Question

The red \(\mathrm{C}-\mathrm{H}\) bond has a high bond dissociation energy because it is doubly allylic.

Short answer

Answer: A red C-H bond has a high bond dissociation energy when it is doubly allylic because the bond is connected to two double bonds in the molecule, causing electron density to be more delocalized. This delocalization leads to increased resonance and stability, which results in a higher bond dissociation energy, making it harder to break compared to a non-allylic C-H bond.

Step-by-step solution

Understand bond dissociation energy

Bond dissociation energy (BDE) is the energy required to break a chemical bond, usually measured in units of kcal/mol or kJ/mol. A high BDE corresponds to a strong chemical bond, which means it requires more energy to break. Conversely, a low BDE suggests a weaker bond that can be broken more easily.

Understand allylic bonds and doubly allylic bonds

An allylic bond is a chemical bond located adjacent to a double bond (C=C) in an organic molecule. Since a double bond has electron-rich regions, the adjacent carbon atoms have an increased electron density. This makes the allylic bond more stable due to the delocalization of electron density. A doubly allylic bond is a bond that has double bonds on both sides, meaning it is connected to two double bonds in the molecule.

Relate the doubly allylic bond to high bond dissociation energy

When a bond is doubly allylic, it means the bond is connected to two double bonds, and the electron density is more delocalized across the molecule. This delocalization makes the bond more stable due to increased resonance, which results in a higher bond dissociation energy. In the case of a red C-H bond, the increased stability due to being doubly allylic leads to a high bond dissociation energy, making it harder to break compared to a non-allylic C-H bond.

Key concepts

Allylic Bond

Understanding the concept of an allylic bond is essential for grasping why some bonds in organic molecules have higher stability and, consequently, higher bond dissociation energies. An allylic bond is particularly interesting because it is not a specific type of bond like a single or double bond, but rather a position in a molecule. Specifically, when we say a bond is allylic, we mean that it is adjacent to a carbon-carbon double bond (C=C).

The carbon atom that holds the allylic bond possesses a p-orbital that overlaps with the π-orbitals of the double bond. This orbital overlap allows the electrons to delocalize over the adjacent structures, providing some of the stability that characterizes double bonds to the allylic position as well. The resulting electron density around the allylic position can create what's known as hyperconjugation, further stabilizing the molecule.

Key Points in Simple Terms

• An allylic bond is not a type of bond but a position next to a double bond.
• Electron delocalization through orbital overlap leads to increased stability.
• Hyperconjugation enhances the stability of the allylic bond.

Chemical Bond Strength

The strength of a chemical bond is commonly expressed in terms of its bond dissociation energy (BDE). In simple terms, the higher the BDE, the stronger the bond, as it requires more energy to break apart the bonded atoms. Various factors can affect bond strength, such as the types of atoms involved, the bond length, and the bond order—the number of shared electron pairs between two atoms.

Chemical bonds are the glue that holds molecules together, and understanding the strength of these bonds provides insights into the molecule's reactivity and stability. Strong bonds are less likely to break under normal conditions, making them crucial in determining a molecule's behavior during chemical reactions. For example, a bond with high BDE will be less reactive toward chemical processes that involve bond breaking, such as combustion or polymerization.

Understanding BDE in Practice

• BDE is a direct measure of bond strength.
• Stronger bonds are less reactive and more stable.
• Factors like bond length and bond order are key determinants of BDE.

Resonance Stabilization

Resonance stabilization is a concept in chemistry that explains how certain molecular structures are more stable due to the delocalization of electrons across different parts of the molecule. This delocalization allows for the electrons to be spread out over a larger volume of space, thereby lowering the overall energy of the molecule. You can think of it as a sort of 'electron sharing' on a molecular level that contributes to stability.

Particularly in the case of our allylic bonds, the presence of adjacent double bonds enables a resonance effect, where the electrons are not fixated in one spot but can move between different bonding and non-bonding p-orbitals. This movement creates several resonance structures, which are different ways of drawing the molecule that depict the various positions of electron density. The actual molecule is a hybrid of these structures and is therefore more stable than any individual resonance structure would suggest.

Applying Resonance Stabilization

• Resonance lowers the energy of a molecule by delocalizing electrons.
• Doubly allylic positions often have enhanced resonance stabilization.
• Resonance structures are tools to visualize electron delocalization.