Question

Draw the structures of an aldopentose and a ketohexose.

Short answer

Aldopentose has 5 carbons with an aldehyde group; ketohexose has 6 carbons with a ketone group.

Step-by-step solution

Understanding Aldopentoses and Ketohexoses

Aldopentoses are carbohydrates with five carbon atoms and an aldehyde group, while ketohexoses are carbohydrates with six carbon atoms and a ketone group. Pentoses and hexoses refer to the number of carbons, and 'aldo-' and 'keto-' specify whether they contain an aldehyde or ketone functional group respectively.

Drawing an Aldopentose

First, identify the backbone structure of an aldopentose which consists of five carbon atoms. Place the aldehyde group (-CHO) at the first carbon. Number the remaining carbons from C2 to C5 and complete the structure by adding hydroxyl (-OH) groups on the rest of the carbons, considering distinct stereoisomers (for example, D-ribose, a common aldopentose, has -OH groups on C2 and C3 in different spatial orientations).

Drawing a Ketohexose

Draw the structure of a ketohexose starting with a six-carbon chain. Place the ketone group (C=O) at the second carbon (C2). Number the carbons from C1 to C6. For example, D-fructose is a common ketohexose which has hydroxyl (-OH) groups in certain configurations across C3 to C5, distinguishing its isomeric forms.

Key concepts

Understanding Aldopentose

Aldopentoses are a type of carbohydrate that have a special place in the world of sugars. They consist of five carbon atoms, which is where the 'penta-' in aldopentose comes from, indicating the number five. One of these carbons is part of an aldehyde group, which is a specific functional group characterized by a carbon atom double-bonded to an oxygen atom and single bonded to a hydrogen atom (written as -CHO). This aldehyde group is located at the first carbon in the chain.
This structure gives aldopentoses their particular chemical properties and reactions. In biology, aldopentoses are essential because they form parts of crucial molecules like ribose, which is integral to RNA. They exist in several forms, depending on the spatial arrangement of the hydroxyl (-OH) groups attached to the carbon chain. This arrangement creates different stereoisomers, which we’ll explore more later on.
Crucial points to remember about aldopentoses include:

• Five carbon atoms
• Aldehyde group at C1
• Stereoisomers due to varied -OH placements

Recognizing these characteristics can aid in understanding related carbohydrates and their roles.

Unraveling Ketohexose

Ketohexoses are intriguing sugars that contain six carbon atoms, distinguishing them in the family of carbohydrates known as hexoses. The prefix 'keto-' refers to the presence of a ketone group, a functional group distinct from an aldehyde, characterized by a carbon atom double-bonded to an oxygen atom within the carbon chain (not at the end, as in aldehydes). In ketohexoses, this ketone group is typically found at the second carbon.
For example, D-fructose is one of the most well-known ketohexoses. It has a six-carbon backbone, with a ketone group at the second position, and the rest of the carbon atoms are attached to hydroxyl groups. The specific orientation and positioning of these hydroxyl groups create different isomers of ketohexoses, which can have varied chemical behaviors and biological activities.
Essential features of ketohexoses include:

• Six carbon atoms
• Ketone group at C2
• Existence of stereoisomers

These sugars are prevalent in various natural sources and are key players in many biochemical pathways.

Discovering Stereoisomers

Stereoisomers are fascinating manifestations of molecules that have the same chemical formula but differ in the three-dimensional arrangement of atoms within their structure. For carbohydrates like aldopentoses and ketohexoses, stereoisomers are defined by the spatial configuration of the hydroxyl (-OH) groups around the carbon atoms.
Two common types of stereoisomers are enantiomers and diastereomers. Enantiomers are perfect mirror images of each other and can’t be superimposed, much like your left and right hands. Diastereomers, on the other hand, are not mirror images and differ at one or more chiral centers without being completely opposite.
Understanding the concept of stereoisomers is critical because it helps explain the diversity and specificity in the behavior of carbohydrates in biological systems. Not only does it influence how these sugars function and react, but it also affects how they are utilized or recognized by enzymes and other biomolecules.
Key points about stereoisomers:

• Same molecular formula, different spatial arrangement
• Includes enantiomers and diastereomers
• Significant effects on biochemical properties

Exploring stereoisomers opens up a deeper understanding of how minute changes in structure can lead to different biological outcomes.