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

(b) hyperconjugation

Step 1: Analyze the Molecules

We start by analyzing the structures of 2-methyl but-2-ene and but-2-ene. 2-methyl but-2-ene has a CH3 group attached to the second carbon, whereas but-2-ene does not have this additional group.

Step 2: Identify the Stabilizing Effect

The extra CH3 group can participate in hyperconjugation, which involves the overlap of sigma electrons with the p-orbital of the double bond, offering additional stability through electron delocalization.

Step 3: Understand Hyperconjugation

Hyperconjugation is sometimes called the no-bond resonance effect and involves the overlap of the sigma bond (C-H) with an adjacent empty p-orbital or π-orbital, stabilizing the alkene due to the dispersal of charge.

Step 4: Determine the Reaction Suitability

Because of the hyperconjugative effect, 2-methyl but-2-ene is more stabilized compared to but-2-ene, which makes it less reactive as the increased stability decreases its tendency to react.

Key Concepts

Hyperconjugation

Hyperconjugation is a fascinating concept in organic chemistry. It explains how certain chemical bonds can enhance the stability of molecules. In simple terms, hyperconjugation refers to the interaction between a sigma (σ) bond—often a C-H bond—and an adjacent pi (π) bond or p-orbital. This interaction, sometimes called the "no-bond resonance effect," allows for the spreading out or delocalization of electrons, which stabilizes the molecule.

In the case of alkenes, such as 2-methyl but-2-ene, hyperconjugation occurs when the C-H bonds in the methyl group overlap with the empty or filled p-orbitals of the carbon-carbon double bond. This overlap leads to enhanced stability not just for 2-methyl but-2-ene itself but for other similar alkenes as well. The stability is a result of the effective sharing of electron density over the entire molecule rather than being confined to one bond or location.

• Increases stability through electron delocalization
• Involves the interaction of σ bonds with π or p-orbitals
• Contributes to decreased reactivity of certain alkenes

This principle of hyperconjugation is key to understanding why certain alkenes are less reactive than others.

Molecular Stability

Stability is a cornerstone concept when studying molecular reactions and behavior. In the context of alkenes, molecular stability often influences reactivity. A stable molecule has less tendency to react because the energy required to break its bonds or alter its structure is high. This is why 2-methyl but-2-ene, with its hyperconjugative effect, shows decreased reactivity compared to but-2-ene.

The presence of additional alkyl groups, such as the CH₃ group in 2-methyl but-2-ene, promotes hyperconjugation, increasing the molecule's stability. This means the more stable the alkene, the less likely it is to participate in reactions, as it already exists in a lower energy, more favorable state.

Moreover, molecular stability isn't solely about hyperconjugation. Other factors also contribute, such as:

• Resonance structures which can distribute charge over a molecule
• Inductive effects which can influence electron distribution through sigma bonds
• Electromeric effects that temporarily redistribute electrons during reactions

Overall, a stable molecular structure will likely exhibit reduced reactivity.

Carbon-Carbon Double Bonds

At the heart of alkene chemistry is the carbon-carbon double bond, a defining feature of these molecules. Characterized by a sigma (σ) bond and a pi (π) bond, the double bond is a source of both strength and vulnerability.

In terms of structure, the carbon-carbon double bond is planar, with regions of electron density above and below the bond axis due to the π bond. This makes it possible for interactions such as hyperconjugation to occur, where sigma bonds can overlap with the electron cloud of the double bond, lending stability.

The pi bond in a double bond is typically more reactive because it is less entangled with the nucleus, making it accessible for reactions. When additional bonds like C-H bonds interact through hyperconjugation, the double bond becomes more stable, decreasing its reactivity—exactly the case in 2-methyl but-2-ene.

• Sigma and pi bonds together define the double bond
• Planar structure allows interactions that influence stability
• The reactivity largely depends on the pi bond's accessibility

The fascinating dual nature of carbon-carbon double bonds—simultaneously a point of weakness and strength—underlines much of alkene reactivity and stability analysis.