Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Which of the following does not have any chiral centre? (a) glyceraldehydes (b) dihydroxyacetone (c) aldopentose (d) ketopentose

Short answer

a) Glyceraldehydes b) Dihydroxyacetone c) Aldopentose d) Ketopentose Answer: b) Dihydroxyacetone

Step-by-step solution

Identify the structure of glyceraldehydes

Glyceraldehyde is the simplest monosaccharide with the formula C3H6O3, having one aldehyde and two hydroxyl functional groups. It exists in two enantiomeric forms (D and L). Its structure is as follows: H-(C=O)-(C(OH)H)-(C(OH)2)-H

Identify the chiral center in glyceraldehydes

There is one chiral center in glyceraldehydes, which is the second carbon atom bonded to the C=O. It has four unique substituents: H, (C=O), (C(OH)2), and (OH): H-(C=O)-\textbf{(C(OH)H)}-(C(OH)2)-H Thus, glyceraldehydes have one chiral center.

Identify the structure of dihydroxyacetone

Dihydroxyacetone is the simplest ketose with the formula C3H6O3, having one ketone and two hydroxyl functional groups. Its structure is as follows: H-(C=O)-(C(OH)2)-H

Determine if dihydroxyacetone has a chiral center

There are no carbon atoms in dihydroxyacetone with four unique substituents. The carbon atom next to the C=O has only three unique substituents: H, (C=O), and (C(OH)2): H-(C=O)-\textbf{(C(OH)2)}-H Thus, dihydroxyacetone does not have any chiral center.

Identify the aldopentose and ketopentose

Aldopentose is a five-carbon monosaccharide with an aldehyde functional group, and all of its isomers have chiral centers. Examples are ribose and arabinose. Ketopentose is a five-carbon monosaccharide with a ketone functional group, and all of its isomers have chiral centers. An example is ribulose.

Choose the correct option

Since dihydroxyacetone does not have any chiral center, the correct option is (b) dihydroxyacetone.

Key concepts

Chiral Center

In organic chemistry, a chiral center is a carbon atom that is bonded to four different substituents. This unique arrangement causes the formation of non-superimposable mirror images known as enantiomers. You can think of chiral centers like your hands; just as the left hand does not match the right when overlapped, chiral molecules have versions that cannot be perfectly aligned.

A molecule can have more than one chiral center, and the presence of these centers is essential in distinguishing stereoisomers. Identifying chiral centers is crucial for understanding the molecule's ability to interact with biological systems, as different enantiomers can have vastly different effects in a biological context. To locate a chiral center, look for carbon atoms connected to four distinct atoms or groups of atoms. This is the reason why glyceraldehyde has a chiral center, while dihydroxyacetone does not; the latter lacks a carbon atom with four unique connections.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, often referred to as single or simple sugars. They have the general formula \(C_n(H_2O)_n\), where "n" is usually a small number like 3 to 7. Monosaccharides are the building blocks of more complex carbohydrates such as disaccharides and polysaccharides.

"Mono" means one, and "saccharide" means sugar, indicating these molecules cannot be broken down into simpler sugars. Each monosaccharide is related to a chain of carbon atoms with a carbonyl group (either an aldehyde or a ketone) and hydroxyl groups attached. The most common monosaccharides are glucose, fructose, and galactose.

• Glucose is an aldose, which means it contains an aldehyde group at the first carbon.
• Fructose, on the other hand, is a ketose because it contains a ketone group at the second carbon.

Monosaccharides also exhibit chirality, as several carbon atoms can be chiral centers, affecting the molecule’s 3D structure and reactivity.

Aldoses and Ketoses

Aldoses and ketoses refer to two categories of monosaccharides differentiated by their functional groups. Aldoses contain an aldehyde group at the first carbon atom, while ketoses have a ketone group at the second carbon.

Aldoses, like glucose and glyceraldehyde, typically have multiple chiral centers, making them capable of forming multiple stereoisomers. For example, aldopentoses are five-carbon sugars with an aldehyde group, such as ribose. Each carbon atom, except the aldehyde (at the end) and primary alcohol carbons (at the other end), is a chiral center.

Ketoses, such as fructose and dihydroxyacetone, feature a ketone group within their carbon chain and also possess the potential for multiple stereoisomers, particularly as the number of carbon atoms increases. Ketopentoses, like ribulose, have the ketone group on the second carbon, with many configurations possible when additional chiral centers are included.

Understanding the difference between aldoses and ketoses helps in identifying how these sugars behave chemically and biologically. While dihydroxyacetone is the simplest form of ketose without a chiral center, sugars generally have one or more chiral centers leading to significant diversity in their structures and functions.