What is the primary difference between Bond Dissociation Enthalpy (BDE) and Average Bond Enthalpy?

The primary difference lies in their specificity. Bond Dissociation Enthalpy (BDE) is the energy required to break a *specific* bond in a *specific* molecule in its gaseous state, forming two neutral radicals. It's a precise value for a particular bond in a given chemical environment. Average Bond Enthalpy, on the other hand, is an average value for a particular type of bond (e.g., C-H) across a range of different molecules. It's a useful approximation for general calculations but lacks the precision of BDE for a single bond cleavage event.

Why are BDE values always positive?

BDE values are always positive because bond breaking is an endothermic process. Energy must be supplied from the surroundings to overcome the attractive forces holding the atoms together in a covalent bond. This energy input is absorbed by the system, leading to a positive enthalpy change. Conversely, bond formation is an exothermic process, releasing energy, and thus has a negative enthalpy change.

How does resonance affect Bond Dissociation Enthalpy?

Resonance significantly affects BDE, particularly in organic molecules. If the radical formed after homolytic bond cleavage can be stabilized by resonance, the energy required to break that bond (BDE) will be *lower*. This is because the resonance stabilization of the resulting radical effectively lowers its energy, making the bond breaking process energetically more favorable. A classic example is the benzylic C-H bond, which has a lower BDE due to the resonance stabilization of the benzyl radical.

Can BDE be used to predict the stability of free radicals?

Yes, BDE is an excellent tool for predicting the relative stability of free radicals. A lower BDE for a particular bond implies that the resulting radical is more stable. For example, the BDE for a tertiary C-H bond is lower than that for a secondary C-H bond, which is lower than for a primary C-H bond. This directly indicates that tertiary radicals are more stable than secondary, which are more stable than primary radicals, a crucial concept in organic reaction mechanisms.

In what state of matter is Bond Dissociation Enthalpy typically measured?

Bond Dissociation Enthalpy is typically measured and defined for bonds in molecules in the *gaseous* state. This is because in the gaseous state, intermolecular forces are negligible, and the energy measured primarily reflects the energy associated with breaking the intramolecular covalent bond itself, without interference from solvent effects or lattice energies present in liquid or solid states.

Bond Dissociation Enthalpy

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Bond Dissociation Enthalpy (BDE), often denoted as $D$ or $DH^circ$ , is the enthalpy change required to break a specific covalent bond in a gaseous molecule, resulting in two neutral fragments, typically radicals. This process is defined for homolytic cleavage, where each atom involved in the bond retains one electron from the shared pair. It is a precise measure of the strength of a particular bo…

Quick Summary

Bond Dissociation Enthalpy (BDE) is the energy required to break a specific covalent bond in a gaseous molecule, forming two neutral radicals via homolytic cleavage. It is always a positive value, signifying an endothermic process.

BDE is a direct measure of bond strength; higher BDE means a stronger bond. It differs from average bond enthalpy, which is an average value for a bond type across various molecules. Factors influencing BDE include bond order, atomic size, hybridization, and crucially, the resonance stabilization of the resulting radicals.

Lower BDE indicates a more stable radical. This concept is vital for understanding radical reaction mechanisms, predicting reaction feasibility, and determining the relative stability of organic intermediates, making it a cornerstone for NEET chemistry.

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Key Concepts

Homolytic vs. Heterolytic Cleavage

Understanding the distinction between homolytic and heterolytic cleavage is fundamental to BDE. Homolytic…

Influence of Hybridization on BDE

The hybridization state of the carbon atom involved in a C-H bond significantly impacts its BDE. As the…

Calculating Reaction Enthalpy using BDEs

BDE values are invaluable for estimating the enthalpy change ( $Delta H_{reaction}$ ) of a chemical reaction,…

Definition — Energy for homolytic cleavage of a specific bond in gas phase, forming neutral radicals.

Sign — Always positive (endothermic).

Relation to Bond Strength — Higher BDE = Stronger bond.

Formula for $\Delta H_{reaction}$ —

\Delta H_{reaction} = \sum D_{broken} - \sum D_{formed}

Radical Stability Order (Inverse of BDE) — Resonance-stabilized (Allylic/Benzylic) > Tertiary > Secondary > Primary > Methyl.

Factors Increasing BDE — Higher bond order, smaller atomic size, higher s-character.

Factors Decreasing BDE — Resonance stabilization of radicals, steric hindrance.

Bonds Dissociate Easily if Radicals Stabilize. (BDE is low if Radicals are Stable).