Bond Dissociation Enthalpy — Revision Notes | Vyyuha Knowledge Platform

⚡ 30-Second Revision

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.

2-Minute Revision

Bond Dissociation Enthalpy (BDE) is the energy required to break a single, specific covalent bond homolytically in a gaseous molecule, yielding two neutral radicals. It's always a positive value, indicating an endothermic process.

A higher BDE signifies a stronger bond. This concept is distinct from average bond enthalpy, which is a generalized value. BDE is crucial for understanding radical stability: a lower BDE implies a more stable radical is formed.

Key factors influencing BDE include bond order (higher BDE for higher order), atomic size (smaller atoms, higher BDE), hybridization (more s-character, higher BDE), and critically, resonance stabilization of the resulting radical (lowers BDE).

For NEET, remember the radical stability order (Allylic/Benzylic > Tertiary > Secondary > Primary > Methyl) and use the formula $\Delta H_{reaction} = \sum D_{broken} - \sum D_{formed}$ for enthalpy calculations.

5-Minute Revision

Bond Dissociation Enthalpy (BDE) is a precise measure of bond strength, defined as the enthalpy change for the homolytic cleavage of a specific bond in a gaseous molecule, producing two neutral radical fragments.

This process is always endothermic, so BDE values are positive. A higher BDE indicates a stronger, more stable bond. For example, the BDE of a C-H bond in methane is about $439,\text{kJ/mol}$ , while a benzylic C-H bond in toluene has a lower BDE (around $375,\text{kJ/mol}$ ) due to the resonance stabilization of the benzyl radical formed.

This specificity distinguishes BDE from average bond enthalpy, which is an average value for a bond type across various molecules. BDE is inversely related to the stability of the radical formed: the more stable the radical, the lower the BDE of the bond that forms it. The general order of radical stability is: Resonance-stabilized (e.g., Allylic, Benzylic) > Tertiary > Secondary > Primary > Methyl. This order is fundamental for predicting reactivity in radical reactions.

Several factors influence BDE: higher bond order (e.g., C $equiv$ C > C=C > C-C) increases BDE. Smaller atomic size (e.g., H-F > H-Cl) increases BDE. Greater s-character in hybrid orbitals (e.g., C-H in ethyne > ethene > ethane) increases BDE. Conversely, resonance stabilization of the resulting radicals and steric hindrance around the bond tend to decrease BDE.

For calculating reaction enthalpies, BDEs are used in the formula: $\Delta H_{reaction} = \sum (\text{BDE of bonds broken}) - \sum (\text{BDE of bonds formed})$ . For instance, in the reaction $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$ , one C-H and one Cl-Cl bond are broken, while one C-Cl and one H-Cl bond are formed.

Summing the BDEs of broken bonds and subtracting the sum of BDEs of formed bonds yields the reaction enthalpy. Mastery of BDE is crucial for understanding organic reaction mechanisms and thermochemical calculations in NEET.

Prelims Revision Notes

Bond Dissociation Enthalpy (BDE) - NEET Quick Notes

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Definition — Energy required to break *one specific* covalent bond in a *gaseous molecule* via *homolytic cleavage*, forming two *neutral radicals*. Denoted as

D

or

DH^circ

.

2

Nature — Always positive (endothermic process). Energy is absorbed to break bonds.

3

Bond Strength — Directly proportional to BDE. Higher BDE = stronger bond.

4

Specificity — BDE is *specific* to a bond in its molecular environment. It is NOT the same as average bond enthalpy.

* Example: BDE of C-H in $CH_4$ is different from C-H in $CH_3CH_3$ .

1

Factors Affecting BDE (and Radical Stability)

* Radical Stability: BDE is *inversely proportional* to the stability of the radical formed. More stable radical $\implies$ lower BDE. * Order of Radical Stability: Resonance-stabilized (Allylic, Benzylic) > Tertiary ( $3^circ$ ) > Secondary ( $2^circ$ ) > Primary ( $1^circ$ ) > Methyl.

* *Benzylic radical example*: $C_6H_5CH_2^cdot$ is highly stable due to resonance with the benzene ring, hence benzylic C-H has low BDE. * *Allylic radical example*: $CH_2=CH-dot{C}H_2$ is resonance stabilized, hence allylic C-H has low BDE.

* Bond Order: Higher bond order $\implies$ higher BDE (e.g., C $equiv$ C > C=C > C-C). * Atomic Size: Smaller atoms $\implies$ stronger bonds $\implies$ higher BDE (e.g., H-F > H-Cl). * Hybridization (s-character): More s-character $\implies$ stronger bond $\implies$ higher BDE.

* Example: C-H in ethyne (sp) > ethene ( $sp^2$ ) > ethane ( $sp^3$ ). * Steric Hindrance: Can weaken bonds $\implies$ lower BDE.

1

Calculation of Reaction Enthalpy ($\Delta H_{reaction}$)

* Formula: $\Delta H_{reaction} = \sum (\text{BDE of bonds broken}) - \sum (\text{BDE of bonds formed})$ * Key: Identify *all* bonds broken in reactants and *all* bonds formed in products. Pay attention to stoichiometry. * Example: For $A-B + C-D \rightarrow A-C + B-D$ $\Delta H = [D(A-B) + D(C-D)] - [D(A-C) + D(B-D)]$

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NEET Relevance — Crucial for:

* Understanding free radical mechanisms (initiation, propagation). * Predicting reactivity and selectivity in organic reactions. * Comparing stability of organic intermediates. * Solving thermochemistry problems involving bond energies.

Vyyuha Quick Recall

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