Organic Chemistry Practice Exam Guidance – NMR, Bond Strength, and Reaction Mechanisms

Q4. Consider all of the different types of C-H bonds in cyclopentene and rank them in order of increasing bond strength.

Background

Topic: Bond Dissociation Energy (BDE) in Organic Molecules

This question tests your understanding of how the environment around a C-H bond affects its bond strength, particularly in cyclopentene. Factors such as hybridization, resonance, and substitution influence BDE.

Key Terms and Concepts:

• Bond Dissociation Energy (BDE): The energy required to break a bond homolytically (producing radicals).

• Hybridization: C-H bonds on sp2 carbons (alkenes) are generally stronger than those on sp3 carbons (alkanes).

• Allylic Position: C-H bonds adjacent to a double bond (allylic) are weaker due to resonance stabilization of the resulting radical.

• Vinylic Position: C-H bonds directly on a double bond (vinylic) are stronger due to sp2 hybridization.

Step-by-Step Guidance

1. Identify the types of C-H bonds in cyclopentene: allylic, vinylic, and aliphatic (on methyl groups).

2. Recall that allylic C-H bonds are generally weaker because the resulting radical is stabilized by resonance.

3. Vinylic C-H bonds (directly attached to the double bond) are stronger due to sp2 hybridization and lack of resonance stabilization.

4. Aliphatic C-H bonds (on methyl groups) are intermediate in strength, typically weaker than vinylic but stronger than allylic.

5. Rank the bond strengths from weakest (lowest BDE) to strongest (highest BDE) based on their positions and the above reasoning.

Try solving on your own before revealing the answer!

Final Answer:

The order of increasing bond strength is: Allylic < Aliphatic < Vinylic.

Allylic C-H bonds are weakest due to resonance stabilization, vinylic C-H bonds are strongest due to sp2 hybridization, and aliphatic C-H bonds are intermediate.