Question

Draw the structure of L-galactose, and then answer the following questions: (a) Which other aldohexose gives the same aldaric acid as L-galactose on oxidation with warm \(\mathrm{HNO}_{3} ?\) (b) Is this other aldohexose a b sugar or an L sugar? (c) Draw this other aldohexose in its most stable pyranose conformation.

Short answer

(a) D-galactose; (b) D sugar; (c) Beta-D-galactopyranose is most stable.

Step-by-step solution

Draw the Structure of L-galactose

L-galactose is an aldohexose, which means it contains six carbon atoms and an aldehyde group. The structure is drawn with the following orientation: OH groups on C2, C3, and C4 are on the left, while the OH group on C5 and the CH2OH group on C6 are on the right. Start with the six-carbon backbone and add the functional groups accordingly.

Identify the Oxidation Product

Aldaric acids are formed by oxidizing both the aldehyde group and the primary alcohol group of an aldohexose to carboxylic acids. Oxidation of L-galactose with warm nitric acid will yield galactaric acid (also known as mucic acid). The structure consists of all OH groups unchanged and aldehyde group at C1 as well as the primary alcohol group at C6 being oxidized to COOH groups.

Find the Other Aldohexose

The aldohexose that gives the same aldaric acid as L-galactose when oxidized is D-galactose. Both L-galactose and D-galactose produce galactaric acid when oxidized with warm nitric acid due to their configuration at all chiral centers except C1 being the same but in opposite enantiomers.

Determine Chirality (b or L Sugar)

D-galactose is the enantiomer of L-galactose with the opposite configuration at the chirality center C5, hence it is a D sugar, not an L sugar.

Draw D-galactose in Pyranose Form

The most stable pyranose form of D-galactose is the beta-D-galactopyranose. When drawing the pyranose form, form a 6-membered ring from carbons 1 to 5 with carbon 6 as a substituent on C5. The hydroxyl groups orient around the ring with C1 OH group in the axial position and others according to their original linear configuration.

Key concepts

Aldohexose

Aldohexoses are a type of monosaccharide, specifically sugars, characterized by having six carbon atoms (hexa) and an aldehyde functional group. Understanding the structure of aldohexoses is fundamental in carbohydrate chemistry.

Key characteristics of aldohexoses include:

• A six-carbon backbone, often written as C₆H₁₂O₆.
• The aldehyde group is located at the first carbon (C1).
• They are stereoisomers with multiple chiral centers.

A familiar example of an aldohexose is glucose, but others like galactose, to which L-galactose belongs, also exist. L-galactose itself is one of the stereoisomers within this group. Understanding the aldohexose structure assists in grasping how sugars like L-galactose interact chemically, forming other compounds such as aldaric acids on oxidation.

Aldaric acid

Aldaric acids are the oxidation products of aldohexoses. They form when both the aldehyde group at the first carbon (C1) and the primary alcohol group at the last carbon (C6) are converted into carboxylic acids through a chemical reaction, typically using an oxidizing agent such as warm nitric acid.

This oxidation process involves:

• Transformation of the aldehyde group (-CHO) to a carboxylic acid group (-COOH) at carbon 1.
• Transformation of the primary alcohol group (-CH₂OH) at carbon 6 to a carboxylic acid group (-COOH).

A practical example is the conversion of L-galactose to galactaric acid (also known as mucic acid) through such a reaction. Interestingly, it is possible to get the same aldaric acid from different aldohexoses, such as L-galactose and its enantiomer, D-galactose, due to their specific configurations and chirality.

Chirality

Chirality is an essential concept in chemistry, especially when discussing molecules like aldohexoses. It refers to the geometric property where a molecule is not superimposable on its mirror image – much like how your left hand is not superimposable on your right hand.

Key points about chirality include:

• Molecules with chirality have at least one chiral center, which is a carbon atom bonded to four different groups or atoms.
• These can exist in "D" or "L" configurations, which indicate the direction the groups are arranged around the chiral centers.

In the case of L-galactose, it has several chiral centers affecting its overall shape and reactivity. L-galactose is the left-handed ("L") isomer while D-galactose is its right-handed ("D") enantiomer. The chirality center at C5 is crucial for determining whether a sugar is classified as an L or D sugar.

D-galactose

D-galactose is the enantiomer of L-galactose, meaning it is a stereoisomer that is a mirror image but not superimposable on L-galactose. In the series of aldohexoses, D-galactose and L-galactose share the same chemical formula but differ in the arrangement of groups leading to different spatial configurations.

Important characteristics of D-galactose include:

• The mirror image orientation compared to L-galactose.
• The classification as a "D" sugar, due to the orientation of the -OH group at the fifth carbon (C5).
• Its ability to be oxidized to the same aldaric acid (galactaric acid) as L-galactose.

D-galactose plays a critical role in biochemistry and serves as a building block in more complex carbohydrates and biomolecules, contributing to its understanding of sugar chemistry.

Pyranose form

The pyranose form is a common structural formation that sugars like aldohexoses can adopt, involving a ring structure. In the pyranose form, the sugar molecule forms a six-membered ring resembling pyran, a structure prevalent in carbohydrates.

Notable features of pyranose forms include:

• Formation of a cyclic hemiacetal via the reaction between the carbonyl group (at C1) and a hydroxyl group within the molecule (typically at C5).
• The six-membered ring generally includes five carbon atoms and one oxygen atom.
• The pyranose form is often observed in a "chair" conformation, which provides stability to the molecule.

For D-galactose, its most stable pyranose form is the beta-D-galactopyranose, where the OH group on the anomeric carbon (C1) projects in an equatorial orientation, minimizing steric hindrance and maximizing interactions that contribute to its stability.