Chemistry for update: Carbohydrates I

Carbohydrates I

Introduction

Taken literally the word carbohydrate means hydrate of carbon. As such, it refers to compounds which may be represented by the formula C_n(H₂O)_n. Thus the molecular formula of D-glyceraldehyde, the simplest carbohydrate, is C₃(H₂O)₃ or C₃H₆O₃, while that of D-glucose, is C₆H₁₂O₆. A more meaningful description of carbohydrates is that they are polyhydroxyaldehydes and ketones. As the interactive model below shows, D-glucose is a pentahydroxyaldehyde.

Definitions

There are a lot of terms used to describe carbohydrates. Here are some of the more common:

sugar- a synonym for a carbohydrate.

saccharide- a synonym for a carbohydrate.

monosaccharide- a sugar that obeys the formula C_n(H₂O)_n. Glucose is an example.

disaccharide- a sugar composed of two monosaccharides. It contains one less H₂O unit than a monosaccharide. Sucrose, table sugar, is a good example:

aldose- a polyhydroxyaldehyde.

aldotetrose- an aldose that contains four carbon atoms. Threose is an example:

ketose- a polyhydroxyketone.

ketohexose- a ketose that contains six carbon atoms. D-fructose is a good example.

reducing sugar- an aldose or its cyclic hemiacetal. These compounds give a positive Tollen's test, i.e. they reduce a basic solution containing silver ions. The metallic silver that is produced forms a mirror on the walls of the container.

pyranose- a cyclic form of an aldose or ketose where the ring contains 6 atoms.

furanose- a cyclic form of an aldose or ketose where the ring contains 5 atoms. The fructose ring in sucrose is a furanose.

anomers- cyclic sugars that are isomers which differ in configuration only at C-1. a-D-(+)-glucopyranose and b-D-(+)-glucopyranose are examples.

epimers- sugars that differ in configuration only at C-2.

Exercise 1

is best described as

an aldopentose

an aldohexose

a ketopentose

a ketohexose

D and L, R and S, (+) and (-): Make Up Your Mind

Let's Start with (+) and (-)

This is easy: (+) means that an optically active sample rotates light in a clockwise direction; (-) means it rotates it in a counterclockwise direction.

and move on to D and L

This is a bit more confusing. Historically rotation in a clockwise direction was called dextrorotation, while counterclockwise rotation was called levorotation. But now the letters D and L describe the stereochemistry at a specific chiral carbon atom in a carbohydrate. In this regard, D-glyceraldehyde (which happens to be dextrorotatory) serves as a reference. This molecule contains one chiral carbon. Hence there are two stereoisomers- enantiomers- possible. They are shown in Figure 1.

Exercise 2 How many of the following sugars are D-sugars?

and conclude with R and S

The letters R and S are the descriptors that the Cahn-Ingold-Prelog rules use to specify the disposition of substituents about a chiral center. In D-glyceraldehyde, the four groups attached to the chiral carbon are H, OH, CHO, and CH₂OH. The priorities of these groups are 4, 1, 2, and 3, respectively. If you orient the model of D-glyceraldehyde so that you are looking down the bond from the chiral carbon to the hydrogen, you will see that in tracing a path from the OH to the CHO to the CH₂OH group (1 to 2 to 3) requires moving in a clockwise direction. By convention, this arrangement is designated as R.

Exercise 3 Specify the stereochemical designations at C-2, C-3, and C-4 of the following sugar:

RRR

RRS

RSR

SRS

SSR

Optical Rotation and Mutarotation

All of the sugars we will consider are optically active. In the case of D-glucose, two crystalline forms of the sugar are common. Both forms are dextrorotatory. One form, which melts at 146^oC, has an optical rotation of +112^o. The other form, which melts at 150^oC, has an optical rotation of +18.7^o. Interestingly, if you dissolve the form that melts at 146^oC in water, its optical rotation gradually decreases from +112^oto +52.7^o. The optical rotation of an aqueous solution of the other form also changes gradually, increasing from +18.7^oto +52.7^o. This change in optical rotation is called mutarotation. Figure 2 animates the process.

Figure 2

The Mechanism of Mutarotation

Figure 2 does not present a complete picture of the mechanism of mutarotation. It ignores the role of solvent. It does not depict any proton transfers. What it tries to emphasize is the change in orientation of the carbonyl group as a result of rotation around the C₁-C₂ bond; if the oxygen attached to C₅ adds to the carbonyl group when it is pointing down, the a-anomer of D-(+)-glucopyranose is formed. If the carbonyl group is projecting upward when ring closure occurs, the b-anomer of D-(+)-glucopyranose is produced.

Figure 3 depicts the equilibria that are involved in mutarotation.

Figure 3

Simultaneous Equilibria

Additional evidence for this equilibrium is available from the ¹H NMR spectrum of an aqueous solution of D-glucose. The anomeric protons of the a and b forms each appear as a doublet. Integration of these signals reveals that the mixture contains 64% of the b anomer and 36% of the a anomer, values that agree with those obtained from optical rotation measurements.

Exercise 4