Question

Galactose, a sugar related to glucose, contains a six-membered ring in which all the substituents except the \(-\mathrm{OH}\) group, indicated below in red, are equatorial. Draw galactose in its more stable chair conformation.

Short answer

Draw galactose with five equatorial groups and one axial -OH group.

Step-by-step solution

Understanding the Chair Conformation

The chair conformation is a three-dimensional model representing the cyclic structure of sugars like galactose. It minimizes steric hindrance by placing many substituents in the equatorial position, which is energetically more favorable than the axial position.

Identifying Axial and Equatorial Positions

In a chair conformation, each carbon atom in the ring has two possible positions for substituents: axial (up or down) and equatorial (pointing slightly outward, closer to the plane of the ring). To minimize steric clash, large groups should be placed in the equatorial position.

Placing Substituents in Galactose

For the galactose molecule, all substituents should be equatorial except for the -OH group marked in red. This means that you will need to draw the galactose ring with five substituents placed equatorially.

Drawing the Ring Structure

Start by drawing the six-membered chair conformation. Make sure to alternate the axial and equatorial positions at each carbon atom for the substituents.

Adding the -OH Groups and Other Substituents

Add the -OH group indicated in red to the axial position. Place the rest of the substituents, such as hydrogen atoms, in equatorial positions, ensuring they maintain the most stable arrangement.

Key concepts

Galactose

Galactose is a simple sugar, closely related to glucose, and is a part of the group of carbohydrates known as monosaccharides. It is a critical component in our diet and forms part of lactose, the sugar found in milk. Like other hexoses, galactose typically exists in a cyclic structural form, rather than as an open chain.
This cyclic form is a six-membered ring, termed a pyranose, due to its stability. Understanding galactose is important as it offers insight into the metabolism of sugars and its role in energy production.
Within the human body, galactose is primarily metabolized in the liver where it is ultimately converted into glucose, providing a crucial energy source for cellular functions. Recognizing the chair conformation of galactose serves as a fundamental basis for studying carbohydrate chemistry and its biological functions.

Axial and Equatorial Positions

In the world of organic chemistry, understanding the axial and equatorial positions in a chair conformation is essential. Imagine a chair conformation as a rocking chair shape, where each carbon atom in the six-membered ring has potential positions for its substituents.

• Axial Positions: These are positions that alternate between pointing upwards and downwards from the plane of the ring. They are perpendicular to the ring.
• Equatorial Positions: These positions are more stabilized as they extend outward along the plane of the ring. This allows large substituents to avoid steric clashes.

In galactose, placing most of the substituents in equatorial positions reduces energy and enhances stability. This is why, except for the selective -OH group, others are positioned equatorially to maintain structural harmony.

Stability of Sugar Molecules

The stability of sugar molecules, like galactose, largely depends on their chair conformation. This conformation ensures that the spatial arrangement of different atoms or groups attached to the carbon ring minimizes potential interference or steric hindrance.
The equatorial position, in particular, provides a more stable environment for large groups. Since energetically favorable conformations reduce repulsion between electron clouds of adjacent groups, they help maintain the sugar structure’s integrity. In galactose, having most substituents in equatorial positions minimizes strain and keeps the molecule in its lowest energy state.
Ultimately, the stability provided by the chair conformation affects how sugars behave and react in biological processes. A stable sugar molecule can more efficiently participate in metabolic pathways, impacting energy production and storage in organisms.

Steric Hindrance Minimization

Steric hindrance occurs when atoms are placed too close to each other, leading to repulsion between their electron clouds. This often results in strained or destabilized molecules. In a chair conformation, minimizing steric hindrance is crucial for sugars like galactose.

• Axial positions, where groups point either up or down from the ring, are more prone to steric hindrance, as they can clash with other substituents on adjacent carbons.
• Equatorial positions, on the other hand, position the groups away from potential clashes. This horizontal extension allows molecules to "breathe," so to speak, giving them more room and leading to a stable conformation.

For galactose, placing substituents in equatorial positions, except for the one specially indicated, effectively lessens steric interference. This careful arrangement plays a key role in reducing stress on the molecular framework, contributing to its overall stability and functionality in various biochemical contexts.