Chapter 7, Problem 7P

Answers to all problems are at the end of this book.. Detailed solutions are available in the Student Solutions Manual, Study Guide, and Problems Book.

Naming Sugars in the (R,S) System Use the inform tit ion in the Critical Developments in Biochemistry box titled "Rules for Description (if Chiral Centers in the (R.S) System" (Chapter 4) to name D-galactose using (R.S) nomenclature. Do the same for L-altrone.

Interpretation Introduction

Interpretation:

The name of$\text{D}-\text{galactose}$and$\text{L}-\text{altrose}$by using$\left(R,S\right)$nomenclature are to be stated.

Concept introduction:

A carbon atom that has four non-equivalent atoms or groups attached to it is known as chiral carbon atom. Chiral carbon centers are also called as asymmetric or stereogenic centers.

The naming of chiral center and geometric isomers are based on Cahn-Ingold-Prelog priority rules. If the priority assigned to each group attached to the chirality center in a molecule is in a clockwise direction, then it is the R-stereoisomer, and if this is counter-clockwise, then it is the S-stereoisomer. R and S-stereoisomer are mirror images of each other.

Answer to Problem 7P

The name of$\text{D}-\text{galactose}$and$\text{L}-\text{altrose}$by using$\left(R,S\right)$nomenclature are$2R,3S,4S,5R-\text{D}-\text{galactose}$and$2R,3S,4S,5S-\text{L}-\text{altrose}$respectively.

Explanation of Solution

The highest priority group in case of a monosaccharide is hydroxyl$\left(\text{OH}\right)$group. Thus, the lowest numbering will be given to the hydroxyl$\left(\text{OH}\right)$group.

The Fischer projection structure for$\text{D}-\text{galactose}$is shown below.

Figure 1

The substituents which extend to the side of$\text{D}-\text{galactose}$are supposed to be bent on the side of viewer. All the other carbon atoms in the Fischer structure is bend away from the viewer.

Thus, in the structure of$\text{D}-\text{galactose}$, the substituents are arranged around each carbon atoms. At$\text{C}-2$position, the priority order of the substituents is$\text{OH}>\text{CHO}>\text{C}-3>\text{H}$. After the rotation of bend structure of$\text{D}-\text{galactose}$molecule, hydrogen atom behind$\text{C}-2$position flips the hydroxyl group at the left position and the aldehyde group at the upper right position. Thus, the priority of the groups is in a clockwise direction. Hence,$\text{C}-2$position is assigned as$R$-configuration.

At$\text{C}-3$position, the priority order of the substituents is$\text{OH}>\text{C}-\text{2}>\text{C}-4>\text{H}$. The carbon at$\text{C}-2$has higher priority due to the presence of an aldehyde group on it. After the rotation of bend structure, hydrogen atom behind$\text{C}-3$position flips the hydroxyl group at the right position and$\text{C}-2$at the upper left position. Thus, the priority of the groups is in an anti-clockwise direction. Hence,$\text{C}-3$position is assigned as$S$-configuration.

At$\text{C}-4$position, the priority order of the substituents is$\text{OH}>\text{C}-3>\text{C}-5>\text{H}$. The carbon at$\text{C}-3$has higher priority due to the bonding of$\text{C}-5$with two hydrogen atoms. After the rotation of bend structure, hydrogen atom behind$\text{C}-4$position flips the hydroxyl group at the right position and$\text{C}-3$at the upper left position. Thus, the priority of the groups is in an anti-clockwise direction. Hence,$\text{C}-4$position is assigned as$S$-configuration.

At$\text{C}-5$position, the priority order of the substituents is$\text{OH}>\text{C}-4>\text{C}-6>\text{H}$. The carbon at$\text{C}-4$has higher priority due to its bonding with other carbon. After the rotation of bend structure of$\text{D}-\text{galactose}$molecule, hydrogen atom behind$\text{C}-5$position flips the hydroxyl group at the left position and$\text{C}-4$at the upper right position. Thus, the priority of the groups is in a clockwise direction. Hence,$\text{C}-5$position is assigned as$R$-configuration.

Thus, the name of$\text{D}-\text{galactose}$that can be assigned by using$\left(R,S\right)$nomenclature is$2R,3S,4S,5R-\text{D}-\text{galactose}$.

The Fischer projection structure for$\text{L}-\text{altrose}$is shown below.

Figure 2

The substituents which extend to the side of$\text{L}-\text{altrose}$are supposed to be bent on the side of viewer. All the other carbon atoms in the Fischer structure is bend away from the viewer.

Thus, in the structure of$\text{L}-\text{altrose}$, the substituents are arranged around each carbon atoms. At$\text{C}-2$position, the priority order of the substituents is$\text{OH}>\text{CHO}>\text{C}-3>\text{H}$. After the rotation of bend structure of$\text{L}-\text{altrose}$molecule, hydrogen atom behind$\text{C}-2$position flips the hydroxyl group at the left position and the aldehyde group at the upper right position. Thus, the priority of the groups is in a clockwise direction. Hence,$\text{C}-2$position is assigned as$R$-configuration.

At$\text{C}-3$position, the priority order of the substituents is$\text{OH}>\text{C}-\text{2}>\text{C}-4>\text{H}$. The carbon at$\text{C}-2$has higher priority due to the presence of an aldehyde group on it. After the rotation of bend structure, hydrogen atom behind$\text{C}-3$position flips the hydroxyl group at the right position and$\text{C}-2$at the upper left position. Thus, the priority of the groups is in an anti-clockwise direction. Hence,$\text{C}-3$position is assigned as$S$-configuration.

At$\text{C}-4$position, the priority order of the substituents is$\text{OH}>\text{C}-3>\text{C}-5>\text{H}$. The carbon at$\text{C}-3$has higher priority due to the bonding of$\text{C}-5$with two hydrogen atoms. After the rotation of bend structure, hydrogen atom behind$\text{C}-4$position flips the hydroxyl group at the right position and$\text{C}-3$at the upper left position. Thus, the priority of the groups is in an anti-clockwise direction. Hence,$\text{C}-4$position is assigned as$S$-configuration.

At$\text{C}-5$position, the priority order of the substituents is$\text{OH}>\text{C}-4>\text{C}-6>\text{H}$. The carbon at$\text{C}-4$has higher priority due to its bonding with other carbon. After the rotation of bend structure of$\text{L}-\text{altrose}$molecule, hydrogen atom behind$\text{C}-5$position flips the hydroxyl group at the right position and$\text{C}-4$at the upper left position. Thus, the priority of the groups is in an anti-clockwise direction. Hence,$\text{C}-5$position is assigned as$S$-configuration.

Thus, the name of$\text{L}-\text{altrose}$that can be assigned by using$\left(R,S\right)$nomenclature is$2R,3S,4S,5S-\text{L}-\text{altrose}$.

Conclusion

The name of$\text{D}-\text{galactose}$and$\text{L}-\text{altrose}$by using$\left(R,S\right)$nomenclature are$2R,3S,4S,5R-\text{D}-\text{galactose}$and$2R,3S,4S,5S-\text{L}-\text{altrose}$respectively.