Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

2, 3 -dimethyl hexane contains .... tertiary.... secondary and \(\ldots . .\) primary carbon atoms, respectively. (a) \(2,4,3\) (b) \(3,2,4\) (c) \(4,3,2\) (d) \(2,2,4\)

Step-by-Step Solution

Answer

The answer is (d) 2 tertiary, 2 secondary, and 4 primary carbons.

Step 1: Understanding the Structure

The compound named 2,3-dimethylhexane means there is a hexane chain (with 6 carbon atoms) as the base structure. There are methyl groups attached at the 2nd and 3rd carbon atoms of the main chain.

Step 2: Drawing the Structure

Draw the structure of hexane: CH₃-CH₂-CH₂-CH₂-CH₂-CH₃ Next, place a methyl group (CH₃) at the 2nd and 3rd carbon position: ``` CH₃ CH₃ | | CH₃-CH-CH-CH₂-CH₂-CH₃ ```

Step 3: Identifying Tertiary Carbons

A tertiary carbon atom is bonded to three other carbon atoms. In the structure, carbons 2 and 3 each connect to three carbon atoms (one from the main chain and two from methyl groups), making them tertiary.

Step 4: Identifying Secondary Carbons

A secondary carbon atom is bonded to two other carbon atoms. In the structure, only the 4th carbon is bonded to two other carbons.

Step 5: Identifying Primary Carbons

Primary carbon atoms are bonded to only one other carbon atom. In this structure, these are the four terminal methyl groups at both ends and the additional two methyl groups attached to carbon 2 and 3.

Step 6: Counting the Carbons

Count the number of each type of carbon: - Tertiary: 2 (at carbons 2 and 3) - Secondary: 1 (at carbon 4) - Primary: 4 (terminal carbons and additional methyl groups) Compare this with the options.

Step 7: Checking Against Answers

As calculated, the structure contains 2 tertiary carbons, 1 secondary carbon, and 4 primary carbons, matching choice d) (2, 2, 4).

Key Concepts

Hydrocarbon Structure

Understanding hydrocarbon structures is fundamental in organic chemistry. Hydrocarbons are compounds entirely composed of carbon and hydrogen. Their simplicity paves the way for a multitude of variations, primarily classified into two types: aliphatic and aromatic. In 2,3-dimethylhexane, we are dealing with an aliphatic hydrocarbon known as an alkane.

Alkanes have carbon atoms connected by single bonds, known as saturated hydrocarbons. They have a general formula of \( C_{n}H_{2n+2} \), where \( n \) is the number of carbon atoms. In hexane, the parent chain has six carbon atoms, hence the term 'hexane.' The parent chain is the longest continuous carbon chain in the molecule.

Substituents, like methyl groups in this case, are branches or side chains attached to the main carbon skeleton. In 2,3-dimethylhexane, a methyl group is attached to the second and third carbon atom of the hexane chain. Understanding how these groups are attached is key to determining the structure.

Carbon Atoms Classification

Each carbon atom in an organic molecule can be classified based on the number of carbon atoms it is bonded to. This classification is crucial in understanding the reactivity and properties of the molecule.

• Primary Carbon: A primary carbon is bonded to only one other carbon atom. In organic compounds, this often makes them less reactive compared to secondary or tertiary carbons. In our structure, primary carbons are found at the terminal ends of the chain and in the methyl groups.
• Secondary Carbon: A carbon atom bonded to two other carbons is called secondary. They are more reactive than primary carbons due to adjacent carbon bonds. In 2,3-dimethylhexane, the fourth carbon is secondary.
• Tertiary Carbon: A tertiary carbon connects with three other carbon atoms. These are usually the most reactive as they are surrounded by other carbon atoms. Carbons 2 and 3, which connect to the main chain and two methyl groups, are tertiary.

Carbon classification helps in predicting the behavior of the compound in chemical reactions and is a significant aspect of organic chemistry.

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system provides a standardized method to name chemical compounds. This system is important in ensuring consistent communication of molecular structures across the globe.

To name a molecule such as 2,3-dimethylhexane using IUPAC standards, follow these steps:

• Identify the Longest Chain: Determine the longest continuous chain of carbon atoms. For 2,3-dimethylhexane, it's a six-carbon chain named hexane.
• Number the Chain: Begin numbering from the end closest to the first substituent. Here, the methyl groups attached are the closest, so numbering starts from there.
• Identify and Name Substituents: These are known as alkyl groups. In this example, methyl is the substituent, positioned on carbons 2 and 3.
• Combine in One Name: Arrange substituents alphabetically if needed and indicate their positions. Use prefixes for multiple identical groups (e.g., di-, tri-). This gives the name 2,3-dimethylhexane.

Using IUPAC nomenclature ensures that every compound has a unique name, preventing confusion in chemical communication.