Question

Incorrect set of \(X\) and \(Y\) respectively when compound is an isomer of isobutane is (A) \(-\mathrm{CH}_{2} \mathrm{CH}_{3},-\mathrm{H}\) (B) \(-\mathrm{CH}_{3},-\mathrm{CH}_{3}\) (C) \(-\mathrm{H},-\mathrm{CH}_{2} \mathrm{CH}_{3}\) (D) \(-\mathrm{H},-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

The incorrect set of \(X\) and \(Y\) for an isomer of isobutane is (D) \(X = -H, Y = -CH2CH2CH3\), as it forms a longer carbon chain that makes the compound pentane instead of an isomer of isobutane.

Step-by-step solution

Understand the structure of isobutane

Isobutane, also known as i-butane or 2-methylpropane, is a branched-chain alkane with the molecular formula \((CH3)_3CH\). It is structured as a central carbon atom bonded to one hydrogen atom and three methyl groups \((-CH3)\).

Analyse the given options

Now, for a compound to be an isomer of isobutane, it should contain the same number of carbon and hydrogen atoms but have a different arrangement of these atoms. Therefore, the options (A), (B), and (C) are valid because they maintain the same number of carbon and hydrogen atoms while changing their arrangement.

Identify the incorrect option

Option (D) is incorrect as the suggested change creates a longer chain of carbon atoms than isobutane. The \(-CH2CH2CH3\) group contains an extra carbon atom, making the compound pentane rather than an isomer of isobutane. Therefore, the incorrect set of \(X\) and \(Y\) for an isomer of isobutane is \(X = -H, Y = -CH2CH2CH3\). So, (D) is the correct answer to the exercise.

Key concepts

Understanding Isobutane

Isobutane is an intriguing molecule found in the realm of organic chemistry, exemplifying the concept of isomerism with its structure. A member of the alkane family, isobutane is otherwise known as i-butane or 2-methylpropane, and it possesses a molecular formula of \( C_4H_{10} \). Diving deeper, its structure is characterized by a central carbon atom bonded to a single hydrogen atom and three methyl groups \( -CH_3 \).

When we visualize isobutane, we see a compact, branched formation rather than a straight chain. This nuance is important as it lends isobutane unique physical properties compared to its straight-chain counterparts. This branching is a key point when considering isomerism because, despite having the same molecular formula as other butanes, isobutane is structurally distinct.

Structural Isomers

Structural isomers are different molecules that share the same molecular formula but differ in the arrangement of their atoms and/or the types of bonds between them. Within the universe of organic chemistry, structural isomerism plays a pivotal role. It's fascinating that such isomers can display remarkably different chemical and physical properties, despite being composed of the exact same types of atoms.

Let's use isobutane as an example from the exercise. Isobutane has structural isomers, all of which maintain the same molecular formula \( C_4H_{10} \), yet vary in their construction. Some may have a linear structure, while others may show different branching patterns. These variations illustrate how structural isomerism offers a kaleidoscope of molecular diversity from a single molecular formula, leading to a spectrum of substances with their unique identities.

All About Alkanes

Alkanes are the simplest class of hydrocarbons, consisting solely of carbon and hydrogen atoms, with single bonds between the carbons. They follow the general molecular formula \( C_nH_{2n+2} \), where \( n \) represents the number of carbon atoms in the chain. In the family tree of organic compounds, alkanes are often considered the base from which other more complex compounds are derived.

The simplest alkane is methane \( CH_4 \), and as the chain grows, we encounter ethane \( C_2H_6 \), propane \( C_3H_8 \), and so on. Alkanes can be categorized into straight-chain and branched-chain alkanes. Isobutane falls into the latter category and serves as a prime example of how branching can occur within an alkane's carbon skeleton, leading to the phenomenon of isomerism that pervades organic chemistry.

Molecular Formula Explained

A molecular formula is the representation of the quantity and type of atoms present in a molecule. For instance, isobutane's molecular formula is \( C_4H_{10} \), which succinctly tells us there are four carbon atoms and ten hydrogen atoms in each molecule of isobutane. This formula is the starting point for understanding a substance's composition.

However, the molecular formula only reveals part of the story. It does not provide information about the molecule's structure—that's where structural formulas come into play. For isobutane and its isomers, although they share the same molecular formula, the structural formulas disclose the varied arrangements of atoms making each unique. These formulas are fundamental to exploring the landscape of organic molecules and navigating the fascinating diversity of hydrocarbons like alkanes.