Problem 140 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following alkenes: 1\. \(\mathrm{H}_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right) \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) 2\. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3}\) 3\. \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) The correct sequence of increasing order of stability of these alkenes is (a) \(3,1,2\) (b) \(1,3,2\) (c) \(1,2,3\) (d) \(2,1,3\)

Step-by-Step Solution

Answer

Sequence of increasing order of stability is (b) 1, 3, 2.

Step 1: Understand the Basic Principle

The stability of alkenes is influenced by the number of alkyl groups attached to the carbon atoms of the double bond. More substituted alkenes (those with more alkyl groups on the carbon atoms of the double bond) are typically more stable due to hyperconjugation and inductive effects.

Step 2: Analyze Alkene Structure 1

For alkene 1, \( \mathrm{H}_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right) \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \), there is a double bond with one hydrogen and the chain \( \mathrm{\left( CH_2CH_3 \right)} \) making it monosubstituted, resulting in a lower stability.

Step 3: Analyze Alkene Structure 2

For alkene 2, \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3} \), it is more substituted with a tri-substituted alkene. The presence of three methyl groups enhances its stabilization through hyperconjugation making it more stable.

Step 4: Analyze Alkene Structure 3

For alkene 3, \( \mathrm{CH}_{3} \mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \), this is a di-substituted alkene. It is less substituted than alkene 2 but more than alkene 1, giving it intermediate stability.

Step 5: Sequence Stability Order

Based on the substitution pattern where alkene 1 is monosubstituted, alkene 3 is di-substituted, and alkene 2 is tri-substituted, the increasing order of stability is alkene 1, alkene 3, then alkene 2.

Key Concepts

Hyperconjugation

Hyperconjugation is a key player in the stability of alkenes. This phenomenon occurs when there is an overlap between the electrons in the sigma bond of an adjacent C-H bond and the empty p orbitals of the double bond in alkenes. This helps distribute electron density over the molecule, lending extra stability to the alkene.

For instance, in alkene 2, \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{C}(\mathrm{CH}_{3}) \mathrm{CH}_{2} \mathrm{CH}_{3}\), we see more hyperconjugation interactions due to the presence of three methyl groups. Each of these methyl groups can participate in hyperconjugation, significantly improving the stability of this alkene compared to ones with fewer methyl groups.

Inductive Effects

The inductive effect is another stabilizing factor for alkenes. It refers to the effect that an electronegative atom or group of atoms can have on the polarity of a molecule.

In alkenes, alkyl groups are known to push electrons (positive inductive effect) towards the double bond. This enhances the electron density around the double bond, providing additional stability. In alkene 2, the multiple alkyl groups contribute significantly to this positive inductive effect, increasing its overall stability.

Conversely, alkene 1 has fewer alkyl groups contributing to this effect, thus making it less stable than alkene 2.

Substitution Pattern

The substitution pattern of alkenes is crucial for determining their stability. More highly substituted alkenes are generally more stable. This is because they have more alkyl groups that can stabilize the molecule via hyperconjugation and the inductive effect.

In the original exercise, we were asked to arrange alkenes in order of increasing stability. Alkene 2 is tri-substituted, meaning it has the most alkyl groups attached to the carbons of the double bond. Alkene 3 is di-substituted, and alkene 1 is monosubstituted. Therefore, the stability order from least to greatest is alkene 1, alkene 3, and alkene 2.

Organic Chemistry

Organic chemistry, a fundamental branch of science, explores the properties, structures, and reactions of carbon-based compounds, including alkenes. Understanding why certain alkenes are stable requires a foundation in organic chemistry concepts like hyperconjugation and inductive effects.

When approaching problems such as determining alkene stability, it's essential to analyze the molecular structure and substitution pattern. These concepts elucidate why certain configurations offer more stability. Organic chemistry lays the groundwork for understanding these phenomena and provides the tools for evaluating chemical behavior systematically.

JEE Main Chemistry

JEE Main chemistry is a critical part of India's Joint Entrance Examination, focusing on topics like organic chemistry. For students preparing for JEE Main, grasifying concepts such as alkene stability is crucial as it frequently appears in exams.

By understanding the principles of hyperconjugation, inductive effects, and substitution patterns, students can predict the stability of different alkenes confidently. Such deep comprehension aids not just in exams but in developing a strong science foundation. This makes organic chemistry an indispensable part of JEE Main preparation.