaccharides Monosaccharides like glucose can also undergo ring-closing hemiacetal formation The equilibrium greatly favors the closed form called pyranose Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.5 Cyclic Structures of Monosaccharides What is the relationship between alpha and beta forms? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.5 Cyclic Structures of Monosaccharides Distinguish between the α and β anomers [a]D = +112.2˚ [a]D = +18.7˚ Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.5 Cyclic Structures of Monosaccharides You should be able to draw monosaccharide structures in their various forms Practice with SkillBuilder 24.3 Draw the chair form D-Galactose Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.5 Cyclic Structures of Monosaccharides Ketoses form both furanose (5-membered) and pyranose (6-membered) rings Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.5 Cyclic Structures of Monosaccharides The furanose form takes part in most biochemical reactions Practice with conceptual checkpoints 24.22 through 24.25 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.6 Reactions of Monosaccharides 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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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? Practice with conceptual checkpoints 24.26 and 24.27 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.6 Reactions of Monosaccharides 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 © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.6 Reactions of Monosaccharides The mechanism of glycoside formation is analogous to the acetal formation mechanism Only the anomeric hydroxyl group is replaced Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

24.6 Reactions of Monosaccharides The mechanism of glycoside formation is analogous to the acetal formation mechanism Practice with conceptual checkpoints 24.28 and 24.29 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e