Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is fast debrominated? (a) BrC1CCCCC1 (b) BrC1=CCCCC1 (c) BrC1C=CCCC1 (d) BrC1C=CC=CC1

Step-by-Step Solution

Answer

Compound (d) is the fastest debrominated due to extensive resonance stabilization.

Step 1: Identify the Compound Structures

First, decode the SMILES notation into structural formulas. - (a) Br-C1CCCCC1 is bromocyclohexane, a saturated six-membered ring. - (b) Br-C1=CCCCC1 is bromocyclohexene, a six-membered ring with one double bond. - (c) Br-C1C=CCCC1 is another bromocyclohexene, but with a different position of the double bond. - (d) Br-C1C=CC=CC1 is a bromocyclohexa-1,3-diene, a six-membered ring with two conjugated double bonds.

Step 2: Evaluate Stability of Intermediates

Debromination often involves the formation of a radical intermediate. Assess the stability of potential radical intermediates: - Saturated rings (a) yield relatively stable radicals but slower due to hyperconjugation availability. - Unsaturated rings (b) and (c) form radicals which are stabilized by single double bonds, providing some resonance stabilization. - Conjugated double bonds in (d) offer the most radical stabilization through extended conjugation and resonance.

Step 3: Consider Resonance and Conjugation

Analyze the resonance and conjugation effects: - Compound (d) has two conjugated double bonds, which provide extensive resonance stabilization to the radical intermediate form after debromination. - This extended conjugation makes the radical in (d) far more stable compared to the other structures, allowing faster debromination.

Step 4: Conclusion Based on Observations

Given the extended resonance and conjugation in compound (d), this compound will be debrominated faster. The resonance offered by conjugated dienes significantly accelerates the reaction by stabilizing the intermediate radical.

Key Concepts

Radical Stability

In organic chemistry, the concept of radical stability is crucial when discussing reactions like debromination. A radical is essentially an atom or molecule with an unpaired electron, making it highly reactive. During debromination, a radical intermediate is often formed. The stability of this radical is key to determining the speed of the reaction.

• Saturated rings, such as those found in bromocyclohexane, generate relatively stable radicals due to hyperconjugation.
• However, radicals formed in unsaturated rings, like bromocyclohexene, benefit from resonance stabilization as they have access to a single double bond.
• The most stable radicals arise from conjugated double bond systems, as seen in bromocyclohexa-1,3-diene. This compound allows for extensive resonance, making the radical highly stable.

Understanding the stability of different radical intermediates provides insight into why certain compounds are debrominated more quickly than others. Conjugated systems enhance radical stability, increasing reaction rate noticeably.

Resonance Stabilization

Resonance stabilization is an important factor in determining the stability of radical intermediates. This concept involves the delocalization of electrons across a molecule, allowing for alternative structures and improved stability.

• For example, single resonance is available in unsaturated compounds like bromocyclohexene, which can delocalize the unpaired electron over a double bond.
• In contrast, compounds like bromocyclohexa-1,3-diene benefit from extensive resonance due to their two conjugated double bonds.

In any resonance-stabilized system, the molecule can adopt several resonant forms. This ability to distribute electron density over a wider area greatly reduces the energy of the radical and enhances its stability.

This stabilization is linear for the radical formed during debromination, making molecules like bromocyclohexa-1,3-diene react more quickly compared to simpler, less resonance-capable structures.

Conjugation Effects

Conjugation refers to the overlapping of p-orbitals across adjacent atoms in a chain, allowing for the delocalization of π (pi) electrons. This effect is instrumental in providing stability to radical intermediates formed during reactions like debromination.

• Conjugation is most prominently observed in compounds like bromocyclohexa-1,3-diene, where alternating double bonds provide a pathway for electron delocalization.
• This results in an extensively stabilized radical intermediate due to the seamless interaction between the π bonds.
• The greater the conjugation, the more stable the radical, hence faster the reaction proceeds.

The conjugation in molecules like bromocyclohexa-1,3-diene allows electrons involved in the radical state to distribute more effectively over the structure, decreasing the overall energy of the intermediate.

Such effects highlight the importance of molecular structure in radical-catalyzed reactions, where conjugated systems enable faster debromination through increased intermediate stability.