Question

Draw and name the enantiomers of the sugars shown in Figure 23-2. Give the relative configuration (D or L) and the sign of the rotation in each case.

Short answer

The enantiomers of glucose are $\mathrm{D}\left(+\right)\mathrm{glucose}$and $\mathrm{L}(‐)\mathrm{glucose}$.

The enantiomers of arabinose are $\mathrm{D}(‐)\mathrm{arabinose}$and $\mathrm{L}(‐)\mathrm{arabinose}$.

The enantiomers of erythrose are $\mathrm{D}(‐)\mathrm{erythrose}$and $\mathrm{L}(+)\mathrm{erythrose}$.

The enantiomers of glyceraldehyde are $\mathrm{D}(+)\mathrm{glyceraldehyde}$and $\mathrm{L}(‐)\mathrm{glyceraldehyde}$.

Step-by-step solution

Enantiomers

Molecules having exact mirror images of itself and has non- super imposable mirror image is known as a chiral molecule. Enantiomers or optical isomers are two mirror images of a chiral molecule.

D or L configuration

If a compound rotates plane polarised light to the right, then it is said to be dextrorotatory $\left(\mathrm{D}\right)$. This is indicated in the name of the compound by the prefix sign $\left(+\right)$. But if a compound rotates plane polarised light to the left, then it is said to be laevorotatory $\left(\mathrm{L}\right)$. This is indicated in the name of the compound by the prefix sign That is, it can be said that $\mathrm{D}‐\mathrm{and}\mathrm{L}‐$ refers to the absolute configuration around the asymmetric carbon atom while (+) and (-) refers to the direction of rotation of the plane polarised light.

Placement of OH group for D and L series of sugars in Fischer projection

For D-series, OH group of the bottom asymmetric carbon is present on the right side in the Fischer projection while for L-series, OH group of the bottom asymmetric carbon is present on the left side in the Fischer projection.

Structure of the enantiomers and their respective names

Glucose is an aldohexose. For glucose, the enantiomers are D(+)glucose and L(+)glucose. The structure of both the enantiomers are shown below.

Arabinose is an aldopentose. For arabinose, the enantiomers are D(-) arabinose and L(-)arabinose. The structure of both the enantiomers are shown below.

Erythrose is an aldotetrose. For erythrose, the enantiomers are D(-)erythrose and L(-)erythrose. The structure of both the enantiomers are shown below.

Glyceraldehyde is an aldotriose. For glyceraldehyde, the enantiomers are D(+)glyceraldehyde and L(-)glyceraldehyde. The structure of both the enantiomers are shown below.