Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Pairs of isomeric polyhydroxy aldehydes differing only in the configuration at first carbon are called (a) anomers (b) enantiomers (c) mesomers (d) epimers

Short answer

The name given to pairs of isomeric polyhydroxy aldehydes that differ only in the configuration at the first carbon is (a) anomers.

Step-by-step solution

Anomers

Anomers are a special type of stereoisomers that are cyclic carbohydrate molecules, differing in the configuration at the anomeric (first) carbon. In this case, the exercise is referring to differences at the first carbon, which aligns with the definition of anomers.

Enantiomers

Enantiomers are a type of stereoisomers that are non-superimposable mirror images of each other. They have different configurations at all chiral centers in the molecule. The exercise specifically asks for compounds that differ only in the configuration at the first carbon, so enantiomers are not the correct answer.

Mesomers

Mesomers are resonance structures of the same compound that share electrons. This term is related to covalent bonding and resonance, which is not applicable to this exercise. Therefore, mesomers are not the correct answer

Epimers

Epimers are a type of stereoisomers that have different configurations at only one of their chiral centers. However, this exercise specifically refers to differences at the first carbon, which is a more specific situation than general epimers. Since Step 1 is the most appropriate option according to the exercise description, the correct answer is: (a) anomers

Key concepts

Stereoisomers

In chemistry, stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms, but they differ in the three-dimensional orientations of their atoms. These differences arise because of variations in how atoms are arranged in space, rather than how they are connected. Imagine two molecules with identical chains of atoms, but one is like a left-handed glove and the other like a right-handed glove. This subtlety makes them stereoisomers.

Stereoisomers are crucial in biology because they can have different chemical reactions and biological activities. For example:

• Enantiomers: These are a type of stereoisomers that are mirror images of each other, like your left and right hands. They cannot be superimposed on each other.
• Geometric Isomers: Arising mainly in compounds with double bonds or rings, these have different spatial arrangements due to restricted rotation.

Understanding stereoisomers can help you appreciate how subtle changes in structure can lead to significant differences in behavior and reactivity.

Epimers

Epimers are an interesting group of stereoisomers. They differ specifically at one chiral center among multiple chiral centers in a compound. Imagine you have a molecule with several chiral centers—the specific arrangement of atoms around one chiral center will be different while the rest remain the same.

Epimers often occur in carbohydrates, where they lead to diversity in sugar structures. Importantly, the specific difference in one chiral center can greatly influence the properties of the molecule, including its taste, reactivity, and how enzymes interact with it. For instance, glucose and galactose are epimers—they vary only at one carbon location. Yet, this small change defines them as separate sugars.

This concept allows chemists and biologists to differentiate and synthesize specific sugars, understanding how their interactions and functions differ due to changes at just one chiral location. Essentially, it's a way to fine-tune molecular properties for specific functionalities.

Carbohydrates

Carbohydrates are essential biological molecules made up of carbon, hydrogen, and oxygen. They are one of the primary sources of energy in our diet and play critical roles in the structure and function of cells:

• Monosaccharides: Simple sugars, like glucose and fructose, these molecules are the building blocks of carbohydrates. They typically have multiple chiral centers, making them interesting from a stereochemical perspective.
• Disaccharides: These are formed by two monosaccharide units joined by glycosidic bonds, exemplified by sucrose (table sugar).
• Polysaccharides: Complex carbohydrates, such as starch and cellulose, are long chains of monosaccharides providing energy storage and structural materials in plants and animals.

Carbohydrates are not only important for nutrition but also for cellular recognition and signaling due to their unique structures, which are determined by the configuration of their chiral centers. This is why studying their structure through concepts like stereoisomers and epimers is important in both chemistry and biology.

Chiral Centers

Chiral centers are key positions in molecules where the arrangement of atoms can lead to different isomers, specifically stereoisomers. A molecule is chiral if it has at least one carbon atom bonded to four different groups, giving it a "handedness". This asymmetry is crucial because it can affect the properties and reactions of molecules.

The most common example in biology and organic chemistry is the carbon atom. If you think of a molecule like a spoke hub with attachments, a chiral center is a hub where all the spokes (or attachments) are different. This allows for different spatial arrangements, leading to various isomers, such as enantiomers and epimers.

Recognizing chiral centers helps you predict and understand the behavior of molecules. For example, they determine whether a sugar is glucose or galactose by spatial orientation, thus affecting taste and metabolism. Identifying these centers is fundamental to designing drugs, synthesizing materials, and understanding biological mechanisms.