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: Identify the Structure of Each Compound

2-methylbut-2-ene has a structure where a methyl group is attached to the second carbon, causing increased branching compared to but-2-ene, which is a linear alkene.

Step 2: Understand Hyperconjugation

Hyperconjugation involves the delocalization of electrons from C-H bonds adjacent to the double bond. It stabilizes the alkene by dispersing charge through the overlap of these bonds with the p-orbital of the double bond.

Step 3: Compare Hyperconjugation in Both Compounds

In but-2-ene, there's more potential for hyperconjugation because there are more C-H bonds adjacent to the double bond to stabilize it. In 2-methylbut-2-ene, the methyl group leads to steric hindrance and less hyperconjugation.

Step 4: Discuss Bond Stability

Increased hyperconjugation in but-2-ene leads to greater delocalization of electrons, making it more stabilized and thus more reactive compared to 2-methylbut-2-ene.

Step 5: Conclude with the Effect

The reduced reactivity of 2-methylbut-2-ene is mainly due to reduced hyperconjugative stabilization compared to but-2-ene.

Key Concepts

Alkene Stability

Understanding the stability of alkenes is fundamental in organic chemistry. Alkenes are hydrocarbons with at least one carbon-carbon double bond. The stability of these compounds depends on various factors, including hyperconjugation, sterics, and electronic effects.
Stability is often discussed in terms of energy. More stable alkenes have lower potential energy and are less reactive towards reactions like hydrogenation or polymerization.

• Hyperconjugation: A key factor affecting alkene stability is hyperconjugation, where electrons are delocalized over the molecule, dispersing energy and increasing stability.
• Steric Effects: Alkenes with less steric hindrance are more stable because bulky groups can increase energy by causing repulsion.

Understanding these principles helps explain why some alkenes are more reactive or stable than others. This lays the foundation for predicting and designing chemical reactions in organic synthesis.

Organic Chemistry

Organic chemistry is the science of carbon compounds. It includes a wide variety of molecules ranging from small hydrocarbons to complex biomolecules. One of its core areas is the study of reactivity and stability, especially within different types of multiple bonds like those in alkenes.

• Reactivity: Alkenes can undergo a range of reactions, from electrophilic additions to polymerizations, due to their reactive double bonds.
• Structure: The arrangement of substituents around a double bond impacts both the molecule's physical and chemical properties.
• Electronics: Concepts like resonance, hyperconjugation, and inductive effects explain reactivity patterns in alkenes.

Grasping these elements offers deeper insight into how and why chemical reactions proceed in particular ways. This makes organic chemistry not only a science of structures but a window into diverse reactions and mechanisms.

Reactivity Comparison

Comparing reactivity among alkenes can illuminate why one compound might participate more readily in chemical reactions than another. This involves understanding the factors that either stabilize or destabilize the alkene's double bond.
The concept of reactivity is closely tied with stability:

• More stable alkenes are less reactive due to lower energy states.
• Hyperconjugation plays a pivotal role, where increased delocalization of electrons can enhance stability and decrease reactivity.
• Compounds like but-2-ene are more reactive because their structure supports greater hyperconjugation compared to more branched alkenes.

By analyzing structural details, chemists can predict not just the reactivity order of alkenes, but also design synthesis pathways that maximize or minimize specific reactions. Understanding these comparisons is crucial for drawing connections between molecular structure and chemical behavior.