Question

Which pair of compound gives meso product on catalytic hydrogenation? (A) Glucose, Mannose (B) Mannose, Galactose (C) Galactose, Allose (D) Erythrose, Fructose

Short answer

The correct answer is (A) Glucose, Mannose, as these compounds would result in a meso product upon catalytic hydrogenation.

Step-by-step solution

Identify the given compounds

The exercise provides four pairs of compounds: (A) Glucose, Mannose (B) Mannose, Galactose (C) Galactose, Allose (D) Erythrose, Fructose

Draw the structures of the compounds

Draw the Fischer projection for each of the given compounds. Note the position of the hydroxyl groups and chiral carbons.

Analyze the stereochemistry of each compound

Analyze the stereochemistry of each compound, focusing on the chiral carbons. Consider the arrangement of the hydroxyl groups and their potential to form a plane of symmetry when a stereogenic center is inverted upon hydrogenation.

Determine the effect of catalytic hydrogenation

Catalytic hydrogenation adds hydrogen to the double bonds in compounds, which usually turns planar sp^2 hybridized carbons into tetrahedral sp^3 hybridized carbons with inverted configuration. Analyze how this process would change the configuration of each chiral carbon in the given compounds.

Identify the pairs of compounds that will yield meso products

Compare the initial configurations of the compounds to their configurations after hydrogenation. Determine which pairs give rise to products with planes of symmetry, indicating that they are meso compounds. After carefully analyzing the stereochemistry of the given compounds and their respective configurations upon catalytic hydrogenation, it can be concluded that:

Answer

The correct answer is: (A) Glucose, Mannose, as these compounds would result in a meso product upon catalytic hydrogenation.

Key concepts

Stereochemistry

Stereochemistry is a fundamental aspect of organic chemistry that deals with the three-dimensional arrangement of atoms within molecules. It plays a crucial role in determining the physical and chemical properties of compounds, as well as their biological activity. By understanding the spatial orientation of atoms, chemists can predict the behavior of molecules during chemical reactions and interactions.
In catalytic hydrogenation, stereochemistry is particularly important because the process can change the configuration of chiral centers within a molecule. Chiral centers are atoms, typically carbon, that have four different substituents, leading to molecules that come in two non-superimposable mirror image forms called enantiomers. Through hydrogenation, the introduction of hydrogen can convert double bonds into single bonds, transforming sp² hybridized carbons into tetrahedral sp³ hybridized carbons, invariably affecting molecular chirality.
Enantiomers have identical chemical and physical properties except for their behavior towards plane-polarized light and their reactions with other chiral compounds. Understanding these changes in stereochemistry is essential when predicting which compounds will yield meso products, which are achiral substances that possess chirality centers but are superimposable on their mirror image due to a plane of symmetry.

Meso Compounds

Meso compounds are a unique class of stereoisomers that, despite containing multiple chiral centers, are achiral due to the presence of an internal plane of symmetry. This plane divides the molecule into two mirror-image halves, negating the optical activity typically associated with chiral centers. An understanding of meso compounds is essential when dealing with the outcomes of reactions like catalytic hydrogenation.
For example, when certain diastereomers undergo hydrogenation, they may become achiral if the process results in the creation of a meso compound. This conversion relies on the creation of a symmetric arrangement of substituents around chiral centers. When predicting the outcome of hydrogenation reactions, identifying potential meso compounds helps anticipate whether a reaction mixture will exhibit optical activity or not. The key characteristic of a meso compound is that it has at least two chiral centers with an internal plane of symmetry and therefore does not rotate plane-polarized light. This feature helps to determine whether a pair of compounds will yield a meso product upon catalytic hydrogenation, as seen in the exercise with Glucose and Mannose.

Fischer Projection

The Fischer projection is a two-dimensional representation of organic molecules that simplifies the understanding of three-dimensional molecular structures, particularly for carbohydrates and amino acids. It is a critical tool employed by chemists to depict the configuration of chiral centers in a molecule. In the Fischer projection, the horizontal lines represent bonds projecting forward (out of the plane of the paper), while the vertical lines represent bonds extending backwards (into the paper).
When drawing the Fischer projection for molecules with chiral centers, it is important to position the most oxidized carbon (typically the carbonyl group in sugars) at the top. Chiral centers are then laid out sequentially from top to bottom. For identifying meso compounds as illustrated in the step-by-step solution, it's vital to compare the Fischer projections before and after hydrogenation, paying close attention to changes in the stereochemistry around chiral centers. A proper Fischer projection can reveal symmetry or lack thereof, thus, providing immediate insight into whether a molecule is chiral, achiral, or meso. Using Fischer projections in the exercise involving Glucose and Mannose helps visualize the necessary plane of symmetry for a compound to be meso upon catalytic hydrogenation.