Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

A hydrocarbon of molecular formula, \(\mathrm{C}_{8} \mathrm{H}_{18}\) gives only one monochloro derivative. The hydrocarbon is (a) 2 -methyl heptane (b) octane (c) \(2,2,3,3\)-tetramethyl butane (d) \(2,2,4\)-trimethyl pentane

Step-by-Step Solution

Answer

The hydrocarbon is 2,2,3,3-tetramethylbutane.

Step 1: Understand Monochloro Derivative

A monochloro derivative is a compound formed when one hydrogen atom in a hydrocarbon is replaced by a chlorine atom. For a hydrocarbon to give only one monochloro derivative, all hydrogen atoms must be equivalent, meaning they are in the same environment.

Step 2: Analyze Options for Symmetry

We need to determine which hydrocarbon among the options has a symmetrical structure, leading all hydrogen atoms to be equivalent. Let's evaluate the options one by one:

Step 3: Examine 2-methyl heptane

The structure of 2-methylheptane is not symmetrical. This molecule has different types of hydrogens leading to multiple monochloro derivatives. Thus, it is not the correct answer.

Step 4: Examine Octane

Octane is a straight-chain alkane without any branching. In this structure, hydrogen atoms are not symmetrically equivalent because of the linear S-C-C-S structure, leading to multiple derivatives. Hence, not the correct answer.

Step 5: Examine 2,2,3,3-tetramethylbutane

This is a symmetrical, highly branched molecule. All hydrogen atoms are in identical environments due to the four identical methyl groups bonded to a central carbon "butane" structure. This results in just one monochloro derivative.

Step 6: Examine 2,2,4-trimethylpentane

The structure of 2,2,4-trimethylpentane has a branching that creates non-equivalent hydrogens, leading to multiple monochloro derivatives. Therefore, it's not the correct option.

Step 7: Conclusion

Upon evaluating the symmetrical properties of the hydrocarbons, 2,2,3,3-tetramethylbutane shows symmetry resulting in a single monochloro derivative, making it the correct choice.

Key Concepts

Hydrocarbons

Hydrocarbons are organic compounds made up entirely of hydrogen and carbon atoms. They are the basic building blocks of organic chemistry. There are different types of hydrocarbons, including alkanes, alkenes, and alkynes.

• Alkanes: These are saturated hydrocarbons with single bonds between carbon atoms. They have the general formula, \({C}_{n}H_{2n+2}\).
  
• Alkenes: These are unsaturated hydrocarbons containing at least one carbon-carbon double bond, with the general formula, \({C}_{n}H_{2n}\).
  
• Alkynes: These are also unsaturated hydrocarbons, but they contain at least one carbon-carbon triple bond, and follow the formula, \({C}_{n}H_{2n-2}\).
  

Understanding hydrocarbons is crucial because they serve as the starting point for many chemical reactions and products, including fuel and plastics.

Isomerism

Isomerism is a phenomenon where two or more compounds have the same chemical formula but different structures or arrangements of atoms. This concept is crucial in organic chemistry because it explains the diversity of organic compounds with similar compositions but different properties.

• Structural Isomerism: This occurs when compounds have the same molecular formula but different structural formulas. It's further divided into chain isomerism, position isomerism, and functional group isomerism.
  
• Stereo Isomerism: This involves compounds with the same structural formula but different spatial arrangements of atoms. It includes geometrical isomerism and optical isomerism.
  

By analyzing isomerism, chemists can predict the number of possible compounds a simple molecular formula can produce, significantly impacting material synthesis in both industrial and academic settings.

Substitution Reactions

Substitution reactions are fundamental in organic chemistry, where one atom or group of atoms in a molecule is replaced by another atom or group of atoms. They are vital in understanding how molecules interact and transform in chemical reactions.

• Nucleophilic Substitution: Here, a nucleophile replaces a leaving group in a molecule. This is common in aliphatic carbon compounds, where a halogen is replaced by a nucleophile.
  
• Electrophilic Substitution: Primarily observed in aromatic compounds, where an electrophile replaces a hydrogen atom in the aromatic ring.
  

Substitution reactions are widely used in the synthesis of complex organic molecules, including the development of pharmaceuticals and polymers.

Molecular Symmetry

Molecular symmetry refers to the balanced distribution of identical parts in a molecule. Symmetry plays a significant role in determining the physical and chemical properties of a compound. In organic chemistry, symmetry can greatly influence a molecule's reactivity and the types of products it can form in reactions.

• Symmetrical Molecules: These are molecules that have identical parts or atoms distributed evenly. A symmetric molecule often leads to fewer possible isomers or reaction products, as seen in 2,2,3,3-tetramethylbutane from the exercise. This molecule's symmetry allows it to form only one monochloro derivative.
  
• Asymmetrical Molecules: These molecules do not have uniformly distributed identical parts, leading to a greater variety of possible structures and reaction products.
  

Understanding molecular symmetry is essential for predicting molecular behavior during chemical reactions, which is especially important in the fields of pharmaceuticals and materials science.