Which of the following statements about the structure of monosaccharides is true in carbohydrate chemistry?

Introduction / Context:
Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex sugars and polysaccharides. Their classification is based on features such as the number of carbon atoms, the type of carbonyl group and the spatial arrangement of substituents around chiral centres. This question asks which statement about monosaccharide structure is true, testing understanding of stereochemistry and basic terminology in carbohydrate chemistry.

Given Data / Assumptions:
- Option A compares aldoses and ketoses based on hydroxyl group position.
- Option B states that monosaccharides can be classified according to spatial arrangement of atoms.
- Option C claims that all monosaccharides contain nitrogen as well as carbon, hydrogen and oxygen.
- Option D describes a six carbon sugar as a pentose.
- Standard definitions for aldoses, ketoses, stereoisomers, pentoses and hexoses are assumed.

Concept / Approach:
The key idea is that monosaccharides are characterised by their carbonyl group type and carbon chain length, and they exhibit stereochemistry through multiple chiral centres. Different stereoisomers, such as D and L forms or epimers, have the same molecular formula but differ in the three dimensional arrangement of atoms. Therefore classification based on spatial arrangement is meaningful. In contrast, aldoses and ketoses actually differ in the position of the carbonyl group, not hydroxyl groups, monosaccharides generally contain carbon, hydrogen and oxygen but not nitrogen, and six carbon sugars are called hexoses rather than pentoses.

Step-by-Step Solution:
Step 1: Evaluate option A. Aldoses have an aldehyde group at the end of the carbon chain, while ketoses have a ketone group usually at the second carbon, so they differ by carbonyl position, not hydroxyl position. Step 2: Evaluate option B. Monosaccharides exist as stereoisomers with different spatial arrangements at chiral centres, allowing classification into D and L series or other stereochemical categories, so this statement is true. Step 3: Evaluate option C. Typical monosaccharides are composed of carbon, hydrogen and oxygen, with the general formula CnH2nOn, and do not necessarily contain nitrogen. Step 4: Evaluate option D. A sugar with six carbon atoms is called a hexose, whereas a pentose has five carbons, so this statement is incorrect. Step 5: Conclude that only option B correctly describes a structural feature of monosaccharides.

Verification / Alternative check:
To verify option B, consider glucose and galactose. They share the same molecular formula but differ in the spatial orientation of groups around a single chiral carbon, making them epimers. Similarly, D glucose and L glucose are mirror image stereoisomers based on overall spatial arrangement. Textbooks systematically classify monosaccharides based on such stereochemistry, confirming that spatial arrangement is indeed a valid classification criterion. This supports the correctness of the statement in option B.

Why Other Options Are Wrong:
Option A misstates the difference between aldoses and ketoses; the key distinction is whether the carbonyl group is at the end of the chain (aldehyde) or internal (ketone), not the hydroxyl positions. Option C incorrectly introduces nitrogen as a necessary element in monosaccharides; nitrogen may appear in amino sugars but these are derivatives, not typical monosaccharides. Option D mislabels a six carbon sugar as a pentose, whereas the correct term for a six carbon sugar is hexose. These inaccuracies contradict basic carbohydrate nomenclature and structure.

Common Pitfalls:
Learners may confuse functional group terminology, mixing up carbonyl and hydroxyl positions, or they may generalise features of modified sugars like amino sugars to all monosaccharides. Another pitfall is misremembering the names of pentoses and hexoses when counting carbons. Paying careful attention to how textbooks define aldoses, ketoses and stereoisomers, and ensuring that the number prefixes match the carbon count, helps avoid these misunderstandings.

Final Answer:
The true statement about monosaccharide structure is Monosaccharides can be classified according to the spatial arrangement of their atoms.