Question

The incorrect IUPAC name is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOC}_{2} \mathrm{H}_{5}\) ethyl butanoate (b) 3-methyl butanal (c) 2-methyl-3-pentanone 2-methyl-3-butanol

Short answer

The incorrect IUPAC name is '2-methyl-3-butanol'. It should be '3-methyl-2-butanol'.

Step-by-step solution

Identify Molecular Structures

First, we need to identify the molecular structures of each compound given in the options.(a) \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOC}_{2} \mathrm{H}_{5} \) corresponds to ethyl butanoate. (b) 3-methyl butanal is an aldehyde derived from butane with a methyl group attached to the third carbon. (c) 2-methyl-3-pentanone is a ketone with a methyl group on the second carbon of a pentane chain and a carbonyl group on the third carbon. The last name given is 2-methyl-3-butanol, which suggests a butane backbone with a methyl group on the second carbon and a hydroxyl (-OH) group on the third carbon.

Check IUPAC Nomenclature Rules

Now, apply IUPAC rules to ensure that the name matches the structure for each. (a) Ethyl butanoate is correctly named as the ester derived from butanoic acid and ethanol. (b) 3-methyl butanal is rightly named; the aldehyde is on the terminal carbon. (c) The name 2-methyl-3-pentanone matches the structure; the ketone group is prioritized at the lower numbered carbon in a chain. '2-methyl-3-butanol' should instead be named as 3-methyl-2-butanol, as the hydroxyl group takes priority in numbering over the methyl group.

Determine the Incorrect Name

Compare each name to the correct IUPAC naming conventions: - Ethyl butanoate (correct) - 3-methyl butanal (correct) - 2-methyl-3-pentanone (correct) - 2-methyl-3-butanol (incorrect, should be 3-methyl-2-butanol) The incorrect IUPAC name is '2-methyl-3-butanol', which doesn't follow the rule of prioritizing functional groups in numbering.

Key concepts

Molecular Structure Identification

When faced with chemical names, the first step is to determine the molecular structure they correspond to. Understanding this helps in visualizing the compound and verifying its name against standard naming rules.
Let's break down the process:

• **Structural Formulae:** Write down the molecular formulae and visualize the connection of atoms to form structural formulae. For example, for ethyl butanoate, represented as \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOC}_{2} \mathrm{H}_{5}\), visualize a linear chain (butane) with an ester linkage at one end.
• **Carbon Backbone:** Identify the longest continuous chain of carbon atoms in each molecule. This establishes the root name of the compound. For ethyl butanoate, the root chain is butane before adding the ester group.
• **Substituents and Functional Groups:** Identify any branches or functional groups. In 2-methyl-3-pentanone, there is a methyl group branch, and the carbonyl group (part of the ketone functional group) is key.

Structuring the molecules mentally aids in matching them to their correct IUPAC name by focusing on each key element of the chemical.

Functional Group Prioritization

Functional group prioritization is crucial in determining how a molecule is named according to IUPAC rules. When multiple functional groups are present, some groups outrank others and affect numbering and naming.
Here are the primary considerations:

• **Hierarchy of Groups:** Certain functional groups, like hydroxyl (\(-OH\)), ketones (\(C=O\)), and esters are given priority in naming. For example, in the case of 2-methyl-3-butanol, the hydroxyl group has higher priority over the methyl substituent, leading to the correct name of 3-methyl-2-butanol.
• **Numbering the Chain:** Start numbering from the end of the chain nearest to the highest priority functional group. In 2-methyl-3-pentanone, the carbonyl group is on carbon 3 because ketones get higher priority than alkyl groups such as methyl.
• **Systematic Order of Naming:** With multiple substituents, always list them as prefixes in alphabetical order while adhering to prioritization rules for numbering.

Prioritizing functional groups helps avoid ambiguity and ensures a standardized approach, crucial for scientific communication.

IUPAC Nomenclature Rules

IUPAC nomenclature rules form the systematic method for naming organic chemical compounds. They ensure consistency and clarity.
When naming compounds:

• **Identify Primary Functional Groups:** Begin by identifying the functional group with the highest priority that adequately describes the compound. This becomes a suffix in the name. For example, the main functional group in ethyl butanoate is an ester.
• **Determine the Longest Chain:** Choose the longest carbon chain that contains the highest priority group. For instance, in 3-methyl-2-butanol, the longest chain containing the \(-OH\) group is butane-based, hence 'butanol'.
• **Assign Locants:** Assign numbers to the carbon atoms in the chain to indicate where substituents (methyl, chloro, etc.) and other functional groups appear. Always start closer to the highest priority group.
• **Stereochemical Descriptors and Isomers:** Include details such as stereochemistry indicators (like cis/trans or R/S) if needed, for compounds with stereoisomers.

Following these rules helps in achieving the correct structure-to-name and name-to-structure correlation, ensuring effective communication in chemistry. Remember, while rules are systematically applied, practice makes recognition and application much more intuitive.