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 of the compound is \(1,1,3,3\)-tetramethyl-but-1-ene.

Step 1: Identify the Longest Carbon Chain

Examine the structure of the compound to identify the longest continuous carbon chain containing the double bond. The compound is given as \(\mathrm{CH}_{3}-\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\). It has a chain of four main carbon atoms in a line which includes the double bond.

Step 2: Determine the Primary Chain and Number It

Name the main carbon chain found in the compound. Since the chain contains four carbon atoms with a double bond, it forms a `but-` chain. Number the carbon atoms starting from the end nearest to the double bond: C1 = \(\mathrm{CH}_{3}\)-, C2 = \(\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\)-, and C3 = \(\mathrm{CH}=\).

Step 3: Locate and Name Substituents

Identify any substituents attached to the main carbon chain. Two \(\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) groups are present as substituents on the main chain. These are at positions 2 and 3 (closest to the double bond). Assign the name `dimethyl` as per their locations.

Step 4: Assemble the IUPAC Name

Combine the elements identified: the number of carbon atoms (but-), the number of the carbon containing the double bond (1-ene), and identify substituents with correct numbering. Write the full name by organizing: \(1,1,3,3\)-tetramethyl-but-1-ene.

Key Concepts

Longest Carbon Chain

One of the first steps in determining the IUPAC name of an organic molecule is identifying the longest carbon chain. The longest carbon chain is the one that includes the maximum number of carbon atoms in a continuous line. However, when a double bond is present, priority is given to the chain that contains this double bond.
This choice can influence the final name, as it determines the root of the nomenclature, the base chain length descriptor, such as but- for four carbons, pent- for five, and so on. Ensure the chain chosen indeed encompasses the double bond to comply with IUPAC guidelines.

Substituents

Substituents are groups or branches attached to the primary chain in a compound. They often include alkyl groups like methyl (\( \text{CH}_3 \)), ethyl (\( \text{C}_2\text{H}_5 \)), and others. Identifying substituents correctly is crucial as they modify the base name of the molecule.

• Identify all branches or groups attached to the longest carbon chain.
• Assign a name to each substituent according to the number of carbons it contains.
• Determine the location of each substituent by numbering the chain from the end nearer to a substituent, or the double bond, if present.

In our exercise, we have methyl groups at several positions, affecting the compound's IUPAC name.

Double Bond Position

The double bond is a key feature in the IUPAC nomenclature of alkenes. It not only affects how you number the chain but also how the structure is named.
The double bond is typically given the lowest possible number in the chain. This is achieved by starting the numbering from the end of the carbon chain closest to the double bond. This approach ensures the position of the double bond is accurately reflected in the compound's name, such as -1-ene or -2-ene, indicating the double bond starts at the first or second carbon, respectively.

Organic Chemistry

Organic chemistry is a vast field focused on the study of carbon-containing compounds. Essential to the discipline is understanding how the structure of a molecule dictates its properties and reactivity.
Part of mastering organic chemistry involves learning IUPAC nomenclature, which standardizes the names of compounds, allowing chemists to communicate clearly. Knowing how to effectively use IUPAC rules aids in predicting compound behaviors and reactions. Understanding these naming conventions is also useful beyond chemistry, in biology and materials science, where the construction of molecules can dramatically impact function.