Problem 184 - A Complete Resource Book in Chemistry for JEE Main

Question

The IUPAC name of (a) 4,4 - dimethyl-5-diethyl pentane (b) 5,5 -diethyl-4, 4 -dimethyl pentane (c) 3 -ethyl-4, 4-dimethyl heptane (d) 1,1 -diethyl-2, 2 -dimethyl pentane

Step-by-Step Solution

Answer

The correct IUPAC names are: (a) Incorrect; should be 3,3-diethyl-2,2-dimethylpentane. (b) Incorrect; should be 3,3-diethyl-4,4-dimethylpentane. (c) 3-ethyl-4,4-dimethylheptane as given. (d) 2,2-diethyl-3,3-dimethylpentane.

Step 1: Identify the Longest Continuous Carbon Chain

For compounds (a), (b), (c), and (d), identify the longest continuous carbon chain. This chain determines the root name of the compound, according to IUPAC nomenclature.

Step 2: Name the Main Chain and Verify Substituents

Based on Step 1, label the main chain and check the substituent groups attached to it. Provide each substituent group a position based on its attachment to the carbon atoms in the main chain.

Step 3: Number the Main Chain Appropriately

Start numbering the main chain from the end nearest to the first substituent group. This helps in assigning the lowest possible numbers to the substituents.

Step 4: Assign IUPAC Names to Each Compound

For each compound, combine the substituent names with their respective positions, followed by the name of the main chain. The substituents should be listed in alphabetical order, regardless of their position numbers.

Key Concepts

Longest Carbon Chain

When naming an organic molecule according to the IUPAC system, the first step is to identify the longest continuous carbon chain in the molecule. This is crucial because it forms the backbone of the compound's name. For example, in compound (c), 3-ethyl-4, 4-dimethyl heptane, the longest chain has seven carbon atoms, hence the root name 'heptane'. Always look for the continuous chain that contains the most carbon atoms. Sometimes chains of the same length are found in different directions. Choose the chain that leads to the simplest set of substituent names or the one with the largest number of substituents.

Substituent Groups

Substituents are atoms or groups of atoms attached to the main carbon chain. They are not part of the longest chain and are typically shown as branches on main carbon chains in structural formulae.
In the compounds given, substituents include methyl groups (as in 4,4-dimethyl) and ethyl groups (as in 5,5-diethyl). In IUPAC naming, each substituent group must be identified and named.
• Methyl groups are made of one carbon atom and three hydrogen atoms: CH₃.

• Ethyl groups consist of two carbon atoms: C₂H₅.
• Each substituent's position on the main chain must also be specified, which brings us to the idea of numbering.

Alphabetical Order of Substituents

The IUPAC system requires listing substituents in alphabetical order, regardless of their position numbers. This is done to maintain a consistent naming format.
• For instance, in the compound 3-ethyl-4,4-dimethylheptane, 'ethyl' comes before 'methyl' even though the methyl group appears earlier in the compound's structure.

• Note that prefixes like 'di-', 'tri-', etc., used to show the number of identical substituents, are not considered in alphabetical order.
• However, 'ethyl' will be listed before 'methyl' because 'e' comes before 'm' in the alphabet.

Numbering Carbon Chain

Once you have identified the longest chain and recognized the substituents, the next step is numbering the carbon atoms in the main chain.
• Start from the end of the chain closest to the first substituent. This rule minimizes the numerical labels of the substituents, leading to the smallest possible numbers in the compound's name.

• For instance, if you can start numbering from either end, choose the one that provides the lower numbers to the substituents.
• If two substituents are equidistant from either end, the substituent that appears first alphabetically will get the lower number.

This careful numbering ensures clarity and uniformity across IUPAC names, assisting in correctly identifying molecular structures.