Problem 111 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The correct decreasing order of priority for the functional groups of organic compounds in the IUPAC system of nomenclature is (a) \(-\mathrm{SO}_{3} \mathrm{H},-\mathrm{COOH},-\mathrm{CONH}_{2^{\prime}}-\mathrm{CHO}\) (b) \(-\mathrm{CHO},-\mathrm{COOH},-\mathrm{SO}_{3} \mathrm{H},-\mathrm{CONH}_{2}\) (c) \(-\mathrm{CONH}_{2^{\prime}}-\mathrm{CHO},-\mathrm{SO}_{3} \mathrm{H},-\mathrm{COOH}\) (d) \(-\mathrm{COOH},-\mathrm{SO}_{3} \mathrm{H},-\mathrm{CONH}_{2^{\prime}}-\mathrm{CHO}\)

Step-by-Step Solution

Answer

Option (d) is correct: -COOH, -SO₃H, -CONH₂, -CHO.

Step 1: Identify Priority Functional Groups

In IUPAC nomenclature, functional groups have a specific order of precedence. For the groups present, Carboxylic acids (-COOH) have the highest priority, followed by sulfonic acids (-SO₃H), amides (-CONH₂), and aldehydes (-CHO).

Step 2: Order the Functional Groups

Arrange the functional groups in decreasing order of priority based on the IUPAC system: Carboxylic acid (-COOH), Sulfonic acid (-SO₃H), Amide (-CONH₂), and finally Aldehyde (-CHO).

Step 3: Match the Order to Given Options

Compare the correct order with the provided options. The correct sequence is found to be: -COOH, -SO₃H, -CONH₂, -CHO.

Step 4: Select the Correct Option

Option (d) presents the order -COOH, -SO₃H, -CONH₂, -CHO, which matches the decreasing priority order of functional groups based on IUPAC nomenclature.

Key Concepts

Functional Groups Priority

In organic chemistry, functional groups play a crucial role in determining the properties and reactivity of compounds. They are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The IUPAC system outlines a hierarchy for these functional groups to provide a standard way of naming compounds. When multiple functional groups are present, the priority dictates which group defines the base name of the compound, and which are treated as substituents.

• Functional groups with higher priority are used as the name's suffix.
• Groups with lesser priority are treated as prefixes or substituents.

This hierarchy ensures that the chemical structure is described accurately and unambiguously. It is important to understand these priority rules to correctly name compounds and communicate their structure to others.

Carboxylic Acids Precedence

Carboxylic acids, denoted by the functional group -COOH, hold the top spot in the hierarchy of functional groups according to IUPAC nomenclature. This high precedence is due to their strong acidic characteristics and their significant influence on the overall properties of compounds.

• The -COOH group is always given priority over other functional groups when naming compounds.
• Its presence as the highest priority group means it often forms part of the main chain in IUPAC names.

Understanding the prominence of carboxylic acids is essential not only for nomenclature but also for appreciating how they drive reactions and interactions in organic chemistry.

Sulfonic Acids in Nomenclature

Sulfonic acids, with the functional group -SO₃H, are next in line for priority following carboxylic acids. Although not as commonly encountered in basic organic chemistry courses, these groups are crucial in industrial and pharmaceutical chemistry.

• Their suffix in nomenclature is "-sulfonic acid," and they take precedence over many other functional groups.
• When naming compounds, a sulfonic acid is given proper acknowledgment, pushing lesser priority groups to prefixes.

This high priority is due to their acidic properties and their structural resemblance to sulfuric acid derivatives, giving them significant importance in the IUPAC naming sequence.

Amides and Aldehydes Ranking

In the IUPAC system, amides (-CONH₂) and aldehydes (-CHO) follow sulfonic acids in the hierarchy of functional group priority. Amides generally come before aldehydes due to their structural influence and the presence of a nitrogen atom.

• Amides are derived from carboxylic acids and hold the suffix “-amide.” They are always named as secondary base parts of a IUPAC name after carboxylic and sulfonic acids.
• Aldehydes, with the -CHO group, are recognized with the suffix "-al" and are often used in simpler organic compound names.

Understanding the relative ranking of amides and aldehydes helps in systematically naming compounds and predicting chemical behavior, giving insight into which reactions might occur prominently or with preference.