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

Question

2,3 -dimethyl hexane contains .... tertiary .... secondary and ..... 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 correct answer is (d) \(2,2,4\).

Step 1: Understand the Molecular Structure

The name "2,3-dimethyl hexane" indicates a hexane chain (6 carbon atoms in a row) with two methyl groups attached. The methyl groups are attached at the 2nd and 3rd positions of the hexane chain.

Step 2: Sketch the Carbon Chain

Draw a straight chain of 6 carbon atoms: C-C-C-C-C-C. This represents the hexane base.

Step 3: Add the Methyl Groups

Attach a methyl group (CH3) to both the 2nd and 3rd carbon atoms of the hexane chain. The structure can be visualized as C-CH(CH3)-CH(CH3)-C-C-C.

Step 4: Identify Carbon Types

A primary carbon is attached to one other carbon, a secondary carbon is attached to two, and a tertiary carbon is attached to three. Check each carbon in the molecule.

Step 5: Determine Tertiary Carbons

The 2nd and 3rd carbons are tertiary, as each is attached to three other carbons. Total tertiary carbons = 2.

Step 6: Determine Secondary Carbons

The 1st and the 6th carbons are secondary as each is attached to two carbons only. Total secondary carbons = 2.

Step 7: Determine Primary Carbons

The methyl groups on the 2nd and 3rd carbons are primary, as they are each only attached to one carbon atom respectively. Total primary carbons = 4.

Step 8: Select the Correct Answer

With 2 tertiary, 2 secondary, and 4 primary carbons, the correct choice is "(d) \(2,2,4\)."

Key Concepts

Alkanes

Alkanes are a fascinating group of organic compounds known for their simplicity and chemical stability. These molecules are typically composed entirely of carbon (C) and hydrogen (H) atoms, connected by single covalent bonds. Because they contain only single bonds, alkanes are often referred to as saturated hydrocarbons. Due to this saturation, they are generally unreactive with many chemical reagents, making them a stable backbone for various chemical applications.

An interesting aspect of alkanes is their chain length. The simplest alkane is methane ( CH_4 ), followed by ethane ( C_2H_6 ) and propane ( C_3H_8 ). As the number of carbon atoms increases, so does the complexity of the alkane. For example, hexane ( C_6H_14 ), which is part of the compound 2,3-dimethylhexane, has six carbon atoms in its straight-chain form. This straight-chain structure and other potential structural branching contribute greatly to the diversity observed in alkanes.

• Physical Properties: Alkanes tend to have low boiling and melting points.
• Applications: Used as fuels (like in natural gas and gasoline).
• Presence in Nature: Found in crude oil and natural gas.

Understanding alkanes is essential for recognizing how basic organic compounds are structured and classified.

Carbon Classification

Carbon atoms in organic molecules can be classified based on the number of carbon atoms they are bonded to. This classification helps in understanding the molecule's structure and reactivity. Let's delve into the three main types:

• Primary Carbon (1°): A carbon atom bonded to only one other carbon atom. For example, in 2,3-dimethylhexane, the carbons in the methyl groups are primary because each methyl group carbon is connected to just one other carbon atom in the main chain.
• Secondary Carbon (2°): These are carbon atoms attached to two other carbon atoms. In the hexane chain, the first and sixth carbons fit this description, being part of the main chain yet only linked to a neighbor on either side.
• Tertiary Carbon (3°): A carbon bonded to three other carbon atoms. In 2,3-dimethylhexane, the second and third carbons are tertiary as they're bonded to the primary methyl carbons as well as their neighbors in the main chain.

This classification is crucial for determining the chemical reactivity and properties of organic compounds. It aids in identifying sites in a molecule that may undergo chemical transformations.

Molecular Structure

The molecular structure of organic compounds plays a significant role in their chemical properties and interactions. For organic molecules such as 2,3-dimethylhexane, the arrangement of carbon atoms forms the backbone of its molecular structure.

Molecular structure can be visualized in several ways, including:

• Linear Structure: Refers to a straight chain of carbon atoms, like the backbone of hexane, which has six sequential carbon atoms.
• Branched Structure: Introduces branches to the main chain, like the methyl groups in 2,3-dimethylhexane attached to the second and third carbons in the chain, causing structural branching.

This structural foundation influences the molecule's behavior in chemical reactions. Furthermore, understanding the molecular structure is important for predicting the molecule's physical properties, such as solubility and boiling point. The arrangement of atoms and type of carbon classification (primary, secondary, tertiary) within the structure provides insights into potential reactions and synthesis pathways.

Consider learning how to sketch these structures to better visualize the relationships between atoms in complex organic molecules.