Question

The IUPAC name of ${\mathrm{CH}}_3\mathrm{COCH}{\left({\mathrm{CH}}_3\right)}_2$ is (A) 1,1 -dimethylpropan-2-one (B) isopropyl methyl ketone (C) 2-methylbutan-3-one (D) 3 -methylbutan-2-one

Short answer

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

Step-by-step solution

Determine the parent chain

Locate the longest continuous chain of carbon atoms in the structure. In this case, the longest chain is 4 carbon atoms long, starting from the carbon on the left (${\mathrm{CH}}_3$) and ending with the carbon on the right ($\mathrm{CH}{\left({\mathrm{CH}}_3\right)}_2$).

Identify the functional group

The functional group in this structure is a ketone, indicated by the carbonyl group ($\mathrm{C}=\mathrm{O}$). Ketones have the IUPAC suffix '-one'.

Number the carbon atoms in the parent chain

To determine the numbering for the longest chain, start at the end of the chain that leads to the earliest occurrence of the functional group. In this case, number the chain from left to right. Therefore, the four carbon atoms in the chain are numbered 1, 2, 3, and 4. The carbonyl group is attached to the second carbon atom in the parent chain.

Identify side chains and their positions

A side chain is present on the last carbon atom in the chain. This side chain is a methyl group (${\mathrm{CH}}_3$). To indicate the position of the side chain in the parent chain, include the carbon number (in this case, 3) before the side chain's name, as '3-methyl'.

Assemble the IUPAC name

Combine all the components from the previous steps to form the IUPAC name: - Parent chain: 4 carbon atoms, which corresponds to 'butane'. - Functional group: A ketone with the suffix '-one'. - Carbon number of the carbonyl group: 2, resulting in 'butan-2-one'. - Side chain with its position: '3-methyl'. Therefore, the complete IUPAC name is: 3-methylbutan-2-one. The correct answer is (D) 3-methylbutan-2-one.

Key concepts

Ketone Functional Group

A ketone is an organic compound characterized by a carbonyl group (C=O) bonded to two other carbon atoms. This carbonyl group is the defining feature that sets ketones apart from other functional groups. Ketones are named using the suffix '-one' in the IUPAC nomenclature.

To identify a ketone in a molecular structure, look for a carbon-oxygen double bond located between two carbon atoms. This is distinct from aldehydes, where the carbonyl group is at the end of the carbon chain. Recognizing a ketone is crucial because it determines the suffix of the compound's name and helps in deciding how to number the carbon chain.

In the provided example, the carbonyl group (C=O) is attached to the second carbon of the four-carbon chain, which confirms the presence of a ketone. Hence, the suffix '-one' is used, indicating that this is a ketone compound.

Carbon Chain Numbering

Numbering the carbon atoms in the longest chain is an essential step in IUPAC naming. The goal is to ensure the lowest possible numbers are assigned to the functional groups and substituents. This helps maintain a standardized naming system.

Begin numbering from the end of the chain nearest to the functional group. For the ketone example, the carbon chain should be numbered so that the ketone group receives the lowest possible number.

• Identify the longest continuous chain of carbon atoms.
• Start from the end closer to the functional group (in this case, the ketone).
• Assign numbers sequentially to each carbon atom.

In the molecule ${\mathrm{CH}}_3\mathrm{COCH}({\mathrm{CH}}_3{)}_2$, number the carbon chain starting from the left, making the ketone group occur at carbon 2. This ensures the correct structural portrayal of "butan-2-one" in the IUPAC name.

Methyl Side Chain Identification

Substituents, such as side chains, are additional groups attached to the main carbon chain that are not part of the longest chain. Identifying these is vital to fully describe the structure of the compound.

A methyl group is a side chain with the formula ${\mathrm{CH}}_3$. In the given structure, methyl groups are side chains attached to the third and last carbon atoms of the main sequence.

• Determine the position of the side methyl group on the main chain through its bond site.
• Use the carbon numbers to indicate its exact location in the name.
• In this case, the methyl group is attached to carbon 3.

The position of the side chain in the structure is indicated by a number before its name, thus the inclusion of '3-methyl' in the complete IUPAC name '3-methylbutan-2-one'.