Deck 24: Carbohydrates

Question 1

Classify the carbohydrate, then indicate the chiral carbons with an asterisk. How many possible stereoisomers can exist for this carbohydrate?

Answer 1

A carbohydrate with an aldehyde group is called an aldose, and carbohydrates are classified by the number of carbons they contain. The carbohydrate shows six carbons and has an aldehyde on carbon 1, so it is an aldohexose. There are four stereocenters, so there will be 2⁴ or 16 stereoisomers.

Question 2

Classify the carbohydrate, then indicate the chiral carbons with an asterisk. How many possible stereoisomers can exist for this carbohydrate?

Answer 2

A carbohydrate with a ketone group is called a ketose, and carbohydrates are classified by the number of carbons they contain. The structure is a ketopentose since it has five carbons and a ketone. There are two chiral carbons, so there are four total stereoisomers possible.

Question 3

Assign the R/S configuration for each of the asymmetric carbons in the carbohydrate.

Answer 3

Recall in a Fischer projection that the vertical bonds are receding from view and the horizontal bonds are coming toward the viewer. In each case, the lowest priority hydrogen is on a horizontal bond, so a shortcut would be to assign priorities and note the direction of increasing priority. Then simply reverse the R/S assignment.

Question 4

Convert the line-and-wedge structure into a Fischer projection, then assign as a D- or L-carbohydrate.

Question 5

Draw an enantiomer of the carbohydrate. Assign the D- or L-configuration to each.

Question 6

Most hexoses found in nature are D-carbohydrates. Comparing a D-carbohydrate with the Cahn-Ingold-Prelog system of nomenclature, select the true statement.

A) A D-carbohydrate will have R configuration.
B) A D-carbohydrate will have S configuration.
C) There is no relationship between the D,L system and the R/S configuration.

Question 7

Identify the relationship between the two carbohydrates. Select all that apply.

A) enantiomer
B) diastereomer
C) anomer
D) epimer

Question 8

Identify the relationship between the two carbohydrates. Select all that apply.

A) enantiomer
B) diastereomer
C) anomer
D) epimer

Question 9

Given the Fischer projection, draw both anomers of the corresponding pyranose ring in a Haworth projection, then label each anomer.

Question 10

Given the chair conformation of a monosaccharide, provide the open-chain form.

Question 11

Ribose is cyclized to the hemiacetal $\beta$ -D-ribofuranose. Draw a curved-arrow mechanism to show the cyclization step.

Question 12

When $\alpha$ -D-glucopyranose is dissolved in water, mutarotation occurs. What has happened?

A) The carbohydrate has converted completely to $\beta$ -D-glucopyranose.
B) The carbohydrate has converted to a mixture of $\alpha$ -D-glucopyranose and $\beta$ -D-glucopyranose.
C) The carbohydrate has converted to its enantiomer.
D) The carbohydrate has changed from D- to L-configuration.

Question 13

D-Altrose can be transformed to another aldose and 2-ketose upon treatment with base. Draw the structure of the aldose and 2-ketose.

Question 14

A methyl D-glucopyranoside contains a methoxy group on which carbon of glucose? Explain the difference between a pyranoside and a pyranose.

Question 15

Predict the major organic product for the reaction. If more than one isomer is formed, just draw one.

Question 16

Devise a two-step synthesis for the transformation.

Question 17

Devise a two-step synthesis for the transformation.

Question 18

A D-aldohexose is treated with HNO₃ to give an optically inactive aldaric acid. Deduce the Fischer projection of the D-aldohexose. There is more than one correct answer.

Question 19

A D-aldotetrose reacts with NaBH₄ to give an optically active product. Deduce the structure of the D-aldotetrose and the product.

Question 20

Two aldohexoses (D-galactose and D-talose) are formed from a Kiliani-Fischer synthesis. Deduce the starting material.

Question 21

The Ruff degradation of two aldohexoses gives D-xylose. Deduce the structures of the two aldohexoses.

Question 22

The Ruff degradation of two aldopentoses gives D-threose. Deduce the structures of the two aldopentoses.

Question 23

The disaccharide maltose is shown below.
a. Classify the disaccharide as a reducing or nonreducing sugar and explain why.
b. Identify the glucoside linkage in maltose and classify each as either alpha or beta.
c. Name the monosaccharides formed when maltose is hydrolyzed in aqueous acid.

Question 24

Identify products A and B from the synthesis.

Question 25

A D-aldopentose forms two aldohexoses, A and B, after a Kiliani-Fisher synthesis. Aldohexose A is oxidized to an optically inactive aldaric acid, while aldohexose B is oxidized to an optically active aldaric acid. The aldopentose can also be oxidized to an optically inactive aldaric acid. Deduce the structures of the aldopentose and the two aldohexoses.