Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Glucose, fructose and mannose form the same osazone because they have the same configurations at all carbon atoms except (a) \(\mathrm{C}_{1}\) and \(\mathrm{C}_{2}\) (b) \(\mathrm{C}_{2}\) and \(\mathrm{C}\), (c) \(\mathrm{C}_{3}\) and \(\mathrm{C}_{4}\) (d) \(\mathrm{C}_{4}\) and \(\mathrm{C}_{5}\)

Short answer

Answer: Glucose, fructose, and mannose have different configurations at carbon atoms C2 and C3.

Step-by-step solution

Understanding Osazone Formation

Osazones are derivatives of reducing sugars formed when they react with phenylhydrazine. The formation of osazones helps to differentiate reducing sugars based on their structure. Glucose, fructose, and mannose form the same osazone because they differ only at the configuration of specific carbon atoms.

Analyzing the Structure of Glucose, Fructose, and Mannose

Glucose, fructose, and mannose are all monosaccharides with six carbon atoms in their structure. Glucose and mannose are aldohexoses, while fructose is a ketohexose. The difference between these three sugars lies in the position of their carbonyl group and the stereochemistry/configuration of their chiral carbon atoms.

Identifying the Carbon Atom Configurations

We need to identify the configurations at which glucose, fructose, and mannose differ. Since they form the same osazone, it means they have the same configuration at the first two carbon atoms (C1 and C2) because this is where osazone formation occurs. Therefore, the differences in configuration must be at other carbon atoms.

Comparing the Given Options

Now, we compare the given options to our findings in the previous steps: (a) C1 and C2: Since osazone formation occurs at these positions, the three sugars must have the same configuration here. This option is incorrect. (b) C2 and C3: Glucose and mannose differ at C2, and fructose differs from them both at C2 due to its keto group and at C3. Therefore, this option is correct. (c) C3 and C4: There are differences at C3, but all three sugars have the same configuration at C4. This option is incorrect. (d) C4 and C5: All three sugars have the same configuration at C4 and C5. This option is incorrect.

Selecting the Correct Answer

Based on the comparison in step 4, the correct answer is option (b) - glucose, fructose, and mannose have different configurations at C2 and C3.

Key concepts

Monosaccharides Structure

Monosaccharides, often referred to as simple sugars, are the most basic form of carbohydrates. They consist of a single sugar molecule and cannot be hydrolyzed into simpler compounds. A key feature of monosaccharides is the presence of a carbonyl group (a carbon atom double-bonded to an oxygen atom) and multiple hydroxyl groups (an oxygen atom single-bonded to a hydrogen atom).

The simplest monosaccharides are trioses, with three carbon atoms, but they can range in size up to heptoses, which contain seven carbon atoms. The most common monosaccharides have five (pentoses) or six (hexoses) carbon atoms. Glucose, a hexose, is a prime example of a monosaccharide with a structure that plays a central role in osazone formation.

Monosaccharides can also be classified based on the position of their carbonyl group. If the carbonyl group is at the end of the carbon chain, it's termed an aldose, and if it's within the chain, it's called a ketose. This distinction is key, as aldoses and ketoses, while both capable of forming osazones, will do so with variations in their structures.

Carbohydrate Stereochemistry

The stereochemistry of carbohydrates refers to the spatial arrangement of the atoms within these molecules. Since most of the carbon atoms in monosaccharides are chiral—meaning they are attached to four different groups or atoms—the arrangement around each of these chiral centers can vary, giving rise to different isomers.

In simple terms, a chiral carbon atom is like a junction that can be connected in various configurations, rather like placing four different shops at a crossroads, each facing a different direction. For sugars, those 'shops' are usually hydrogen atoms or hydroxyl groups (-OH), and each different arrangement is a particular stereoisomer.

The arrangement of these hydroxyl groups defines the sugar's specific properties and its identity. For instance, just changing the position of one -OH group can convert one sugar, like glucose, into another, like mannose. This specific stereochemistry is crucial during biochemical processes and also determines the outcome of reactions such as osazone formation, where only some parts of the molecule are involved, thus providing a shared characteristic among different monosaccharides.

Aldohexoses and Ketohexoses

Aldohexoses and ketohexoses are both subcategories of hexoses, which are monosaccharides containing six carbon atoms. Aldohexoses have an aldehyde group (-CHO) at one end of the carbon chain, typically at the first carbon atom (C1). An example of an aldohexose is glucose, which is a crucial energy source in living organisms.

Ketohexoses have a ketone group (C=O) in the second position of the carbon chain (C2). Fructose is the most well-known ketohexose and is found in many fruits, sweetening syrups, and honey. The main difference between these two types of hexoses is the location of their carbonyl group.

The concept of osazone formation showcases how, despite these initial structural differences, aldohexoses and ketohexoses can share aspects of their chemistry. When forming an osazone, the carbonyl group along with the second carbon atom's hydroxyl group, react, leading to a structure that's common in both aldohexoses and ketohexoses like glucose, fructose, and mannose, at least in the relevant part of the molecule for this specific reaction.