Problem 49 - A Complete Resource Book in Chemistry for JEE Main

Question

The IUPAC name of the compound \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) is (a) \(1,1,3,3\)-tetramethyl-but- 1 -ene (b) \(1,3,3\)-trimethyl-pent \(-2\)-ene (c) \(2,2,4\)-trimethylbut- 4 -ene (d) \(2,4,4\)-trimethylpent- 2 -ene

Step-by-Step Solution

Answer

The IUPAC name is (d) 2,4,4-trimethylpent-2-ene.

Step 1: Identify the Longest Carbon Chain

Find the longest continuous chain of carbon atoms that includes the double bond. In this compound, the longest chain including the double bond has 5 carbon atoms. This indicates the base name would contain "pent-" for the number of carbon atoms.

Step 2: Number the Carbon Chain

Number the carbons in the chain starting from the end closest to the double bond, ensuring the double bond gets the lowest possible number. Here, numbering from left to right gives us: 1, 2, 3, 4, 5, with the double bond starting from carbon 2, hence it is a "pent-2-ene".

Step 3: Identify and Position Substituents

Identify any substituents attached to the chain. There are three methyl groups at carbons 2, 4, and 4 (two on the same carbon). This is indicated as 2,4,4-trimethyl.

Step 4: Assemble the IUPAC Name

Combine all the parts together. You have the base name "pent-2-ene" and the substituents "2,4,4-trimethyl". Putting it together results in the name "2,4,4-trimethylpent-2-ene".

Key Concepts

Longest Carbon Chain

When identifying the IUPAC name of a chemical compound, the first step is to determine the longest continuous carbon chain. This chain forms the backbone of the compound, and its length determines the base name. For example, in the given compound, the longest chain that includes the double bond has five carbon atoms.
This leads us to use 'pent-' in the name, indicating a five-carbon backbone. It is crucial to include the double bond in this chain as it significantly influences the compound's name and properties. Finding the longest chain should always prioritize including any double or triple bonds present in the structure.

Numbering the Carbon Chain

Once the longest carbon chain is identified, the next step is numbering the carbons to provide a unique address for each carbon and any double bonds or substituents. Begin numbering from the end that results in the lowest possible number for the double bond. In this example, numbering from the left assigns the double bond its optimal position, beginning at carbon 2. Therefore, this chain is referred to as 'pent-2-ene'. Proper numbering ensures minimal ambiguity, with IUPAC rules giving priority to double bonds over other substituents when assigning numbers.

Identifying Substituents

Identifying and correctly positioning substituents is vital for the final IUPAC name. In our compound, substituents are the groups attached to the main carbon chain that are not part of it. Here, we have three substituents, all methyl groups.

• One methyl group is at carbon 2.
• Two methyl groups are at carbon 4.

Thus, these groups are combined as '2,4,4-trimethyl' to indicate their location and repetition. Knowing how to identify and number substituents aids in accurately building the IUPAC name.

Assembly of IUPAC Name

Combining all these elements results in the full IUPAC name. Begin with combining the substituents with their respective positions followed by the base name of the compound, which includes the position of any double bonds. In this example, start with '2,4,4-trimethyl' to denote the positions and type of substituents. Next, append this to the 'pent-2-ene', which not only indicates a five-carbon backbone but also specifies the position of the double bond. This leads to a comprehensive IUPAC name: '2,4,4-trimethylpent-2-ene'. This methodical assembly ensures that chemists across the world can precisely understand the compound's structure from its name.