Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is the first member of monosaccharides? (a) \(\mathrm{CH}_{2} \mathrm{OH}-\mathrm{CHOH}-\mathrm{CHO}\) (b) COC(=O)CO (c) COC(O)C(=O)CO (d) \(\mathrm{CH}_{2} \mathrm{OH}-\mathrm{CHOH}-\mathrm{CHOH}-\mathrm{CHO}\)

Step-by-Step Solution

Answer

Option (a), glyceraldehyde, is the first member of monosaccharides.

Step 1: Identify the Structure of Monosaccharides

Monosaccharides are the simplest form of carbohydrates consisting of carbon (C), hydrogen (H), and oxygen (O) atoms, usually in a ratio of 1:2:1, following the formula \ \ \[\text{(CH}_2\text{O)}_n\]\ \ where represents the number of carbon and oxygen atoms. The simplest monosaccharide is an aldose or ketose with three carbon atoms, known as a triose.

Step 2: Evaluate Given Options for Simplicity

We need to identify which option presents the simplest structure fitting the definition of a monosaccharide. Looking at the options, option (a) is the molecular structure of glyceraldehyde, \(\mathrm{CH}_{2} \mathrm{OH}-\mathrm{CHOH}-\mathrm{CHO}\). It consists of three carbon atoms, fitting the formula for a triose. Options (b) and (c) are presented in SMILES notation and do not follow a simple triose pattern due to their complexity and extra groups. Option (d) is a tetrose with four carbon atoms, exceeding the simplest form.

Step 3: Determine the Simplest Monosaccharide

Compare option (a) glyceraldehyde with the other options. Glyceraldehyde fits the description of a simple monosaccharide with three carbons, called a triose. In comparison, options (b) and (c) are structurally more complex and option (d) contains more carbon atoms than are found in the simplest monosaccharides.

Step 4: Conclusion of the First Monosaccharide

Based on the analysis, glyceraldehyde (option a), \(\mathrm{CH}_{2} \mathrm{OH}-\mathrm{CHOH}-\mathrm{CHO}\), is the simplest form of monosaccharide, thus making it the first member in this category.

Key Concepts

Molecular Structure of Carbohydrates

Carbohydrates are organic compounds consisting mainly of carbon, hydrogen, and oxygen atoms. Typically, they have a general chemical formula represented as \( ext{(CH}_2 ext{O)}_n\), indicating each molecule includes these elements in a 1:2:1 ratio. This formula underscores the basic structure common to all carbohydrates. Depending on the number of carbon atoms, a carbohydrate can range from simple monosaccharides to more complex forms. Envision carbohydrates as versatile building blocks, which can link to form bigger structures like disaccharides and polysaccharides.
One peculiarity of monosaccharides, the simplest form, is that they cannot be broken down into smaller carbohydrates. They are the fundamental units representing complexity from a chemical perspective.
To classify carbohydrates, we also consider their length and functional groups, which help in determining whether the saccharide is an aldose or a ketose. Though monosaccharides are simple, they are critical as they play central roles in energy storage and supply in biological systems.

Triose Glyceraldehyde

Triose glyceraldehyde is a key example of a molecule classified under the family of monosaccharides. The term 'triose' itself refers to monosaccharides comprising three carbon atoms. Glyceraldehyde, with the formula \( ext{CH}_2 ext{OH}- ext{CHOH}- ext{CHO}\), exemplifies this. It is an aldose, meaning it contains an aldehyde functional group, characterized by the -CHO at one end. As a triose, glyceraldehyde is significant for various biochemical pathways.
Biochemically, triose molecules like glyceraldehyde play noticeable roles in metabolism. For instance, through glycolysis, glyceraldehyde-3-phosphate is a pivotal intermediate. Such trioses help convert glucose, the body's chief energy source, into energy and useful intermediates for other biochemical reactions. Despite its simplicity, glyceraldehyde's importance cannot be underestimated as it is integral to numerous cellular processes.

Aldose and Ketose

The classification of monosaccharides into aldoses and ketoses is essential for identifying their chemical properties and biological roles.
The primary distinction between them lies in their functional groups:

• **Aldose:** Contains an aldehyde group (characterized by -CHO) at the end of the carbon chain. This places the double-bonded oxygen on the terminal carbon.
• **Ketose:** Contains a ketone group (characterized by C=O) usually on the second carbon atom. This means the double-bonded oxygen is within the carbon skeleton.

These functional groups influence the chemical reactivity and behavior of the sugars. For example, glyceraldehyde is a simple aldose due to its aldehyde group. Conversely, dihydroxyacetone is the simplest ketose.
Understanding the distinction between aldose and ketose sugars aids in both the fundamental comprehension of biochemistry and the application in life sciences, such as in metabolic pathways, where the molecules undergo different processes depending on their group.