Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are incorrect statements? (a) IUPAC names of succinic acid is ethane 1, 2 -dicarbolxylic acid. (b) Enantiomorphs possess identical biological properties. (c) Tetrahydrofuran is a heteroaromatic compound (d) Generally homologues are not isomers.

Step-by-Step Solution

Answer

Statements (a), (b), and (c) are incorrect.

Step 1: Understanding the Concepts

Before evaluating the statements, we need to understand each term. For (a), succinic acid is a dicarboxylic acid, and its IUPAC name should indeed be examined. For (b), enantiomorphs are mirror-image isomers, also known as enantiomers. For (c), tetrahydrofuran is a five-membered ring with one oxygen atom. Aromaticity rules in heteroaromatics must be checked. For (d), homologues are organic compounds differing by a CH2 group but may or may not be isomers.

Step 2: Evaluate Statement (a)

The IUPAC name of succinic acid is Butanedioic acid, which is definitely not 'ethane 1,2-dicarboxylic acid'. So statement (a) is incorrect.

Step 3: Evaluate Statement (b)

Enantiomorphs or enantiomers, while having identical physical properties (e.g., boiling, melting points), and often similar chemical properties in non-chiral environments, can have vastly different biological properties due to chiral sensitivities. Therefore, statement (b) is incorrect.

Step 4: Evaluate Statement (c)

Tetrahydrofuran (THF) is a cyclic ether consisting of a five-membered ring with one oxygen atom. It is not aromatic because it lacks the continuous overlap of p-orbitals required for aromaticity. Therefore, statement (c) is incorrect.

Step 5: Evaluate Statement (d)

Homologues belong to the same homologous series and thus have similar chemical structures. They are not typically considered isomers of one another, as isomers have the same molecular formula but different structures. Therefore, statement (d) is correct.

Key Concepts

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system is the standard method used to name chemical compounds. It provides a systematic way to name both simple and complex structures. This system uses a set of rules to create a unique and universally accepted name for every compound. This can help in accurately identifying a compound's structure.

Let's take succinic acid as an example. Its traditional name gives limited insight into its structure. However, its IUPAC name, butanedioic acid, reveals a four-carbon structure each with carboxylic acid groups attached at both ends.

To understand such naming schemes, small steps can be helpful:

• Identify the longest carbon chain: This forms the base name (e.g., "but" for four carbons).
• Determine functional groups and their position: For example, the term "dioic" indicates two carboxylic groups.
• Apply prefix or suffix rules as needed: Mak sure all components like additional substituents or multiple bonds are included.

Mastering IUPAC nomenclature enables you to decode and construct intricate organic structures based on their names.

Enantiomers

Enantiomers are a type of stereoisomer, where molecules are mirror images of each other but cannot be superimposed, much like left and right hands.

Despite their similarity, enantiomers have fascinating and important distinctions, especially when it comes to biological systems. While they share identical physical properties like melting and boiling points, in the context of living organisms they may interact with chiral environments in drastically different ways.

Here's how:

• Chirality: This property determines how they interact with other chiral molecules, including proteins and enzymes.
• Biological Activity: For instance, one enantiomer of a drug might be therapeutic, while the other could be less effective or even harmful.
• Optical Activity: Enantiomers rotate plane-polarized light in opposite directions, identified as either "left" (levorotatory) or "right" (dextrorotatory).

Understanding enantiomers is crucial for fields like pharmaceuticals where the correct enantiomer can be the difference between a successful treatment and a faulty therapeutic attempt.

Aromaticity

Aromaticity is a concept used to explain the special stability and unique properties of certain cyclic compounds, often hydrocarbons. These structures typically involve resonance, where electrons are delocalized over a pi (π) system.

For a compound to be considered aromatic, certain criteria must be met:

• Planarity: The atoms in the compound usually need to be in the same plane.
• Continuous π-electron cloud: There must be uninterrupted overlap of p-orbitals around the ring.
• Hückel's Rule: A compound must have a (4n+2) number of π-electrons, where n is a whole number.

Tetrahydrofuran (THF), although cyclic, lacks these characteristics, especially the continuous π-electron system. Therefore, it does not qualify as aromatic.

Aromatics are essential in chemistry, offering insights into stability and reactivity that aren't apparent in non-aromatic compounds.

Homologous Series

A homologous series in chemistry is a group of organic compounds that have a similar general formula, differing only through the addition of a specific repeating unit, typically -CH₂-.

While homologues resemble each other closely and display consistent trends in physical properties such as boiling points and solubility, they actually are not isomers. Isomers, in contrast, have identical molecular formulas but different structural configurations.

Some key attributes of homologous series include:

• Gradation in Properties: As you move through the series, there is a predictable change in properties, often seen in gradual but systematic shifts in boiling points and densities.
• Similar Chemical Behavior: Members react similarly due to the presence of the same functional group.
• Ease of Studying: The predictable change simplifies studying chemical behavior across a range of similar compounds.

The concept of homologous series helps chemists infer the behavior of unknown compounds by comparison to known ones in the series.