Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

The IUPAC name of \(\mathrm{CH}_{3} \mathrm{COCH}\left(\mathrm{CH}_{3}\right)_{2}\) is (a) 4-methyl isopropyl ketone (b) 3 -methylbutan-2-one (c) isopropylmethyl ketone (d) 2-methylbutan-3-one

Step-by-Step Solution

Answer

The IUPAC name of \(\mathrm{CH}_3\mathrm{COCH}\left(\mathrm{CH}_3\right)_2\) is 3-methylbutan-2-one.

Step 1: Identify the Functional Group

The compound given is \(\mathrm{CH}_3\mathrm{COCH}\left(\mathrm{CH}_3\right)_2\). The functional group present here is a ketone because of the \(\mathrm{CO}\) group.

Step 2: Longest Carbon Chain Determination

Identify the longest carbon chain that includes the ketone group. In this compound, the longest chain consists of four carbon atoms: \(\mathrm{CH}_3-\mathrm{CO}-\mathrm{CH}-\mathrm{CH}_3\).

Step 3: Number the Carbon Chain

Assign numbers to the carbon atoms in the chain starting from the end nearest to the ketone group to give the lowest possible number to the ketone carbonyl carbon. Thus, the carbonyl carbon receives position 2.

Step 4: Identify and Name the Substituent

The remaining part, \(\mathrm{CH}_3\), attached to the third carbon of the chain, is a methyl group. Hence, it is a substituent and will be included in the name.

Step 5: Assemble the IUPAC Name

Combine the information from the previous steps: The main chain has four carbons (butan-), a ketone on the second carbon (-2-one), and a methyl group on the third carbon (3-methyl). This results in the name: 3-methylbutan-2-one.

Key Concepts

Ketone

A ketone is an organic compound featuring a carbonyl group (C=O) attached to two carbon atoms. This specific functional group is critical when naming ketones. In the given molecule \(\mathrm{CH}_{3}\mathrm{COCH}\left(\mathrm{CH}_{3}\right)_{2}\), the carbonyl group is flanked by two carbon atoms, confirming it is indeed a ketone. Ketones are characterized by the carbonyl group located internally in the carbon chain, distinguishing them from aldehydes, where the carbonyl group can be found at the end of the chain. Recognizing the presence of this functional group is the first step in accurately naming the compound using IUPAC nomenclature.

Longest Carbon Chain

The longest carbon chain in organic compounds helps determine the main name in IUPAC nomenclature. In our example, \(\mathrm{CH}_3\mathrm{COCH}\left(\mathrm{CH}_3\right)_2\), the primary task is to find the longest continuous chain of carbon atoms that includes the functional group. Here, we find the chain consists of four carbon atoms: \(\mathrm{CH}_3-\mathrm{CO}-\mathrm{CH}-\mathrm{CH}_3\). Identifying the longest chain ensures that the ketone group is part of the main compound name and provides the backbone for substituent locations. This step is crucial in ensuring the structural accuracy revolving around carbon differentiation.

Substituents

Substituents are atoms or groups of atoms attached to the main carbon chain which can influence compound naming. In our compound example, \(\mathrm{CH}_3\) attached to the third carbon serves as a substituent. This is identified as a methyl group, which is a common and simple hydrocarbon substituent. When naming a compound:

• Identify all substituents on the main carbon chain.
• Note the specific carbon each substituent is attached to.

Substituents are important as they clearly define structural differences between molecules of otherwise similar carbon backbones and contribute to the full IUPAC name.

IUPAC Naming Rules

IUPAC naming rules are a systematic way to name organic compounds, ensuring each name is unique and descriptive. These rules serve as guidelines:

• Identify the main functional group, which in this example is a ketone.
• Determine the longest continuous carbon chain that includes the functional group.
• Assign numbers to the carbon atoms in the chain starting from the end nearest to the ketone group to give the functional group the lowest possible position.
• Identify and name any substituents, noting their position on the chain.

For our compound, applying these rules leads to naming it as **3-methylbutan-2-one**. The naming precision offered by these rules is invaluable for chemists communicating complex molecular structures.