Problem 17 - A Complete Resource Book in Chemistry for JEE Main

Question

2-methyl but-2-ene is less reactive than but-2-ene. This is due to (a) resonance (b) hyperconjugation (c) electromeric effect (d) inductive effect

Step-by-Step Solution

Answer

The decreased reactivity is due to (b) hyperconjugation.

Step 1: Understanding the Structures

First, we need to understand the structures of 2-methyl but-2-ene and but-2-ene. In 2-methyl but-2-ene, a methyl group is attached at the second carbon, while but-2-ene is simply a four-carbon chain with a double bond between the second and third carbon.

Step 2: Identify Stabilizing Effects

We must identify which stabilizing effects are present in the molecules. Hyperconjugation and inductive effects are stabilizing contributions in alkenes. Resonance and electromeric effects don't typically contribute directly to alkene stability as much.

Step 3: Analyzing Hyperconjugation

In 2-methyl but-2-ene, the methyl group provides additional hyperconjugation, which involves delocalization of electrons from the C-H bonds of the methyl group to the double bond, leading to greater stability.

Step 4: Analyzing the Effect of Substituents

The presence of additional alkyl groups in 2-methyl but-2-ene enhances hyperconjugation, increasing electron density on the double bond. This results in a more stable but less reactive compound compared to a simple alkene like but-2-ene.

Step 5: Determining Effect on Reactivity

Increased stability due to hyperconjugation lowers the reactivity of 2-methyl but-2-ene compared to but-2-ene, answering the problem question.

Key Concepts

Hyperconjugation

Hyperconjugation plays a crucial role in influencing alkene stability. It is a stabilizing interaction that occurs when the electrons in a sigma (σ) bond, such as a C-H bond, are delocalized into an adjacent empty p orbital or a π bond orbital. This process allows for the spreading out of electron density over multiple atoms, thereby increasing the stability of the molecule.

In the context of alkenes, hyperconjugation provides additional stabilization to a carbon-carbon double bond. For example, in 2-methyl but-2-ene, the methyl groups are capable of hyperconjugating with the π system of the double bond. Each methyl group can contribute to multiple hyperconjugative structures, stabilizing the alkene.

Thus, alkenes with more alkyl substitutions experience a greater hyperconjugation effect, which leads to increased stability. This explains why 2-methyl but-2-ene demonstrates greater stability than but-2-ene, as it has more opportunities for hyperconjugation.

Inductive Effect

The inductive effect is another important factor contributing to the stability of alkenes. It refers to the transmission of charge through a chain of atoms in a molecule by electrostatic induction.

Unlike hyperconjugation, the inductive effect depends on the electronegativity of the substituents rather than their ability to delocalize electrons. Alkyl groups, being electron-donating, are known to push electrons towards the double-bonded carbons. This donation increases the electron density along the alkene's carbon atoms, which enhances the molecule's stability.

• Positive inductive effect (+I): occurs when electron-donating groups (EDGs) are present.
• Alkyl groups are typically +I groups, stabilizing the alkene through increased electron density.

In 2-methyl but-2-ene, the alkyl substituents provide this +I effect, thereby contributing further to the overall stabilization of the alkene structure.

Alkene Reactivity

The reactivity of alkenes is inversely related to their stability. This means that the more stable an alkene, the less reactive it tends to be.

Alkenes like 2-methyl but-2-ene are more stable due to factors like hyperconjugation and the inductive effect. While this increased stability makes the molecule less likely to react, it also serves as a protection mechanism against unwanted reactions.

The lower reactivity of more substituted alkenes, like those in the 2-methyl series, is largely because the electron-dense environment around the double bond hampers the approach of electrophiles. In practical terms, this means:

• Alkenes with more alkyl groups are less reactive to addition reactions.
• Less substituted alkenes, like but-2-ene, are more reactive due to less stabilization from hyperconjugation and the inductive effect.

In conclusion, while both stability and reactivity are important aspects of alkene behavior, the factors that stabilize an alkene usually lead to a decrease in its reactivity.