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What is a bond dissociation energy (BDE)?
What is a C-H Bde?
How do Bde bonds differ with hybridization?
How can bond breaking be quantified using the bond dissociation energy?
What is homolytic bond dissociation energy?
Is bond dissociation energy correlated to free radical stability?
The bond-dissociation energy (BDE, D 0, or DH°) is one measure of the strength of a chemical bond A−B. It can be defined as the standard enthalpy change when A−B is cleaved by homolysis to give fragments A and B, which are usually radical species.
- Quantifying Free Radical Stability
- Why Does Water Have A Higher Bond Dissociation Energy Than Methane?
- Bond Dissociation Energy Is Correlated with Free Radical Stability
- Factor #2: Free Radicals Are Stabilized by Resonance.
- Factor #3: Free Radicals Are Stabilized by Adjacent Atoms with Lone Pairs.
- Factor #7: Electron Withdrawing Groups Destabilize Free Radicals.
- Summary: Quantifying Free Radical Stability with Bond Dissociation Energies
- Notes
- References and Further Reading
Over the last two postswe’ve been going through the factors which affect the stability of free radicals. The bottom line is that radicals are electron deficient and that any factor which either helps to donate electron density to the half-filled orbital, or to spread that unpaired electron out over a larger volume(a.k.a “delocalize” it) will stabil...
Let’s look at a quick representative example. Take two molecules – methane (CH4) and water (H2O). Which has the weaker bond to H ? Thinking back to some of the chemistry we’ve talked about earlier, such as acid base reactions, it might be tempting to say that O–H is weaker than C–H, since we can think of many strong bases which will deprotonate wat...
The bottom line for this post is that bond dissociation energy is correlated to free radical stability. Low bond dissociation energies reflect the formation of stable free radicals, and high bond dissociation energies reflect the formation of unstable free radicals. [EDIT: Caveats apply when extending this discussion beyond the scope discussed in t...
Note the difference in bond strengths between the (primary) C-H bond of propane and of the alkyl C-H bond of propene. The sizeable difference [~13 kcal/mol] is a reflection of the greater stability of the resonance-stabilized “allyl” radical. Although not directly comparable, look at the C-H bond strength when it is adjacent to two alkenes [76 kcal...
[This is a subtle point!]. Note the difference in bond strengths between the C-H bond of methane [104 kcal/mol] and that of methanol [95 kcal/mol]. In between we have the C-H bond of fluoromethane [101 kcal/mol]. Note that even though fluorine is more electronegative than H, the presence of the lone pairs on F is actually stabilizingrelative to H.
To isolate this effect it’s important to look at examples where the electron-withdrawing group cannot donate a lone pair to the radical (see factor #3). One good example is comparing the C-H strength in ethane vs. trifluoroethane.
Hopefully it’s clear by now that where C-H bonds are concerned, by examining bond dissociation energies, we can discern trends in free-radical stabilities. This will be of prime importance in understanding selectivityin free radical reactions: “which free radical forms?”. A subtle point is that it is also important in understanding fragmentation pa...
Note 1. More caution is required than I had previously indicated regarding the main thesis of this post – that free radical stabilities are solely reflected by bond strengths. They depend on the stability of bothreactant and reagent. Edits are indicated inline. For more discussion see bottom section.[ TL;DR – the general trends in this post are val...
Shortcomings of Basing Radical Stabilization Energies on Bond Dissociation Energies of Alkyl Groups to Hydrogen Andreas A. Zavitsas, Donald W. Rogers, and Nikita MatsunagaThe Journal of Organic Che...On the Advantages of Hydrocarbon Radical Stabilization Energies Based on R−H Bond Dissociation Energies Matthew D. Wodrich, W. Chad McKee, and Paul von Ragué SchleyerThe Journal of Organic Chemistr...The Radical Stabilization Energy of a Substituted Carbon-Centered Free Radical Depends on Both the Functionality of the Substituent and the Ordinality of the Radical Marvin L. PoutsmaThe Journal of...A Single Universal Scale of Radical Stabilization Energies Does Not Exist: Global Bond Dissociation Energies and Radical Thermochemistries Are Described by Combining Two Universal Scales Andreas A....He explains how to identify Delta H, the activation energy, and the transition state on a reaction coordinate diagram. He then introduces and defines bond dissociation energy (aka bond...
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The products of homolytic cleavage are radicals and the energy that is required to break the bond homolytically is called the Bond Dissociation Energy (BDE) and is a measure of the strength of the bond.
Jan 23, 2023 · The products of homolytic cleavage are radicals and the energy that is required to break the bond homolytically is called the Bond Dissociation Energy (BDE) and is a measure of the strength of the bond.
Mar 12, 2019 · – A bond dissociation energy is the ΔH° for a specific kind of reaction—the homolysis of a covalent bond to form two radicals. – Because bond breaking requires energy, bond dissociation energies are always positive numbers, and homolysis is always endothermic.
The amount of energy needed to break a given bond to produce two radical fragments when the molecule is in the gas phase at 25 °C is a quantity called the bond strength, or bond dissociation energy (D). Each specific bond has its own characteristic strength, and extensive tables of such data are available.
