Bond Dissociation Enthalpy — Prelims Strategy

Prelims Strategy

To ace NEET questions on Bond Dissociation Enthalpy, a multi-pronged strategy is essential. Firstly, master the core definition: BDE is for homolytic cleavage, forms neutral radicals, and is always positive.

Understand its distinction from average bond enthalpy – BDE is specific, average bond enthalpy is generalized. Secondly, memorize the radical stability order: Resonance-stabilized (Allylic, Benzylic) > Tertiary > Secondary > Primary > Methyl.

This is critical because BDE is inversely proportional to the stability of the radical formed. A lower BDE means a more stable radical. Thirdly, understand factors affecting BDE: Higher bond order, smaller atomic size, and higher s-character in hybrid orbitals generally increase BDE.

Resonance stabilization and steric hindrance generally decrease BDE.

For numerical problems, meticulously list all bonds broken in reactants and all bonds formed in products, along with their respective BDE values. Use the formula: $\Delta H_{reaction} = \sum (\text{BDE of bonds broken}) - \sum (\text{BDE of bonds formed})$.

Pay close attention to stoichiometry. For conceptual questions, focus on identifying the type of radical formed and its stability. Trap options often involve confusing homolytic with heterolytic cleavage, or misapplying the radical stability order.

Practice identifying the 'weakest link' (lowest BDE) in a molecule for radical reactions. Always double-check your calculations and conceptual reasoning.