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 is 3-methylbutan-2-one, option (b).

Step 1: Identify the Functional Group

The compound \( \mathrm{CH}_{3}\mathrm{COCH}\left(\mathrm{CH}_{3}\right)_{2} \) contains a ketone group, which is characterized by a carbonyl group (C=O) bonded to two hydrocarbon groups.

Step 2: Determine the Longest Carbon Chain

Identify the longest continuous carbon chain that includes the carbon of the carbonyl group. In this compound, the longest chain is four carbons long, hence it is a butane derivative.

Step 3: Number the Carbon Chain

Begin numbering the longest carbon chain from the end nearest to the carbonyl group, giving the carbonyl carbon the lowest possible number. Thus, the carbonyl carbon is at position 2, and the numbering is as follows: 1-\(\mathrm{CH}_{3}\), 2-(=O), 3-\(\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\).

Step 4: Identify and Name the Substituents

The side chain at position 3 is a methyl group (\(\mathrm{CH}_{3}\)), making it a '3-methyl' substituent on the butanone chain.

Step 5: Write the IUPAC Name

Using the information about the functional group and substituents, combine them with the base name corresponding to the longest chain. The IUPAC name is determined as 3-methylbutan-2-one, which corresponds to option (b).

Key Concepts

Ketone Functional Group

The ketone functional group is a key component in organic chemistry and plays a crucial role in naming compounds with the IUPAC system. Ketones are organic compounds characterized by a carbonyl group, which is a carbon double-bonded to an oxygen ( C=O ) within their structure.
What makes a ketone distinct from other carbonyl-containing compounds, such as aldehydes, is that the carbon atom in the carbonyl group is bonded to two other carbon atoms. This positioning of the carbonyl group is essential when identifying ketones.
In the compound CH_3COCH(CH_3)_2 , the ketone functional group can be found by locating the carbonyl carbon bonded to other carbon atoms. This group is central to determining the primary functional feature of the compound and helps guide the naming process in the IUPAC system.
Understanding the ketone functional group assists in not only naming the compound correctly but also in predicting its reactivity and interactions within chemical reactions.

Longest Carbon Chain

Identifying the longest carbon chain in a molecule is a crucial step in assigning the correct IUPAC name. This chain represents the backbone of the molecule.
When dealing with a ketone, the longest chain must include the carbonyl group. It is not simply a measure of the maximum number of carbon atoms but rather ensuring that the functional group is a part of the chain.
In the compound CH_3COCH(CH_3)_2 , the longest chain includes four carbon atoms and is recognized as a butane backbone. This forms the basis of the compound's name, where the chain length combined with the presence of the ketone group indicates that the compound belongs to the 'butanone' family.

• Ensure the carbonyl's carbon is part of the main chain.
• Identify branch points as potential substituents.
• Consider all continuous paths to determine chain length.

This step is imperative for ensuring the compound's structure and the presence of substituents are accurately reflected in the name.

Carbon Chain Numbering

Once the longest carbon chain is determined, the next step in the IUPAC naming process is numbering this chain. Numbering the carbon atoms correctly ensures that the functional groups and substituents can be accurately identified and reported in the name.
In the case of ketones, numbering should start from the end of the chain that will give the carbonyl group the lowest possible number. This ensures the importance of the carbonyl group is highlighted in the name.
For CH_3COCH(CH_3)_2 , numbering starts from the end nearest to the ketone group, giving it number 2. The carbon at position 3 harbors a methyl substituent, which must be noted in the IUPAC name. Thus, the correct position of all groups are reflected, leading to an accurate representation of the molecule's structure in its name.

• Always prioritize the numbering for the highest priority functional group, e.g., ketone.
• Ensure substituents like methyl groups are numbered for their position.
• The lowest set of locants is preferred.

Correct numbering is paramount for clarity and is a fundamental step in precisely naming organic compounds.