Unveiling the Molecular Mechanism of n-Bromobutane Synthesis Experiment: A DFT Study for Undergraduate Organic...

The synthesis of n-bromobutane from n-butanol is a classic undergraduate organic chemistry experiment, primarily intended to illustrate the bimolecular nucleophilic substitution (S_N2) mechanism. However, this experiment is commonly plagued by low yields and the formation of byproducts (e.g., n-butene and di-n-butyl ether), which confuse students. To reveal the molecular origin of these competitive pathways, this study employs density functional theory (DFT) calculations to systematically investigate the reaction mechanism under acid catalysis. Four potential reaction pathways were explored: S_N2 substitution, E2 elimination, intermolecular etherification, and a high-energy E2 pathway. The computational results indicate that the S_N2 pathway to n-bromobutane is kinetically and thermodynamically favorable due to its low energy barrier. In contrast, the E2 elimination pathway possesses a higher energy barrier (18.8 kcal/mol vs. 13.5 kcal/mol for S_N2), explaining why elevated temperatures favor the formation of n-butene. Moreover, the etherification pathway was found to be the most energetically demanding, consistent with the trace amounts of di-n-butyl ether observed experimentally. These findings provide a quantitative molecular-level rationale for the strict temperature control and standardized reagent addition sequences in the laboratory protocol. By visualizing the potential energy surfaces, this computational approach bridges the gap between theoretical mechanism and practical operation, offering a valuable pedagogical tool for enhancing student understanding.